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Bell & Howell Information and Leaming 300 Norlh Zeeô Road, Ann Arbor, MI 481 06-1 346 USA
-521 -0600
AN lNVESTlGATlON O F Z R AND Tl-BEARING
ALKALI ALUMINOSILICATE GLASSES: SOLUBlLlTY EXPERIMENTS, RAMAN
SPECTROSCOPY AND 2 3 ~ ~ NMR ANALYSES
by RoSert A. Man
May, 1998
A thesis submitted to the Faculty of Graduate Studies and Research in partial
fuifillment of the requirements of the degree of Ooctor of Philosophy
Department of Earth and Planetary Sciences
McGill University
Monléal, Québec
Canada
O Robert A. Man - MCMXCVlll
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The solubility of Zr-bearing minerals in peralkaiine, H,O-saturated alkali
aluminosiiicate melts with or without added F reaches a maximum of about 4 and 3.5 W.%
23-0, respectively at approximately 57 wt% Si02 The saturating phase for melu with SiO2
content above this threshold is zircon. In the halogen-free experiments, the saturating phase
for melts with lower silica content is wadeite (K2ZrS &O9), while ZQ crystallizes from melts
with I wt.% added F. Experiments with Cl-bearing melts indicate no maximum solubility
of Zr minerals; the solubility remains a: 2 to 2.2 wt.% Zr02 across a wide compositional
range. The saturating phases in the Cl-bearing melts are the same as those of the F-bearing
melts and the transition from ZrO, to zircon occurs at the same composition. The positive
dope of the wadeite saturation curve on X,-X,, plots for the halogen-- melts suggests
that increasing SiOl activity increases the solubility of wadeite. This behaviour of wadeite
would not be predicted based on a simple dissolution mechanism. Therefore, a more complex
equilibriurn involving melt structure is proposed. Observations that zircon is scarce, whereas
el pidite (N- &O,, -3H2 O) and other alkali and alkalineearth Wconosilicates are abundant
in the Strange Lake peralkaline intrusive complex in northem Quebec/Labrador indicate that
the parent magma did not saturate with a Zr-bearing mineral until it cooled to a low
temperature, probably less than 600 OC.
Peralkaline Ti,Zr-bearing sodium duminosilicate glasses have been analyzed by
Raman spectroscopy to determine the effect of CI on glas stxucture. The spectra of the Ti-
bearing glasses show a significant difference between the CI-free and the Cl-bearing
composition. The Cl-free glass spectrum contains a strong, asymmetrïc peak at 900 cm"
which is associated with Ti in five-fold coordination. This peak is shified to higher
frequency and becomes more symmetric with the addition of 0.3 wt% CI. Deconvolution
of the high-fiequency waveband suggests that differences between spectra result from the
contribution of a peak at 945 cm-'. This peak is believed to be the result of Ti-O vibrations
in fûliy-polymerized titanate tetrahedra. It is proposed that the addition of Cl destabiiizes
[ T i in favour of tevahedral coordination as a result of cornpetition between Cl and titanate
groups for aikalis. The spectra of Zr-bearing glas show a strong peak at 980 cm-' which is
not seen in spectra of the base glass, indicating a vibrational mode associated with a
zirconosilicate stmcturai environment. There is no discemible difEerence between the
Raman spectra of Zr-bearing glasses with and without added Cl, indicating that the local
structure around Zr is insensitive to the presence of Cl.
=Na NMR MAS analyses of a suite of Na-duminosilicate glasses with NdAi = 2 and
v w n g SiO, content has revealed a trend toward more negative chemical shifi ( p a t e r
shielding of the nucleus) as the glas structure becomes more polymerized, Le. the average
number of non-bndging oxygen atoms per tetnhedron (NBOm decreases. This trend is
obsemed only for glasses with NBO/T s 0.3. For more polymerized glasses no change in
chernical shift is measured. Past studies have indicated a strong correlation between
increasing Na-O bond length and increasingly more negative .=Na chemical SM. This
suggests that for Na-aiurninosilicate glasses with NBO/T s 0.3, Na-O bond lengths would
be expec ted to increase as polymerization increases.
La solubilité des minéraux de Wcmium dans les liquides silicates peralcalins, saturés
en H20 et formés d'aiuminosilicates alcalins, aux quels on a ajouté ou non du fluor, atteint
respectivement un maximum à environ 4 et 3.5% poids de ZrO, à environ 57% SiO,. La
phase qui sature les liquides ayant un contenu en SiO, au-dessus de ce seuil est le zircon.
Dans les expériences sans halogène, la phase qui sature les liquides avec un un contenu plus
faible en SiO, est la wadéite (I(2ZrSi3O,), tandis que le ZrO, cristallise à partir de liquides
avec I % de fluor. Les expériences avec des liquides contenant du chiore n'indiquent pas de
maximum de solubilité pour les minéraux de zirconium; la solubilité reste entre 2 et 2.2%
ZrO, à l'intérieur d' une vaste gamme de compositions. Les phases qui saturent les liquides
chlorés sont les mêmes que pour les liquides fluorés, et la transition de ZrO, au zircon se
produit à la même composition. D'après la pente positive de la courbe de saturation de la
wadéite sur un diagramme Xm2-XSa2 pour les liquides sans halogène, une augmentation de
l'activité de la silice augmente la solubilité de la wadéite. Ce comportement de la wadéite
est inattendu si on s'entient à un simple mécanisme de dissolution. Par conséquent, un
équilibre plus complexe impliquant la structure du liquide semble indiqu6. Dans le complexe
intrusif perdcalin de Strange Lake dans le nord du Québec-Labrador, le zircon est peu
abondant, tandis que I'elpidite (Na&3,0,,-3H20) et d'autres zirconosilicates alcalins et
dcaiino-terreux le sont davantage, indiquant que le magma parent n'était pas saturé avec des
minéraux de zirconium jusqu'à ce qu'il évolue vers une basse température, probablement
moins de 600 OC.
Des verres d'aiuminosilicates de sodium avec Na/Ai > I et dopés avec Ti ou Zr ont
été analysés par spectroscopie de Raman afin de déterminer l'effet du chlore sur la structure
du verre. Les spectres du verre contenant du titane montrent une différence importante entre
les compositions possédant ou non du chlore. Le spectre du verre sans chlore contient un
important pic asymétrique à 9 0 cm-' qui est associé avec le [ q i . Ce pic est déplacé à une
plus haute fréquence et devient plus symétrique avec l'addition de 0.3 % de chiore. D'après
la déconvolution des bandes de hautes fréquences, les différences entre les spectres résultent
de la contribution d'un pic à 945 cm-'. Ce pic résulterait de vibrations Ti-O dans les
tétraèdres de titane dans la trame. L'addition de chlore déstabiliserait le IqTi en faveur de la
coordinance tétraédrique à cause de la compétition entre le chlore et les groupes de titane
pour les alcalins. Le spectre du verre contenant du zirconium montre un pic important à 980
cm-' absent dans les spectres du verre de base, indiquant un mode de vibration associé à la
structure du zirconosilicate. Il n'y a pas de différence discemable entre les spectres de Raman
des verres contenant du Zr avec ou sans chlore. indiquant que la structure locale autour du
zirconium est insensible à la présence du chiore.
Des analyses de "Na NMR MAS d'une série de verres d'aluminosilicates de sodium
avec NdAi = 2 et un contenu variable de SiOz ont révélé une tendance vers un dédoublement
chimique plus fortement négatif (plus grand camouflage du noyau) lorsque la structure du
verre devient plus fortement polymérisée, Le. le nombre moyen d'atomes d'oxygène non liés
à deux tétraèdres divisé par le nombre de tétraèdres (NBO/T) diminue. Cette tendance est
observée seulement pour les verres avec NBO/T 5 0.3. Pour les verres plus fortement
polymérisés, aucun changement dans le dédoublement chimique n'est décelé. Des études
précédentes ont montré une forte corrélation entre la longueur de la liaison Na-O et un
dédoublement chimique du %a de plus en plus fortement négatif. Pour les verres
d'aluminosilicates de sodium avec NBOm 5 0.3, les longueurs de la liaison Na-O devraient
donc augmenter avec une augmentation du degré de polymérisation.
Acknowledgments
1 wish to acknowledge the many individuals who contributed much to this thesis.
First of ali, 1 would like to thank my supervisors, Don Baker and AE. "Willy" Williams-
Jones. They provided guidance and fmanciai support, which were absolutely essentid to the
success of this undertaking. 1 owe them much. Numerous people provided technical expertise
and their patience is greatly appreciated. Glenn Poirier was always very helpful as I stniggled
to acquire good electron microprobe analyses of my diffcult samples. Andrew Vreugdenhil
spent many hours with me collecting Raman spectra and 1 am very grateful for his efforrs.
Fred Morin provided the NMR MAS analyses. 1 dso wish to thank Linda Reven, Grant
Henderson, Bob Martin and Michel Bouley who provided valuable insights.
1 have many fnends here at McGiU University whose support and insight were
invaluable in the completion of this endeavor. In particular, 1 would Iike to acknowledge
Pierre Hudon, Claude Daipé, Soûad Guemina, Leyla Houssain, Lawrence Yane, Sandy
Archibald, Jim Davies and Kate Ault. Thank you, my fnends.
Finally, for their love and support, 1 wish to thank my parents and my beloved Vice.
This thesis is as much a result of their efforts as mine.
....................................... ............... 3 -3 -2 Raman spectroscopie analyses .... 48 3.4 Results ............................ .- ....... ................................................... 48
........................................................................... 3 -5 Discussion ........ .... ............... .. 61 3.6 Conclusions ................................................................................................... 70
Chapter 4: "Na NMR MAS Analyses of Na-aluminosilicate Glasses with Varying SiO,
.............................................................................................................................. Content. 72
4.1 Abstract .......................................................................................................... 73 4.2 Introduction ................................................................................... .. .......... 74
.................................................................................. 4.3 Experimental Procedure -74 4.4 Results ............................................................................................................ ..75
....................................................................................... ............ 4.5 Discussion .. 80 ...................................................................................................... 4.6 Conclusions 89
Chapter 5: ALkali Zirconosilicate Phase Equilibria in the Systern: N~~o .K~o .AI~o~&~o~ .
................................................................. ....................................................... H.OG ....... -90
................................................................................................... 5.1 Introduc tion... 91 ................................................... ......................... 5.2 Experimental Procedure ....... 92
5.3 Results ......................................................................................................... 94 ...................................................................................... 5.4 Discussion .......... ... 96
...................................................................................................... 5.5 Conclusions 97
Chapter 6: ..................................................................................................................... Conclusions -98
...................................................................................................... 6.1 Conclusions 99 .................. 6.2 Contributions to Knowledge ... ................................................ 100
............................................................... 6.3 Recommendations for Future Work 101
........................................................................................................................ References 103
....................................................................................................................... Appendix 1 127
........................................................................ Appendix 2 ....................................... 129
....................................................................................................................... Appendix 3 131
vii
Preface This thesis is divided into six chapters: an introductory chapter, three manuscnpts,
a chapter reporting the results of phase equilibriurn experiments which will not be submitted
for publication and a concluding chapter. Each of the three manuscripts is CO-authored by my
thesis supervisors Drs. Don Baker and A.E. Williams-Jones, who provided advice on
research methodology and assisted in interpretation of results. Dr. Andrew Vreugdenhil of
the Department of Mining and Metallurgy at McGill is the third author of Chapter Three.
He contributed his expertise in Raman spectroscopy and perfomed the anaiyses with my
assistance. All interpretation of Raman spectra are, however, the sole responsibility of this
author. Glenn Poirier provided assistance in the collection of the electron microprobe data
reported in this thesis, although the author performed the anaiyses. Dr. Fred Morin of the
Department of Chemistry at McGill performed the nuclear magnetic resonance analyses
reported in Chapter Four.
The foiiowing is an excerpt from the "Guidelines Concerning Thesis Preparation" as
required by the Facuity of Graduate Studies and Research at McGill University:
Candidates have the option of including, as part of the thesis, the text of one or more papers submitted or to be subrnitted for publication, or the clearly- duplicated text of one or more published papers. These texts must be bound as an integral part of the thesis-
If this option is chosen, comecting texts that provide logical bridges between the different papers are rnanda!ory. The thesis must be written in such a way that it is more than a mere collection of manuscnpts; in other words, results of a senes of paper must be integrated.
The thesis must s a conform to al1 other requirements of the "Guidelines for Thesis Preparation". The thesis must include: A Table of Contents, an abstract in English and French, an introduction which clearly States the rationaie and objectives of the study, a review of the litenture, a finai conclusion and summary, and a thorough bibliography or reference list.
Additional matenai must be provided where appropriate (e.g. in appendices) and in sufficient detail to U o w a clear and precise judgement to be made of
the importance and ongiaaiity of the research reported in the thesis.
In the case of manuscripts CO-authored by the candidate and others, the candidate is required to make an explicit statement in the thesis as to who contributed to such work and to what extent. Supervisors must attest to the accuracy of such statements at the doctoral oral defense. Since the task of the examiners is made more difficult in these cases, it is in the candidate's interest to make perfectly clear the responsibilities of di authors of the co- authored papers.
Chapter 1
General Introduction
1.1 Introduction
P e r a h h e , felsic rocks represent a smdl proportion of the total volume of igneous
rock in the crust. However, such rocks have received a disproportionate share of scientific
and economic interest due to their having elevated concentrations of rare metals such as Zr,
Y, Nb and rareearth elements (REE). Adding to the curious nature of these rocks is the fact
that these rare metais are generally found in unusuai minerais such as REE fluorocarbonates
and alkali and aikaline earth Wrconosilicates- Whiie it has long been thought that these
minerais might serve as useful petrogenetic indicators, efforts to employ them in this mannet
have been hampered by the absence of thermodynamic data. Those attempts which have
been made have mostiy been limited to chemographic analysis (Williams-Jones and Wood,
1992; Marr and Wood, 1992) or synthesis experiments (Maurice, 1949; Baussy et al., 1974;
Caruba, 1975; Canrba et al., 1973; Christophe-Michel-Lévy, 196 1 ; Haschke, 1975; Haschke
and Eyring, 1971; Shunhua er al., 1986; Christensen, 1973; Chai and Mroczkowski. 1978;
Kutty et al., 1978; Tareen and KUW, 1980; Wakita and Kinoshita, 1979; Metz. 197 1). One
notable exception is the phase equilibna study of Cume and Zaleski (1985) which
constrained the pressure and temperature (P-T) conditions of the reaction:
elpidite (N@r!3i6O,,-3H20) - viasovite (Na7ZrSi,0,,) + 2 quartz + 3 H20
Given the relative paucity of reliable phase equilibria data, this project was
undertaken to elucidate the physico-chernical conditions of crystaiiization of the alkali
zirconosilicates and other Zr-bearing minerals at magmatic conditions and to describe the
structure of magmas from which these minerals are derived. It was found that two minerals;
wadeite (K2ZrSi,0,) and zircon (ZrS iO,), are predominant at these conditions. There fore,
in Chapter 2, the solubility of these mineds has been measured for several perallcaline melts.
Chapten 3 and 4 are Raman spectroscopy and = ~ a NMR MAS studies focused on the
structure of such melts.
1.2 Mineralogy of the Alkali Zirconosilicates
Aikali zirconosilicates have been reported in most perallcaIine rocks around the world
including reknowned loçalities such as Lovozero, Russia, Ascension Island, Ilimaussaq,
Greenland and Mont St. Hilaire, Canada (Table 1.1). It should be noted that many alkaiine
earth and mixed alkaii-alkaline earth zkconosilicates have also k e n discovered. In ail, there
are over two dozen zirconosilicates which have been given approval by the IMA
Commission on New Minerds and Mineral Names.
For most of the minerals in Table 1.1, there has only k e n one or two reponed
occurrences and none of these exceptionally rare minerais hâs ever been successfÛiiy
s yn thesized. The more common aikali zirconosilicates do warrant some afrention, ho wever,
as attempts to study h e m experïmentally have met with Limited success. These minerais
include: elpidite, vlasovite, catapleiite, parakeldyshite, dalyite and wadeite.
EIpidite has been reported at Lovozero, Russia (Neronova and Belov, 1963) as weU
as Mont S t. Hilaire, Quebec (Chao, 1967) and Strange Lake, QuebecLabrador (Miller,
1986). while vlasovite has k e n reported at Lovozero (Tikhonenkova and Kazakova, 1961),
Ascension Island (Fleet and Cam, 1967), Kipawa, Quebec (Gittins et al., 1973) and Strange
Lake (B irke tt et al., 1992). Several successfui syntheses of elpidite and vlasovite have k e n
done (Christophe-Michel-Lévy, 196 1; Baussy et al., 1974; Camba, 1975, Currie and Zaleski,
1985). The upper thermai stability Mt of elpidite has been established at 1.0 kbar
(595d°C) and 2.0 kbars ( 6 4 4 ~ I " C ) at which point it breaks down to vlasovite + quartz + H,O (Currie and Zaleski, 1985).
Catapleiite is another relatively common Na-zirconosilicate which has k e n reported
at several localities including Strange Lake (Birkett et al., 1992). Catapleiite has been
successfully synthesized at temperatures of 350 to 450°C and at a pressure of 0.7 kbar
(Baussy er al., 1974). although no reversed experiments have been performed with this
Mineral 1 Composition 1 Reference(s)
elpidite
vlasovite
catapleiite
gaidonnayite
hilairi te
parakeldys hi te
petarasite
te rs kite
wadeite
dalyite
umbite
khibinskite
kostylevite
Table 1.1 Naturaliy-occumng Na and K-zirconosiiicates. References are ris follows: 1. Cannillo et al- (1973); 2. Tikhonenkova and Kazakova (1962); 3. Chao et al. (1973); 4. Chao and Watkinson (1974); 5. Chao (1985); 6. Chao et al. (1974); 7. Rade and Mladeck (1977); 8. Chao et al. (1980); 9. Ghose et al. (1980); 10. Khomyakov et al. (1983a); 11. Prider (1939); 12. Henshaw (1955); 13. Van Tassel (1952); 14. Khomyakov et al. (1983b); 15. Khomyakov and Voronkov (1973); 16. Khomyakov et al. (1974); 17. ïiyushin et al. (198 1); I 8. Khomyakov et al. (1983~); 19. Boggs and Ghose (1985); 20. Ghose and Thakur (1985).
phase.
Parakeldyshite is one of two phases which had originaily b e n identified as keldyshite
(Gerasimovskii, 1962). Later workea detemllned that the original materid described by
Gerasmovskü actually contaïned two trichic phases, NaJrSi,O, (Phase II) and another with
indeterminate composition, but probably involving significant substitution of Na by H,O
(Phase 1) (Khomyakov et al. 1975; Khalilov et al. 1975). A second occurrence of Phase II,
which was named parakeldyshite (Khomyakov et al. 1975), was reported at Lagendalen,
Norway (Raade and Mladeck, 1977). Several successful syntheses have k e n accomplished
(Baussy et aï., 1974; Carub;r, 1975, Currie and Zaleski, 1985).
Wadeite and dalyite are the two most cornmon K-zirconosilicates, although the
number of reported occurrences is relatively smaü compared to that of Na-zirconosilicates.
Wadeite was first reported in the West Kimberley area of Australia (Prider, 1939) and was
fully characterized by Henshaw (1955). Wadeite has been successfully synthesized using
severai techniques (Caruba, 1975; C m b a et al. 1973). Dalyite was fmt reported at
Ascension Island (Van Tassel, 1952) and has been successfuily synthesized in one study
(Camba, 1975). No reversed experiments with K-zirconosilicates have ever k e n performed.
1.3 Raman Spectroscopy of Silicate Melts
Fundamental to any study of alkali zirconosilicate crystaliization is understanding the
nature of the silicate melt. Raman spectroscopy is a valuable technique for detemining melt
structures. At the most basic level, components of a silicate melt are thought of as network-
formers and network-modifiers. Network-fonning cations are those which contribute to the
nework of linked teuahedn. which make up a silicate melt Silicon is the most hpo-t of
these eiements, but aluminum as weil as boron and, in some cases, titanium act as network-
fomers &o. Al1 other cations act as network-modifiers which means that they alter the
structure of the meit in their vicinity. This alteration involves the transformation of bridghg
oxygen a t o m (those oxygen atoms that Link tetrahedra) to non-bridging oxygen atoms
(oxygen atoms which are part of a tetrahedron, but are bonded to one or more network-
modifien). This process, known as depolymerization, has a tremendous effect on melt
properties such as viscosity and difisivity.
A signifcant contribution to our kmwledge of the structure of silicate melts has been
provided by Raman scattering spectroscopy. When an incident beam of light, usually visibie
laser Light, is directed toward a sûmple, a s m d fiaction of that light is scattered. Most of the
scattering is elastic, known as Rayleigh scattering, and results in no change in wavelength.
However, a very small fraction of the scattered light results fiom inelastic interaction with
a vibrational mode (McMillan and Hofmeister, 1988). This is known as- aman scattering
and the scattered light in this case is shified to longer wavelengtb (Stokes scattering) or
shorter wavelength (anti-Stokes scattering).
The inelastic interaction of the incident photons with certain molecular vibrational
modes which genemtes Raman scattering arises because the electric field associated with
incident light oscillates at a frequency, v', much higher than the oscillation frequency for any
given molecular vibration, vi, which lies in the range 1012 to 10i4 Hz (McMillan and Wolf,
1995). The electronic response to this osciiiating electric field is a time-dependent, induced
dipole moment. /~,,(t), which is a function of electric field strength, E,, the oscillation
frequency and a proportionality constant, a, known as the molecular polarizability (McMillan
and Wolf, 1995):
The molecular polarizability depends on the relative positions of nucleü w ithin the molecule,
and therefore, the vibrationai frequency. For srnail vibrational displacement, qi, a may be
represented by a Taylor series (Wilson et al. 1955):
Substituting this expression into that for the induced dipole moment:
/lin, ( t ) = îXo Eo C O S 27W*t +
The new t e m which appear in the expression represent scattered light with frequencies
iower (va - vi)-and higher (v' + vi) than that of the incident beam, Stokes and anti-Stokes
Raman scattering respectively (McMillan and Wolf, 1995).
Interpretation of the Raman spectra of glasses and melts involves cornparison of the
spectra to those of crystaiiine matends with well-defined structures. The Raman spectra of
aikali and aikaline-earth silicate and durninosilicate @asses and rnelts have k e n studied
thoroughiy (Brawer and White, 1975; Fukumi et al. l99U; Furukawa et al. 198 1 ; Matson et
al. 1986; McKeown et al. 1984; McMillan, 1984a,b; McMiilan et al. 1982; McMUan et al.
1992; Mysen. 1990sb; Mysen, 1995; Mysen and Frantz, 1992; Mysen and Frana, 1994;
Mysen et al. 1985; Seifert et al. 1982). Therefore, many of the recent Raman snidies bave
concentrated on the structural effects of transition met& such as Y and Zr (Ellison and Hess,
1994) and Ti (Alberto et al. 1995; Henderson a d Fleet, 1995; Mysen and Neuville, 1995)
or halogens such as F (Mysen and Virgo, 1985a,b). The Raman study undertaken as part of
this thesis is intended to expand on these recent efforts and to include CI, a halogen known
to have a si gnificant effect on silicate melt properties (Hirayama and Camp, 1969; Baker and
Vaillancourt, 1995), but which has not, to date, been studied with this technique.
1.4 Sotid State Nuclear Magnetic Resonance
Nuclear magnetic resonance (NMR), like Raman spectroscopy, is a useful tool for the
study of silicate melts, and the two techniques are complirnentary. NMR is used to observe
changes in the local environment around a specific nucleus as a result of changes in the
electronic density function, unlike Raman spectroscopy, which detects the presence of
vibrations associated with certain bonds.
1.4.1 Basic Theory
Al1 sub-atomic particles possess 2 spin quantum number (1) of '/t. The manner in
which the spins pair up determines the net spin. Isotopes with an even number of both
protons and neutrons (i.e. "c, 160) have a net spin, 1 = 0, while those with an odd number
of both protons and neutrons (Le. '"N) have a net spin equal to some integer. It is those
nuclides with an odd number of protons and an even nurnber of neutrons or vice versa which
have a net spin 1 = !4, or some multiple thereof that are useable for nuclear magnetic
resonance (NMR). The list of NMR-active isotopes includes 'H, I3C, "O, =Na, "Al,
' 9 ~ i , 3 1 ~ and 35C1 to name a few.
Nuclear spins possess angular momentum, 1. which has discrete orientations in the
presence of an applied magnetic field. This gives rise to 21 + 1 spin States, which are
degenerate in the absence of a magnetic field. Spinning charges create a magnetic moment,
p. In the case of a spinning nucleus, the expression for this magnetic moment is:
where h is Planck's constant and y is the gyrornagnetic ratio, a quantity specific to the
nucleus and different for each isotope. The magnetic moment and the spin angtdar
momentum behave as parallel vectors, that is, the angles between the vector of the appIied
magnetic field, H, and those of the magnetic moments are fixed and quantized. However,
the moments are not static and they precess amund the extemal magnetic field at a frequency
given by the equation:
This expression, known as the Larmor equation, states that the Larmorfrequency, o,, is
directly proportional to the applied magnetic field.
The energy levels of the nuctear spin states are split by Zeeman interaction. This is
illustrated for h e simplest case, I = Yi, in Fig. 1. la. Two energy -1evels. referred to as the a
and p states appear with magnetic spins of +!A and -!4 respectively. The net magnetization
of al1 spins in the lower energy a state describes a vector parallel to that of the applied
rnagnetic field, while the net magnetization of the higher energy P state is antiparaliel. This
is illustrated in Fig. 1. lb. Note that the number of spins in the a state is higher than in the
p state, resuiting in an overall net magnetization parallel to the externat magnetic field. The
distribution of spins shown in Fig. 1. lb has been exaggerated for clarity. In fact, hE is srnall
relative to thermal motion and (Np - NJN, = IO-'.
A nuctear magnetic resonance experirnent is perfonned by subjecthg a sample in an
Fig. 1.1 A) Nuclear spin energy level diagram for a nuclide with spin, 1 = S. AE is directly
proportional to the intensity of the applied magnetic field, Ho. B) Graphical representation
of spin orientations for a nuclide with 1 = Yz The siightiy larger number of spins in the lower
energy cc state result in a net rnagnetization, w, paraiiel to the appiied field.
Fig. 1.2 The application of a transverse RF field, H,, results in an effective magnetization
vector, H,, precessing about the axis of the applied magnetic field, H,. Within a rotating
fnme and at resonance (Le. = q), H8 = 8, and precesses about the axis of the RF field.
applied magnetic field as descnbed above to a transverse radio fiequency (RF) field. This
generates a second magnetic field vector, Hl, perpendicular to H, and rotating at frequency,
Q, in the x-y plane. Therefore, the nuclear spins see an effective magnetic fieid, E&, as
shown in Fig. 1 . 2 ~ At resonance, when Q equals the Larmor fiequency, the spins wili only
see H~ = H, and the net magnetization now precesses around H, (Fig. 1 -2b). The angle of
rotation around H l is called the nutation angle, 0, and is controlled by the time duration of
the RF pulse. In other words, a 90" pulse is one of suficient duration to rotate the net
magnetization by 8 = 90". Afier the RF pulse, the net mapetkation retums to its
equilibrium position. The RF coi1 used to emit the RF pulse now serves as a receiver as it
detects the sinusoidally decaying transverse magnetization resulting from the relaxation of
the net magnetization. This signai is c d e d the free induction decay 0). The FID is
converted from a time domain signal to a frequency spectmm using a Fourier transfom.
1.4.2 NMR of geological materials ami rnagic angle spinning (MAS)
There are three important interactions which cornplicate interpretation of NMR
results when dealing with minerals and glasses. These are 1) dipole-dipole interaction, 2)
c hemical shift aniso tropy and 3) quadrupolar interaction (Kirkpatnck, 1988). di pole-dipole
interaction is an interaction between the dipole moments of individual nucleii. This is
illustrated in Fig. 1.3. The spins in nucleus j see the applied magnetic field, Ho, resulting in
a net magnetization. pj. However. nucleus j is also influenced by the dipole moment of
nucleus i, resulting in a second magnetization vector Bi. Note that the magnitude of the
interaction is inversely proportional to the cube of the internuclear distance. Chernical shift
anisotropy is non-sphencal electronic shielding at individuai sites. Quadrupolar interaction
affects NMR-active nuclides with 1 > '/z. These include many geologically important
isotopes such as "0. UNa, *'AI, 35C1 and "K. This interaction arises because of a non-
sphericai distribution of spins in the nucleus. As noted earlier, in the presence of an extemal
magnetic field, there SI + 1 spin states. Therefore, for a nuclide with 1 = 3/2, this results in
four spin states.
The diffïculties in interpretation of NMR spectra which result from these interactions
c m be mitigated by a technique known as magic angle spinning (MAS). This methoci
involves spinning the sample at an angle around the axis of the applied magnetic field. By
doing this, the dipole-dipole interaction, the chemicai shift anisotropy and the fmt-order
quadrupolar interaction are reduced to zero (Kirkpatnck, 1988). A superficial understanding
of why this technique works cm be derived from Figure 1.3. Note that the equation for the
interaction (in this case, dipole-dipole interaction) includes the term (3cos28-l), where 0
represents the ange between the extemal magnetic field vector and the p ~ c i p a l axis of the
interaction. At 8 = 54.7" (the magic angle), that term becomes zero. Rotation of the sample
at that angle relative to the applied field causes the average angle of the interaction to equal
54.7 O resulting in greatly improved fine resolution. This effect applies to all solids, although
it is less striking for amorphous materials due to stnictunl disorder (Kirkpatrick, 1988).
Fig. 1.3 Gnphical representation of dipole-dipole interaction. Nuclide 'j' is influenced by
the applied magnetic field, I&, as well as the dipole moment of nuclide 'i'. The influence
of the dipole-dipole interaction results in a second magnetic vector, Bi, which is proportional
to Urij3.
1.5 Notation
There are two types of notation associated with melt structure used in this thesis
which require some explanation. The fmt of these is Q" notation. This notation appiies to
silicate and aluminate tetrahedra in a glasdmelt network. The value of 'n' is an integer
between O and 4 inclusive and refers to the number of bridging oxygen atoms present in the
tetrahedron, that is, the number of oxygen atoms which are bonded to two tetrahedra.
Therefore, a fuily-polyrnensed network in which al1 tetrahedra are linked consists entirely
of Q* stmctures.
The second notation refers to the coordination number of a given ion and is presented
as ["'A in which n is the coordination number and A is the symbol of the element. This
notation is dso used to denote coordination numbers in crystdine materials.
1.6 Thesis Organization
Three of the chapters have been or will be submitted for publication. Chapter 2 is
a study of the solubility of Zr-bearing minerais in Na,K-aluminosilicate melts with
alkali/Al> 1 as an analogue for naturai felsic, peralkaline magmas. Among the findings
reported in this manuscript is the observation that adding s m d amounts of CI to a H,O-
saturated, perallcalïne melt significantly lowers the solubility of Zr miner& in such a
meit, relative to those with no Cl. A stnictural change in peralkaline melts resulting fiom
the addition of Cl is implied. Chapter 3 is a Raman spectroscopie study of Zr and Ti-
bearing aikali aluminosiiicate glasses, with and without added Cl. Ti-bearhg glasses
were included because Ti and Zr are chemically-similar (Le. both are high field-strength
elernents; similar valency) and because a prominent peak associated with Ti-O bonding
has been reported in previous Raman spectra (Mysen and Neuville, 1995; Henderson and
Fleet, 1995).
The results of the solubility experiments in Chapter 2 indicate a maximum
solubility of Zr-bearing minerals at about 57 wt.% SiO, for H20-saturated peraikaline
rnelts with and without added F. This dso represents the compositional threshold
between the stability field of zircon and that of wadeite (KZZrSi,O,) [for F-fke melts] or
ZrO, [for F-bearing melts]. Since alkalis are believed to play an important role in the
dissolution mechanism of Zr in perakaiine melts, an investigation of the structural
environment of Na as a function of SiO, was undertaken. Chapter 4 is a =Na nuclear
magnetic resonance magic-angle spinning (NMR MAS) study of Na-aluminosilicate
giâsses. NMR MAS was chosen as the andyticd technique for this smdy since it directIy
observes the nuciide of interest, whereas Raman spectroscopy analyses changes in overidi
glas stmc ture resulting from cornpositional ciifferences.
Collectively, this thesis represents an effort to establish physical and chernical
constraints on the stability of zirconium-beiuhg minerals in perdkaline, magmatic
environments. It also includes the application of Raman spectroscopy and NMR MAS
techniques to the study of perdkaline giasses in order to place solubiiity and phase
equiiibria experirnents in a context of melt structure.
Chapter 2
Chemical Controls on the Solubility of Zr-bearing Phases in
Peralkaline Magmas and Application to the
Strange Lake Intrusion, Canada
Robert A. Marr, Don R. Baker, A.€. WilliamsJones
Department of Earth and Planetasr Sciences, McGil University, 3450 University Street, Montreal, Québec, H3A 2A7
2.1 Abstract
The solubility of Zr-bearing mùierals was measured in H,O-saturated, peralkaline
haplobasaltic, haplosyenitic and haplogranitic melts CkF at 800 OC and 1 kbar. In
halogen-free and F-bearing melts, the solubiiity of Zr-besiring phases reached a maximum
of 4 and 3.5 W.% ZrO, respectively in melts with at 57 to 60 wt.% silica- No such maximum
was observed for C I - b e h g compositions which became saturated in a Zr-bearing phase at
~ r O f "" concentrations of 2 to 2.2 wt.%. A silica concentration of 57 to 60 wt.8 represents
a threshold abovewhich zircon is the saniraihg Zr-bearing phase for ail compositions. For
compositions with SiO, content below this range, wadeite (K,ZrSi,O,) crystalkes in the
halogen-free experiments while ZrOz is the saturating phase for the F and Cl-bearing run
products. The positive slope of the wadeite saturation curve on XmIrO?-&i, plots suggests that
increasing silica activity increases the solubility of wadeite. Since this behaviour of wadeite
is not expected based on a simple dissolution mechanism, a more complex equilibnum
involving different structural units is proposed. Observations that zircon is scarce, whereas
elpidite (Na&!%@,,-3H20) and other alkali and alkaline-earth zirconosilicates are abundant
in the Strange Lake perakaline intrusive complex in northern QuebecLabrador indicate that
the parent magma did not saturate with a Zr-bearing mineral until it cooled to a low
temperature, probably less than 600°C.
2.2 Introduction
Peralkaline, felsic rocks have k e n a focus of study by petrologists and economic
geologists because of the exotic minerais cornmonly associated with them and their potential
as ore deposits of rare metals such Zr, Y, Nb and the rareeuth elements (RE). The unusual
rnineralogy of these rocks has long held promise of providing a means for constraining the
pressure, temperature and chernical conditions of crystallization. The wide array of alkali
and alkaiineearth zirconosilicates reported in peralkafine intrusions (e-,o., Chao er al,, 1967;
Semenov, 1972; Harris, 1983; Currïe, 1985; Birken et al., 1992; Marr and Wood, 1992; Sdvi
and Williams-Jones, 1995) is particularly promising in this respect.
Cume and Zaleski (1985) pubiished the fmt phase-equilibrium resuits involving
alkali zirconosilicates and consuained the reaction:
Na,ZrS i,O,,BH,O (elpidite) * NaJrSi,O,, (vlasovite) t 2 S i 4 + 3H,O
at 0.5, 1 .O, 1.5 and 2.0 kbars. These experiments were limited to the system Na@-Si0,-ZQ-
H,OiCl. Marr and Wood (L992) later produced P-p,, (as an analogue for P-T reactions
involving hydrated phases) petrogenetic grids for the systems: Si0,-Na&03-H,0 and SiO2-
CaZrO,-H,O, To our knowledge, however, no attempt has been made to utilize phase
equilibria involving alkaii zirconosilicates to constnin petrogenetic conditions in systems
more closely approximating those of natural peralkaline magmas.
The solubility of Zr-bearing rninerals in peralkaline melts is much greater than in
peraluminous or metaluminous melts. Watson (1979) conducted the earliest systematic
study of zircon solubility in such melts. For perdkaline felsic rnelts, he suggested a solution
mechanism involving the formation of alkali zirconosiiicate' complexes of the fom:
A,ZrSi,O,,,, in which A represents an alkali metal. The existence of such complexes was
supported by Raman spectroscopie analyses (EUison and Hess, 1994). which indicate the
presence of SiO, tetrahedra with one non-bridging oxygen atom shared by one Zr and two
K ions in anhydrous, Zr-bearing, K-alutninosilicate glasses. The purpose of our snidy is to
ex tend the work of Watson ( 1979). by including S i02 under-saturited peralkaline
compositions, such as are found in well-known localities as Mt- St- Hilaire, Canada and
Lovozero, Russia. The effects of Cl and F on the solubiiity of Zr also were hvestigated, and
the results applied to the petrogenesis of the Strange Lake intrusion.
2.3 Experimental Procedures
We chose as starting rnaterids, glasses with compositions ranging from 50 to 70
wt.% SiO2, and with Na+K/Ai and Na/K values of about 1.4. These compositions span the
range comrnonly found in peralkaline intrusions. Additionally, results on the more Si0,-nch
samples c m be compared with the results of Watson (1979) who performed s idar
experiments on compositions ranging from 68 to 79 W.% SiO,. Glasses were prepared using
reagent-grade SiO2, GO,, NhCO,, &Cq, NaCl, NaF and Zrq . The carbonates and oxides
were ground together and decarbonated at 750°C for 24 hours, then reground and fused in
air at 1450°C for only one hour to avoid volatilization of Na. The resulting giasses were
then ground a third time, with NaF or NaCl added as appropriate.
For each experiment, 0.01 g of sample was sealed in a 02x1 -2 cm gold capsule with
2 PL of distilled water. The experiments were performed for 2 weeks at 800°C and 1 kbar
in cold-seal pressure vessels, utilizing Ar as a pressure medium. The pressure vessels were
air-quenched, achieving subsolidus temperatures in approximately 20 S. After the
expenments, the capsules were reweighed to ensure that there was no loss of volatiles. Only
those samples that tiad excess water upon opening were retained.
The samples were analyzed using a JEOL 8900 electron microprobe. The results of
these analyses are presented in Table 2.1. Crystalline phases were analyzed with a beam
current of 20 nA and a beam width of 5 prn to minimize damage to standards and samples.
The instrument was calibrated using zircon (Zr), albite (Na,Si,Al), orthoclase (K), fluorite
(F) and scapolite (Cl) standards. The beam width was increased to 20 pm for glasses. Glas
SiO,
A1203 Na,O
&O ZQ aZr0,
F =O
total
K+Na/AI
SiO, AI203
Na20 &O
drû, total
K+Na/AI
SiOz
Na20
60 Zr02 atm2
CI =O
total
K+Na/Al
Table 2.1 : Compositions of melts saturated with a Zr-bearing phase. Errors reported are for one standard deviation. Experiments denoted by "r" were reversed (see text). Sampies RMG6a and RMG6c are run products of the variable duration experiments shown in Fig. 2.2. These experiments were performed for one and three weeks respectively. Totals do not iaclude loss of dissolved H20 during analysis.
standards with NdSi values similar to the samples were used to calibrate the instrument for
alkalis, Al and Si where analyses of glasses were performed. This was done to account for
the effects of Na loss during analysis.
2.4 Results
S yn thetic wadeite (&ZrSi,O,) satunted the me1 t in experiments employing low-silica
glasses with no added halogens. Wadeite appeared as large (up to 50 pm-diameter), equant
crystds with weli-defmed boundaries; it was accompanied by ieucite and nepheline in some
experiments and small clusters of unreacted ZrO, were occasionaiiy detected in the cores of
wadeite crystals. Leucite exhibited Little solid solution, with Na representing about 5 mol.%
of total akalis. Nepheline displayed a very nearly ideal (Na,K)AI,Si,O,, composition. The
addition of F or Cl destabilized wadeite at 800 OC and ZrO, crystallized instead, appearing
as cIusters of smd , rounded grains. Since the source of Zr was ZrO,, the presence of this
phase in the run products did not inspire confidence that equiiibnurn was achieved.
Therefore, a senes of experiments were performed using F-bearing glasses at 650°C and
1 kbar for one week. These experiments yielded synthetic wadeite as observed in the
halogen-free expenments (Figure 2.la). This material was then ground and used in
experiments at 800°C for 2 weeks and the results compared to the initiai results at those
conditions. Zirconia reappeared as the saturating Zr-bearing phase in the reversed
experiments as elongated, euhedrd crystals 10 to 15 prn in length (Figure 2.lb). The Zr
concentrations in melts formed in the reversed experiments (Table 2.1) were similu to those
in the other experiments, but dways slightly Iower. The equilibrium concentration of Zr is
presumed to be between measurements of the forward and reversed experiments.
Disequilibrium concentrations of Zr in the melts are deemed to be much less Likely for chose
experiments in which the starting Zr-bearing phase and the saturating phase were different.
Reagent ZrO, would fmt have to dissolve iato the melt and then crystailize the new phase
Fig. 2.1. Secondary electron images of a) wadeite crystal with nepheline from sample RMGlr
at 650°C and 1 kbar after one week and b) ZrO, cqstal from the sarne sample at 800°C and
1 kbar after two weeks.
To further establish that the Zr content in the rnelts reached equiiibrium, or at least steady
state after two weeks, one sample (RMGo) was selected for a series of variable duration
experiments at 800°C and 1 kbar. The results, presented in Figure 2.2, indicate that there is
no change in ZrO, content in the melt from two to three weeks. Since Watson (1979) also
found that two weeks was a sufficient duration for experiments on zircon solubiiity, we
accept that these results represent good evidence that steady state was achieved.
Zircon was the saturating phase in ai i experiments with SiO2 contents above o r
approximately equd to 55 wt-96. Occasionaily small amounts of unreacted ZQ were
observed as ctusters of rounded grains similar to those described above, each grain measuring
about 1 or 2 p m in diameter. They do not comprise euhedrd crystals iike those observed in
the reversed, F-bearing experiments. Chemicd analyses of run products (Table 2.1; Figure
2.3) indicate a maximum solubility of -4 W.% ZrO, at 55 to 60 wt.% SiOl for the haiogen-
free and -3.5 W.% ZrOS in F-bearing experiments. As mentioned, this SiO, concentration
represents a threshold above which zircon is the saturating phase for al1 experiments (Figure
2.3). The solubility of Zr-bearing phases in the Cl-bearing run products is much lower, near
2 wt.% ~ r 0 ~ " ~ " , and shows no such maximum across the entire range of S i 4 content
-2.5 Discussion
2.5. I Zr minerals coexisting wirh -0-saturated melts
The positive slope of the curve representing the solubility of Zr in the low silica
region of the halogen-free experiments in Figure 2.3 is puzzling. It suggests that as the
activity of silica in the melt increases, the precipitation of wadeite is suppressed. This cannot
be reconciled with the reaction:
K20m" + Z I ~ ~ ~ ~ " + 3s i02mc'i 4 ~ ~ Z r ~ i ~ 0 ~ - (1)
because increased %, would favour wadeite crystallization and lower Zr& concentrations
Fig. 2.2. Concentration of ZrOl in the melt as a function of run duration for sample RMG6.
- - E R O F k-sreprese~t 0~eatandaddeviaUan.- - - - - - - - - - - - - - -
12 16
Time (days)
Fig. 2.3. Solubility of Zr-bearing phases as a function of silica concentration for peraikaline
alkali duminosilicate @asses with no added halogens (round syrnbols), with adcied F (square
syrnbols) and with added Cl (triangular symbols). The holIow circle represents data fiom
Watson (1979). A second-order polynomid was Gtted to the data for zircon-saturated, F-
karing and halogen-free experiments. Linear fits have been applied to aii other data. Dashed
lines represent extrapolations of these regressions. Error bars represent one standard
deviation.
ZrO, or wadeite satüration I zircon saturation
f
-saturated (this study) b -saturated (Watson, 1979)
Only with a strong decrease in the alkali activity would this reaction be compatible with our
results. Given the relarively low enthalpy of mïxïng in the binary alkali-silica systems [Le.
me tas table Iiquid-iiquid immiscibility gap (Haller et al., 1974)], this seems unlikel y.
Therefore, a more complicated explanation must be proposed, One possibility is the
presence of a high-silica, alkali zirconosilicate complex in the melt. Such alkali
zirconosiiicate complexes have been proposed by Watson (1979) and by Eiiïson and Hess
(1994) with (Na,K)/Zr varying from 2 (Ellison and Hess, 1994) to 4 (Watson, 1979).
Ellison and Hess (1 994) attributed the difference in aiktli/Zr ratio to the higher temperature
at which their experiments were performed and possibly a different coordination geometry
for Zr in the two studies as a result. Unfortunately, the approaches taken by Eiiison and Hess
(1994) and Watson (1979) did not allow the number of SiO, tetrahedra associated with these
melt complexes to be detennuied. The Raman spectra of Ellison and Hess (1994) can only
provide indirect information on the local environment around Zr ions in a glas or melt by
detecting changes in the structure of the network of (Si&-tetrahedra resulting from its
presence, whereas solubility studies like those of Watson (1979) provide no information
about melt structure. Thus, the presence of a high-siiica complex is speculative.
An alternative approach is to mode1 a reaction in terrns of structural units. Eiiison
and Hess (1994). concluded, on the bais of Raman spectra, that a Q3 structure (a SiO,
tetmhedron with one non-bridging oxygen atom) is involved in the aikali zirconosilicate melt
complex. Assurning this to be the case, we have used (NaK)Jr[3Si0,]m"t to represent the
aikali zirconosilicate complex, based strictly on charge balance. For simplicity, we have also
assumed ideal niixing among al1 species in the melt. Because the structure of wadeite is
based upon three-membered rings of QZ SiO, tetrahedra (Henshaw, 1955), we may write its
formula as (Na,K)2ZrZr[3Si0,]c~? Using this notation, we can write an equation that
schematicaily accounts for the increase in Zr concentrations in wadeite-saturated melts as
silica concentration increases:
Kz~r[3Si~s]m"' c K J ~ D S ~ O ~ ] ~ ~ + ~ [ S ~ O J ~ ~ ' (2)
Written in this way, the reaction proceeds to the left as the SiO, concentration increases,
consistent with the solubiiity data reported in this study.
2.52 The effect of halogens on solubility of Zr-bearing rninerals
The addition of hdogens significantly rtffects the solubility of Zr-bearing phases in
perdkaline melts. As indicated in Figure 2.3, the saturation curves of Zr-bearing minerals
in fluorinated and halogen-free, H,O-saturated melts are similar, indicating that F and H,O
affect Zr in a simiiar manner in the melt. However, there are ciifferences in the solubilities
of Zr-bearing phases in the F-beuing and F-free melts that must be addressed The solubility
of Zr-bearing phases in F-bearing, low-silica glasses is approximately equal to that in
halogen-free compositions, whereas the solubility of zircon is decreased in the F-bearing
glasses relative to the F-free glasses for the hi@-silica compositions. This observation must
be related to the effect of F on melt structure and alkali activity because direct complexation
of F with Zr is unlikely (Farges, 1996). Previous expenments have demonstrated that the
addition of F to metaluminous, haplogranitic melts increases the solubility of zircon
(Keppler, 1993). This increase results from the creation of additional non-bridging oxygen
atorns as aluminofluoride complexes are formed (Mysen and Virgo, 1985; .Kahn et al., 199 1 ;
Schaller er al., 1992). k is not clear whether such a mechanism is applicable in peralkaline
melts, however, as the presence of alkalis in excess of what is needed to charge-balance Al
may favour direct alkali-F bonding. The observation in this study that the presence of F
appears to decrease zkcon solubility for the more Sioz-rich compositions bears this out,
suggesting that (Na,K)-F bonding may be involved.
As in the halogen-free expenments, a positive slope is also observed for the
saturation curve of 210, in the F-bearing experiments below the threshold of zircon
saturation (Figure 2.3). As ZrO, is the satunting phase and given the reaction:
(~a,K),Zr(Si,0,),""' * + (Na,K),SiO,m'L + 5Sio2me't (3)
it is clear that increasing the activity of SiO, or aikalis in the melt would favour an increase
in the solubility of Zr02' The solubility of Zr-bearing phases increases with the addition of
F in the low-silica melts, but the different saturating phases in the F-free (wadeite) and F-
bearing (ZrO,) ex periments make cornparison less meaningful.
The reason for the difference in saturating phase between the low-silica F-free and
F-bearing compositions is unclear. Wadeite crystallues from the F-bearing melts at 650°C,
but is replaced by a at 800°C. ïndicating that F lowers the upper stabiiity iirnit of wadeite.
Based on the reaction:
K , Z ~ S ~ , O , C ~ ~ * ZrOZm + KzSi03"t + 2 ~ i 0 , ~ " ' (4)
the destabilization of wadeite in favour of ZrO, could be achieved by lowering the activity
of either silica or rtlkalis. Although the results of this study do not indicate which of these
mechanisms is responsible for the observed ciifferences between the F-free and F-bearing
experiments, it should be noted that the (Na,K)-F complex we propose could result in a
decrease in the availability of alkalis to interact with the duminosilicate network.
Chlorine lowers the solubility of zircon and ZQ across most of the compositional
range investigated in this study. Furthemore, the solubiiity maximum observed in the
hdogen-free and F-bearing compositions is not present in the CI-bearing experiments. It has
been known for sorne time that Cl affects melt properties differently than F or H20.
Viscosity studies of (NoCa) and (K,Ba)-silicate melts by Hirayama and Camp (1969)
indicated that the addition of F resulted in a decrease in viscosity, while Cl increased it.
Since viscosity is a reflection of the degree of polymerization, the observed increase in
viscosity mems that Cl polymerizes a silicate melt, at least for alkali and alkaiine-earth
silicate compositions. A m e n t diffusion study lends further support to this proposal.
Activation eaergies for diffusion determined by Baker (1993) were Iowered by the presence
of fluorine, whereas the addition of chiorine increased the activation energy of ciiffision.
Baker (1993) proposed that these results are due to decreases and increases in the degree of
melt polymerization produced by the presence of F and Cl respectively. Solubility and
partitioning experiments (Metrich and Rutherford, 1992; Webster, 1992) provide evidence
of a solution mechanisrn for CI which could account for this behaviour. The results of these
studies suggest that Cl forrns meft complexes with network-modifying alkalis (at l e s t for
compositions with alkalis/Alz 1) rather than with network-forming Al as is the case with F
(Mysen and Virgo, 1 9 8 5 ~ Kohn et al., 1991; Schaller et al., 1992). This solution mechanism
would lower the activity of aikalis and, therefore, be expected to reduce the solubility of Zr-
bearing phases by destabilizing alkali zirconosilicate melt complexes. Given the signifîcant
decrease in solubility of Zr-bearing minerals in Cl-bearing meits, it is clear that more
investigation is needed of the effect of Cl on the concentration of high field-strength
elements (HFSE) in perdkaline magmas.
2-5-3 Applications to the Strange Lake peralkaline granite
The mid-Proterozoic Strange Lake (Lac Brisson) peralkaline granitic pluton on the
QuebeciLabrador border may represent the extension into Labrador of Gardar anorogenic
igneous activity (Cume, 1985). The pluton comprises two main rock types; a hypersolvus
granite and a volatile-saturated, typicdly altered, subsolvus granite (Nassif, 1993). The
hypersolvus granite consists of mesoperthitic alkali feldspar, interstitial quart. and late-
crystallizing arfiedsonite with accessory fiuorite, zircon, thorite, aenigmatite and
as trop hylii te.
The subsolvus granite is either equigranular or porphyritic. It contains idiomorphic
quartz, arfvedsonite (occurrhg as phenocrysts in the porphyritic granite) and two alkali
feldspars. Accessory minerals in the subsolvus granite include late zircon, elpidite
(Na,ZrSi60,,-3H,O), annstrongite (CaZrSi,O,,-2.5H20), gittinsite (CaZrSi,O,), kainosite
(Ca,[Ce,Y,HREE],Si,O ,,[CO3] .HO) and calcite. Pegmatites and miarolitic cavities are
associated with the subsolvus granite, ixidicating that it crystaüized under water-saturated
conditions.
Elpidite has been identified as the primary, magmatic Zr-bearing minerai in the
subsolvus granite (Salvi and Williams-Jones, 1995). It appears as doubly terrninated
orthorhombic prisms up to 3 mm in length that may be flow-siligned (Miller, 1996). It shows
varying degres of rirn and core replacement by gittinsite or ~unstrongite. Anhecirai elpidite
ais0 has been reported in the porphyritic granite either as single crystals interstitial to quartz
and feldspar or as large aggregates of smdi p i n s (4 .1 mm in diameter) (Salvi and
Williams-Jones, 1995).
Althougfi elpidite was the prïmary, magmatic Zr-bearing mineral at S trange Lake, the
experïments performed in this study have yielded only zircon, ZrO, and &ZrSi,O,. This
suggests that the upper stability limit of elpidite is exceeded at the P-T conditions of these
experiments. This is consistent with the results of Currie and Zaieski (1985), which
indicated that the transition from elpidite to vlasovite + quartz + H20 at P(H20) = 1 kbar
occurs at 595 & 4°C. The inferred pressure of crystakation at Strange Lake, based on fluid-
inclusion data (Salvi & Williams-Jones, 1992), is 0.7 kbar, assurning Ph, = Pmm. At this
pressure, the upper thermal stabiliv limit of elpidite is about 57S°C, a low temperature for
a magma, but close to the approximate lower-tempenture iimit suggested by Nassif (1993)
of 590°C for the subsolvus-granite-fonning magma at Strange Lake. The solidus temperature
proposed by Nassif ( 1993) is based on that reported for a topaz-bearing quartz keratophyre
(ongonite) Erom Mongolia (Kovalenko et al., 1971) with a high F content similar to the
Strange Lake intrusion. Thermometric studies of melt inclusions in topaz from this locality
indicated a solidus between 550 and 600°C (Naumov et al., 1971), which Nassîf (1993)
estimated to be 575°C at 1 kbar and 590°C at 0.7 kbar. Thus, the solidus temperature is not
well-constrained. Furthemore, the Mongolian ongonite is not peraikaiîne. Experiments have
indicated that perdkaline rnelts have solidi signifïcantly lower than those of hapiogranitic
melts (McDoweil and WyUie, 1972). Indeed, some unusual granitic and syenitic rnelts have
been reported with solidus tempentures below 500°C (London et cil., 1989; Piotrowski and
EdgarJ970). Therefore, lowenng the proposed solidus temperature for the Strange Lake
subsoIvus magma can be justified.
Since zircon is the saturathg phase at 800°C and 1 kbar, granitic intxusions which
contain elpidite as a magmatic phase must not have been saturated until late in the
crystdlization process or must have crystdized from a Iow-temperature magma. Textural
evidence fiom the subsoIvus granite (Salvi and W'iams-Jones, 1995) and related pegmatites
(Miller, 1996) indicate that in the case of Strange Lake, elpidite crystaiiized early. Based on
the chemopphy of the common aikali zirconosilicate (AZS) minerais, which generally lie
on the SiO2-(Na,K)JrO, pseudobinary ( M m and Wood, 1992), zircon is an indifferent
phase and is, therefore, stable at the same P-T conditions as elpidite, wadeite, vlasovite and
other cornrnon AZS minerais. Consequently, if zircon crystallized from a magma at a
temperature above the upper stability b i t of elpidite, it would remain stable as the magma
cooled to a tempenture at which elpidite would also be stable. indeed, small arnounts of
early zircon with a reaction rim of vlasovite or elpidite have been reported in the
hypersolvus granite at Strange Lake (Birkett et al., 1992). These may represent localized
pockets i t which zircon saturated early or may be xenocrysts. Furthermore, vlasovite at
Strange Lake commonly displays a reacîion rirn of elpidite in contact with quartz (Birkett
et al., 1992). This sequence indicates a reaction senes of zircon - vlasovite - elpidite as
temperature decreased.
Thus, presence of magmatic alkaii ùrconosilicates in peralkaline granites implies
either a lower initial Zr content in the melt or a lower melt tempenture at the tirne of
emplacement than if zircon is present. Three possible paths of melt evolution are indicated
in Fig. 2.4. Here we assume that Zr behaves incompatibly until either zircon or elpidite
saturation in the meit. Path A illustrates the evolution of a relatively high-temperature
magma with a relatively hi@ initial Zr content. As the magma cools, the concentration of Zr
in the me1 t increases until zircon saturates. Path B , indicates how a high-temperature magma
with a low initial Zr content would evolve differently from that illustrated by path A. Once
again, the magma becomes enriched in Zr as it cools. However, saturation in zircon does not
occur and the melt continues to cool until elpidite saturates. An alternative path that could
lead to elpidite saturation is represented by B,. A magma that has a high initial Zr content,
but is emplaced at a lower temperature could avoid saturation in zircon and crystallize
elpidite. It is this third alternative which is the most compatible with the petrographic
textures observed at Strange Lake.
This mode1 must be viewed as highly qualitative, since it does not take into account
the effect of variable agpaitic index (alkWAl); the index will increase in a fkactionating,
peralkaline, felsic melt. Nor does it consider pressure which has a significant effect on the
stability field of elpidite, especiaily for pressures below 1 kbar (Currie and Zaleski, 1985).
However, the mode1 does provide some insight into the evolution of the peralkaline felsic
magmas. High concentrations of Zr can be reached in the late stages of magmatic
crystallization from melts with reiativety low initial Zr content. On the other hand, high
initial Zr concentrations may result in early saturation of zircon, which would be dispersed
throughout the magma, yielding final Zr contents comparable to those of magmas with low
initial concentrations.
2.6 Conclusions
1) For H,O-saturated, peralkaiine, Zr-rich, aikali duminosilicate melts with 55 W.%
SiOl or more, zircon is the saturating phase at 800°C and 1 kbar, whereas wadeite appears
in melts with lower silica contents.
2) The addition of F Lowers the upper thermal stabiiity limit of wadeite in favour of
Fig. 2.4. Schematic representation of cooling paths for three perdkaline magmas assuming
Zr behaves as an incompatible element. Magma A represents a relatively high temperature
melt with high initial ZrOz content which becomes saturated in zircon. Magmas B, and B,
represent melts at high T and low initial ZrO, content, and low T and high initial Zr&
content, respectively, which avoid early saturation in zircon and crystallize elpidite.
ZrO, in low siiica melts and decreases the solubiiity of zircon in hi&-silica melts
3) There is a maximum in the solubility of Zr-bearing minerals at ktween 55 and 6û
wt.% SiO2 for water-satunted perdkaline melts with and without added F.
4) The addition of Cl serves to depress the solubility of Zr-bearing phases,
panicularly in the compositional range where a maximum is observed in Cl-& experiments.
This may be the result of lower alkali activity due to alkali-Cl complexing.
5) Fluorine appears to destabilize aikali zirconosiiicate meIt complexes, possibly as
a result of aikaiï fluoride cornpiexation.
Introduction to Chapter 3
In Chapter 2, it was reported that the addition of CI to perdkaline melts depresses the
solubility of Zr-bearing minerais across a wide range of SiO, content. Ellison and Hess
(1994) indicated the presence of a peak in the Raman spectra of K,Zr-silicate glasses which
is associated with Si-O stretching vibrations in Si-O-Zr bridges. This suggests the possibility
of using Raman spectroscopy to determine how CI affects the solution mechanisrn of Zr.
Ti-bearing alkali silicate glasses have also k e n studied by Raman spectroscopy
(Mysen and Neuville, 1995; Henderson and neet, L995), but the pezk appearing in the
spectra of Ti-karing @asses is associated with Ti-O vibrations. Therefore, Raman analyses
of Ti-bearing glasses with added Cl may serve as a useful cornparison since any changes
observed in the spectra would be the result of some direct effect on Ti-O bonding.
Chapter 3 is a Raman study of Zr and Ti-bearing alkali aluminosilicate glasses with
and without added Cl. The objective is to determine changes in the local structural
environment of these two metais in glasses and, by inference, melts which resuIt from adding
Cl.
Chapter
The Effect of CI on the Structure of Zr and Ti-bearing alkali
aluminosilicate glasses
Robert A. Marr', Don R. Baker', Andrew Vreugdenhi12 and A.€. VftJliamsJones'
'Dept of Earth and PIanetary Sciences, McGill University, Montreal, QC 'Oept of Mining and Metallurgy and Dept of Chemistry, McGill University, Montreal, QC
3.1 Abstract
Na-alurninosilicate glasses wi th Na/Al> 1 and added Ti or Zr have been analyzed
by Raman spectroscopy to determine the effect of Cl on g las structure. The spectra of
the Ti-bearing glasses show a significant difference between the Cl-free and the C1-
bearing composition. The Cl-free glas spectrurn contains a strong, asyrnmetric peak at
900 cm-' which is associated with Ti in five-fold coordination. This peak is shifted to
higher frequency and becornes more symmetrïc with the addition of 0.3 wt.% Cl.
Deconvolution of the high-frequency waveband suggests that ciifferences between spectra
are the result of a contribution from a peak at 945 cm-'. This peak is believed to be the
result of Ti-O vibrations in fully-polymerized titanate tetrahedra It is proposed that the
addition of Cl destabilizes [''Ti in favor of tetrahedral coordination as a result of
cornpetition between Cl and titanate groups for alkalis.
The spectra of Zr-bearing @as show a strong peak at 980 cmaL which is not seen
in spectra of the base glass, indicating a vibntional mode associated with a
zirconosilicate structure. There is no discernible difference between the Raman spectra of
Zr-bearing glasses with and without added Cl, indicating that the local structure around
Zr is insensitive to the presence of CL.
3.2 Introduction
Viscosity studies of CI- and F-bearing alkali-alkiiiine-earth silicate rnelts indicate a
decrease in viscosity with added fluorine, but an increase with added chlorine (Hirayama and
Camp, 1969). These viscosity measurements can be interpreted to indicate that F
depolymerizes silicate melts, whereas CI increases the degree of melt polperization. This
increase in viscosiq with addition of Cl was also observed by Baker and Vaillancourt (1995),
in a study comparing the viscosity of a CI-bearing albitic melt with that of pure albite melt
at 1400°C and 1.5 GPa. A recent diffusion study lends funher support to this proposal;
activation energies for difision detennined by Baker (1993) were lowered by the presence
of fluorine, whereas the addition of chlorine increased the activation energy of diffusion.
Baker proposed that these data reflect an increase in the degree of melt polymerisation due
to the presence of Cl. Recent solubility and partitionhg experiments provide evidence for
a solution mechanism for CI which could account for this behaviour (Metrich and
Rutherford, 1992; Webster, 1992). The results of these studies suggest that Cl forms melt
complexes with network-rnodifjring aikalis (at l e s t for compositions with alkalis/Alz 1)
nther than with network-forming AI as is the case with F (Mysen and Virgo, 1985a; Kohn
et al., 199 1; Schaller et al., 1992).
While the effect of Cl on the physical properties of silicate melts and glasses is well-
established, the changes in network stmcture responsible for these changes have not yet been
detected. Raman and NMR MAS ('?U, '%i and = ~ a ) analyses of albite glass with 0.5 wt.%
Cl (Baker and Vaillancourt, 1995; Baker, unpubl. data) indicate little change in the spectra
as compared with those of pure albite glass, suggesting little discemible change in glas
structure. Yet, the effect of Cl on viscosity must result in some measure from structural
change.
One possible approach to examine the effect of Cl on melt stmcture is to study it
indirectly by analyzing samples with an additional element which may interact with CL.
Phase equilibna in the system N%O-Si0,-Zr02-H2W1 suggests that Zr may be such an
element (Currie and Zaleski. 1985). Efforts to synthesize sodium zirconosilicates revealed
that the source of Na affected which phase would crystdlize. Specificdiy it was reported
that pwtkeldyshite (NaJrSi20,) woulci appex if NaCl was used as opposed to vlasovite
(NaJrSi,O, ,) if N-CO, was used as the source of N a This suggests that Cl may alter the
local environment around Zr. The siniilarity in chernical behavior between Zr and Ti irnplied
by thek proximity in the periodic table indicates that the latter rnay be affected similariy by
CI.
Raman spectra have been reported for Zr-bearing potassium silicate glasses (Ellison
and Hess, 1994) and Ti-bearing sodium silicate glasses and melts (Mysen and Neuviiie,
1995). In both of these studies peaks were reported which were not observed for the base
glasses free of Ti or Zr, but were associated specifically with the transition metal studied.
In light of the fact that these peaks appear even with only a few weight percent of Ti or Zr,
it is possible that the addition of Cl, which has a Iow solubility compared to F or H,O, may
have some noticeable effect on the spectra. The purpose of this study is to examine the
Raman spectra of Ti- and Zr-bearing Na-aluminosiiicate glasses. By this approach, it is
hoped that the effect of added Cl on the glas structure around these two met& may be
de termined.
3.3 Experimental Procedure
3.3. I Sumple Preparation
Samples were prepared from reagent grade Na,CO,, SiO, N203, ZrO,, TiOz and
NaCl. Carbonate and oxide powden were ground together and decarbonated at 750°C for
48 houn. The sample material was ground and fused at 1450°C for one hour in air, then
reground with added NaCl as appropriate, sealed in Pt capsules and fused at 1600°C and 1.5
GPa for two hours in a piston-cylinder apparatus using NaCl-crushable Al,03-fused silica
assemblies (Hudon et al., 1994). The samples were prepared at high pressure to ailow for
high temperature homogenization of the $lasses without CI volatilization. The effect of
pressure on the structure of the samples is expected to be negligïble. Mysen et al. (1983) has
s h o w only srnail changes in the average T-0-T angle and the absence of new peaks in the
Raman spectra of Na-duminosilicate glasses as pressure is increased hm 1 atm to 3.0 GPa
The sample compositions were verified by electron microprobe analyses using a 40 pm beam
diameter and a 2 nA current calibrated with g l a s standards. Using these andytical
conditions, Na migration was eliminated. The results are reported in Table 3.1. It should be
noted that while no water was added to the samples, previous studies in Our laboratory have
indicated that a srnall amount of dissolved water, on the order of 0.5 wt.%, is inevitable in
melts prepared in this manner. Glas samples were examined opticaily and those devoid of
inclusions were retained for Raman analysis.
3.3.2 Raman Spectroscopie Analyses
The 5 14.5 nm line of an Ar' laser was used for excitation at a power of 200 mW. The
spectrometer was a JY Spex 750M single monochrometer with a Liquid N,-cooled CCD
detector. Spectra were acquired by averaging 10 iotegrations of 20 s each. Slit width was set
at 300 Pm. The monochrometer was calibrated ushg MMT [(C,H4CH3)Mn(CO),] which
provides strong Raman peaks across the entire frequency range of interest.
The background was removed manuaily and the spectra were smoothed with a 9-point
smoothing function. Peak intensities were nomdized using the 470 cm-' peak in order to
allow cornparison of the high-frequency region between 850 and 1200 cm".
3.4 Results
nie spectra of the base glasses with no added Zr or Ti (Fig. 3.1) show a band in the
low frequency region in which two peaks are observed, a large peak at 485 cm" and a
I - - -
RML RMLC RMLT RMLTC RMLZ RMLZC
- - - - -
total 99.86 99.67 99.56 9959 99.67 9934
Table 3.1. Electron microprobe analyses of g l a s samples. Al1 compositions are averaged from ten or more analyses. Errors represent one standard deviation.
Fig. 3.1. Raman spectra of base glasses: RML and RMLC, Cl-free and CI-be;uing,
peraikaline Na-duminosilicate glass, respectively.
much smaller one at approximately 560 cm? The large peak is not weH-understood although
it has been noted that it is associated with a large displacement of oxygen (Galeener and
-Mikkelson, 198 1) and it h a , therefore, been interpreted to represent symmetric vibrations
in the plane bisecting Si-O-Si bridges (McMillan et al., 1982). nie srnalier peak at 560 cm-'
is believed to represent the "breathing" mode of three-membered ring structures (Matson et
al., 1986; Kubicki and S ykes, 1993). This peak appears at about MW) cm'' in spectra of fused
silica, but the presence of Al tetrahedra in these ring structures shifts it to Iower fkquencies.
in the medium frequency range, there is an asymmetric peak at 780 cm". ui fused
silica and albite glas, this peak is found at approximately 800 cm", with probable
components at 790 and 830 cm" (Mysen et al., 1982). This peak is beiieved to represent
vibration of silicon within its oxygen cage with Little associated displacement of oxygen
(McMillan et al., 1982). The position of this peak in less polymerized @asses is shified to
lower frequencies due to the decrease in rigidity of the network.
There is a large, asymmetric peak in the high-fiequency region, between 850 and
1200 cm? This band has a peak at 1060 cm-' with a shoulder at a lower frequency. A peak
at about 1100 cm-' in alkali silicate @asses is generaiiy assigneci to vibrations associated with
Q' structures, that is, silicate tetrahedra with one non-bridging oxygen (Brawer and White,
1975; Virgo et al., 1980; Furukawa er al., 198 1 ; Mysen and Frantz, 1994), although it has
been suggested that it may also be associated with alkali-oxygen bonding (Fukumi et al.,
1990; McMillan et al., 1992). This peak shifts to lower frequency in alkali aluminosilicate
$asses as the AU(Al+Si) ratio increases (Mysen, 1990). The shoulder probabiy indicates the
presence of a significant number of Q' structures (SiO, tetrahedra with .two non-bridging
oxygen atoms) which are manifested in Raman specua by a peak at 950 cm" (Brawer and
White, 1975; Furukawa et al., 198 1; McMilIan, 1984a; Mysen and Frantz, 1994). It should
be noted that there is no demonstnble ciifference between the spectra of the Cl-free and CI-
bearing compositions in Fig. 3.1.
The addition of Zr has a sipiîlcant effect on the Raman spectra in the hi&-fkquency
region (Fig. 3.2). A new peak emerges ar 980 cm-' which may correspond to the 1010 cm-'
peak reported by Ellison and Hess (1994). Etlison and Hess proposed that tbis peak
represents the Si-NB0 stretch of a species in which the non-bndging oxygen is shared by
a Zr and two aikali ions. The shift in the position of this peak to lower frequency may be a
result of the effect of Al-containhg tetrahedra reducing the overail rigidity of the glass
network; the compositions studied by Ellison and Hess did not contain Al. Altematively, the
choice of dicati metai used in the glasses studied may account for the difference in peak
positions; in this study, Na was use& while Ellison and Hess used K in the preparation of
their glasses. This latter explmation is deemed less plausible, however, as significant
structural change due to the larger K+ ion seems unlikely in the open structure of an alkali
duminosikate g las .
Very little difference is observed between the CL-bearing and Cl-free gIasses in Fig.
3.2. A slight broadening of the high-frequency band toward higher wavenumbers may
indicate an increase in more highly-polyrnerized species, but the difference is too small to
be definitive. The spectra of the Ti-bearing glasses (Fig. 3.3) show the strong peak at 1060
cm", as well as a prominent peak at 9 1 O to 930 crd. This latter peak is more prominent, less
asymmetrical and shifted to higher frequency in the Ci-bearing glass. A comparative
deconvolution of the high-frequency region of the base glass and the Ti-bearing glasses is
presented in Fig. 3.4. The deconvolution of the base glass spectmm (Fig. 3.4A) was
performed assuming the presence of two peaks, the positions of which were lefi
unconstrained. The peak positions and intensities derived are indicated in Table 3.2.
Examination of the spectra of the Ti-bearing glasses indicates the presence of a third major
peak at approximately 900 cm''. Therefore, the deconvolution of these spectra was
perfomed assuming three peaks, with the positions of the peaks at 950 and 1060 cm-'
Fig. 3.2. Raman spectra for samples RML, RMLZ and W C . These samples are the base
glass, the base composition with added Zr and with added Zr and Cl, respectively. The peak
at 975 cm-' observed in the Zr-bearing compositions is associated with an alkali
zirconosilicate melt complex involving a Q~ structure with the loue non-bridging oxygen
being shared by one Zr and two alkali ions (Ellison and Hess, 1994).
cm-
Fig. 3.3. Raman specua for samples RMli, RMLT and RMLTC. the base glass, with 1.9
wt.% added Ti and with Ti and 0.3 wt.% added CI respectively. A prominent peak appears
at 900 cm-' in the Ti-bearing glasses which is associated with Ti-O bonds.
I 800 cm-'
Fig. 3.4. Deconvolution of the high-frequency region of the Raman specua for (A) the base
glass and the Ti-bearing samples. (B) RMLT and (C) RMLTC. The increase in intensity of
the peak at approximately 950 cm*' may result from an increase in the number of fuiiy-
polymerised, titanate tetrahedn as a result of added Cl. See Table 2 for exact peak positions
and intensi ties.
1m 1 la, cm"
IRML RMS: 0.0 172
1 RMLT RMS: 0.0186
- -
position
1 RMLTC RMS: 0.0288
949 0.320 28.8
intensity
position
position intensity integral
integral
intensity integral
Table 3.2. Positions and intensities of peaks fitted CO spectra of the high-fiequency region of the base glass and Ti-bearing glasses with and without added Cl. The residuai root-mean- square (RMS) error is included for each sarnple. Peak positions were unconsvained in the specûum of the base glass. The positions acquired for the base giass were used to constrain the 950 cm" and 1060 cm'' peaks in the spectra of the Ti-bearing glasses.
899 0.833 53.8
constrained to approximately those of the base glas. The results in Fig. 3.4 indifate that the
addition of Cl serves to increase the 950 cm-' pedc relative to those at 900 and 1060 cm-'.
The peak at 900 cm-' also appears diminished relative to that at 1060 cm", although the
difference is much more subtle and the validity of the observation more uncertain. Once
again, exact peak positions and intensities are indicated in Table 3.2.
In addition to the clifferences between the spectra of Ti-bearing and base glass in the
high-f~quency region, the presence of a low-fkquency shoulder on the 485 cm" peak is also
observed. This shouider is more prominent in the CI-bearing sample, but is present in both
Ti-bearing spectra. Henderson and Fleet (1995) repoaed that sorne peaks of anatase and
rutile (polymorphs of T i 0 3 occur in this region of the spectrum. However, optical analysis
of the Ti-bearing glasses indicated no evidence of phase separation which might account for
the appearance of peaks associated with @ ? ~ i .
3 5 Discussion
It is clear fiom the spectra in Figs. 3.2 and 3.3 that the addition of Cl has a significant
effect on the structure of Ti-bearing akali duminosilicate glasses and little discernible effect
on the structure of Zr-bearing glasses with similar compositions. This results from d i f f e ~ g
solution mechanisms for these metals in silicate melts. In the case of Zr, it has been proposed
that in silicate melts in which (Na+K)/N > 1, Zr forms alkali zirconosilicate meit complexes
of the form (Na,K),ZrSi,02+h,,, (Watson, 1979, Ellison and Hess, 1994). Some
disagreement exists between the two studies regarding the ratio of total akalis to Zr in the
melt cornplex, but Eiiison and Hess (1994) suggest that this is a function of the g las
transition temperature, that is, the temperature range across which a supercooled melt
becomes rigid and, thereby, "freezes" the melt structure in the gassy state. The (1-x)
K,O-xZr0,*4SiO, glasses analyzed by Eliison and Hess were prepared at 1600 to 1800°C,
while the experiments of Watson (1979) were performed at 700 and 800°C. Since the glass
transition temperature is a function of the cooling rate, the high temperature, rapidquench
experiments of Ellison and Hess would have a higher glass transition temperature than those
of Watson and would, therefore, have the melt structure at a higher temperature "fiozen"
into the glas. Eiiïson and Hess proposed that a peak which appears at 1010 cm-' with the
addition of Zr to the base glas represents a Q' structure in which the lone non-bridging
oxygen is shared by one Zr and two K. A peak at approximately this same location is
indicated in both spectra presented in Fig. 3.4.
Raman spectra of Ti-beanng Na-siiicate glasses and meIts showed a peak at 880 cmeL
which increases in prominence with increasing Ticontent (Mysen and Neuviiie, 1995). A
similar peak is reported by Hendenon and Fleet (1995) at 843 cm". The authon of the latter
study compared the spectra of titaniferous Na-silicate glasses to those of several crystalline
Ti-bearing compounds with differing coordination geometries for Ti. Based on this
cornparison, they concluded that the peak at 843 cm-' results ikom vibratious associated with
Ti-O bonding in tetrahednl coordination at low TiO, concentrations. A slight shift to higher
frequency with increasing titanium concentrations was reported by Hendenon and Fleet,
which they proposed resulted from Ti in five-fold coordination. A recent XANES study with
a more polymensed composition confmed the presence of primarily [ q i in titaniferous Na-
silicate giasses (Farges, 1997). The coordination geometry proposed for [''Ti consists of a
square pyramidal structure with a short Ti=O bond and four long Ti-O bonds (Farges et al.,
1996; Farges, 1997; Cormier et al., 1997). Bond valence-bond length correlations of the
titanyl unit (Ti&) favor a non-bridging oxygen bonded to 3 or 4 low field strength ions such
as alkaiis (Farges et al., 1996). The rernaining oxygens are probably bridging oxygens
bonded to s ikate tetrahedra or a mix of silicate and titanate tetrahedra (Fig. 3.5).
In the current study, a large peak appears around 900 cm'' for the Ti-bearing glasses,
which is not seen in the base glasses. It seems likely that this peak results from a vibrational
mode associated with Ti-O bonding, but the different peak positions reported by Henderson
Fip. 3.5. A possible bonding mode1 for Ti in five-foId coordination based upon bond valence-
bond lene& correlations of Farges et al. (1996). The structure iiIustnted is that of NfliSiO,
and is similar to fiesnoite (BaJiS&O,). Titanium in this structure is bonded to both bridging
and non-bndging oxygens (modified after Farges et aL, 1996).
and Fleet (1995), Mysen and Neuville (1995) and the current study îndicate differing bond
strengths in each case. The base composition in Henderson and Fieet was N@i03, while that
of Mysen and Neuville was N@iO,. a composition which would result in a more
polymerized melt structure compared to NqSiO,. As a shift of 37 cm-' exists in peak
positions between the two studies for the same concentration of dissolved Ti, it is concluded
that the iower degree of polymerisation in the glass compositions of the former study is
responsible for the shift to iower frequency. C o r n p a ~ g the resuiü of these previous studies
to those of the present study lends further credence to diis conclusion. The glasses used in
this study are much more polyrnensed than those of either of the previous studies and the
peak associated with Ti-O bonding is shifted to still higher frequency. A comparison of peak
position as a function of the average number of non-bndging oxygens per tetrahedra
(NBOm, assuming T= [Si+Al] is presented in Fig. 3.6. Given the uncertainty in establishing
the exact peak positions when dealing with the broad bands produced by silicate glasses, the
trend shown in Fig. 3.6 appears linear. The NBO/T values presented are for base glasses as
the effect of Ti is diff~cuit to quanuS. However, if the effect is constant for alI three studies,
then the comparison is vdid assuming approxirnately equal concentrations of dissolved Ti.
The relatively low sensitivity of peak position to Ticoncentration reported by Mysen and
Neuville (1995) and Henderson and Fleet (1995) gives a high degree of confidence in
interpolating peak positions for Ti concentrations similar to that in this study.
Henderson and Fleet (1995) also produced Raman spectra for Ti0,-SiO, glasses.
They reported that the addition of Ti to vitreous silica results in the appearance of peaks at
945 and 1 LOO cm" which were interpreted to represent the reaction of the silicate network
with Ti. The peaks were interpreted to represent the stretching vibrations of Si-NB0 bonds
in Q' and Q3 structures respectively. resulting from disruption of the silica tetrahedral
network by Iarger Ti-tetrahedn. A later EXAFSKANES study (Henderson and Fieet, 1997)
indicated that the addition of Ti to fused siiica has little effect on the network. This suggests
Fig. 3.6. Cornparison of the peak position for the peak associated with Ti-O bonding as a
function of the number of non-brid=eg oxygen per tetrahedron. NBOiT values are for base
glass compositions and are, therefore. only approximate for Ti-bearing compositions.
- This study
\ Mysen and Neuville (1 995)
Henderson and Flee
that the Raman peaks at 945 and 1 100 cm" represent vibrations associated with Ti-BO
(bridging oxygen) bonds in fully-polymerised tetrahedra. However, EXAFS/XANES may
not be able to distinguish fully-polymerised silica tetrahedra fkom those with one or two non-
bridgïng oxygens (G. Henderson, perscomm.). Therefore, the interpretation that the peaks
at 945 and 1100 cm-' represent @ and @ structures (Henderson and Fieet, 1995) cannot be
d e d out. In both cases, these peaks are associated with the presence of fully-polymerized
Ti-tetnhedn, either directly as a result of vibrations of Ti-O bonds or due to the appearance
of Q2 and Q3 stnrctures involving Si andlor Al.
The addition of Ti to the base glass composition in this study resutts in an increase
in the peak at 950 cm" in addition to the appearance of a new peak at 900 cm-' (Fig. 3.4).
A further increase in this peak is observed with the addition of Cl. Since no change is
indicated in the spectra of the base glas or the Zr-bearing glas with the addition of Cl, this
increase in intensity must be rdated to changes in the local structure around Ti. The fact that
Hendenon and Reet (1995) reported a peak at 945 cm-' resulting from the presence of [''Ti
in fused silica suggests that the increase in intensity of the peak at 950 cm-' may also indicate
the presence of tetrahedrdy-coordinated Ti.
The solubility of Ti, like that of Zr, is greatly enhanced in natural silicate melts in
which alkaii/Al> 1 (Gwinn and Hess, 1989). Furthemore, Ti often crystallizes unusual aikali
titanosilicate minerals such as narsarsukite [Na2(Ti,Fe)Si,0,,(OH~] and aeni=patite
[~a,Fe;*TiSi,0~J from such rnelts, in much the same way that Zr forms alkaii
zirconosiiicates such as catapleiite ~a&Si,O,-SH,Oj and wadeite [KZZrSi,Og] (Salvi and
Williams-Jones, 1990). It is not unreasonable to suggest, therefore, that Ti forms alkali
titanosiiicate complexes in alkali-rich silicate melts, in much the same way as Zr forms alkali
zirconosilicate melt complexes. Indeed, just such a complex of the form K,Ti,Si,O,, was
proposed for Ti-bearing, hi&-silica, K-aluminosilicate glasses (Dickinson and Hess, 1985).
The I s i~ i structure proposed by F-es et al., (1996) may be thought of in terms of an alkali
titanosîiicate melt complex. One of the three possible bond-valence models they proposed
requires the presence of some Ti-O-Si M a g e s and al1 their models require the titanyl unit
(Ti=O) to bond witb several alkalis in order fulfd its bond-valence requirements- Thus, it is
possible that if Cl bonds preferentiaily with alkalis in the melt structure, a cornpetitive
situation could arise between CI and an a h d i titanosilicate melt complex. The composition
of the Cl-bearing titaniferous glas (RhUTC) from Table 3.1 has only 3.2 mol% Na,O in
excess of that needed to charge-balance Ai. This same glas has 1.9 mol% TiO, and 0.6
mol% Cl. Thus, if Cl cornpetes effectiveiy for Na, Ti may be forced to alter its soiution
mechanism from five-fold coordination in an alkaii titanosilicate melt complex to network-
forming, tetrahedral-coordination.
The finding that adding Cl to the Zr-bearing glass has little effect on the Raman
spectra must be addressed. It is generally accepted that Zr is 6-fold or 8-fold-coordinated in
silicate glasses and melts. Such coordination geometries do not require that one of the
oxygen atoms be bonded only with akalis to satisfy its bond-valence requirements as is the
case in a [']Ti structure. Therefore, it is reasonable to conclude that such a coordination
geometry would be far less sensitive to cornpetition from Cl for alkalis. Furthemore, the
large ionic radius of Zr4 makes it impossible for it to revert to tetrahedral coordination.
Therefore, addition of Cl to the melt would not cause any detectable change in the intensity
or position of any Raman peak associated with Zr.
The phase equiiibria study of Cunie and Zdeski (1985) indicated that Cl exerts some
control over the crystallization of allcali zïrconosiiicate phases. Since the results of this study
indicate that no discernibte change in structure results from adding Cl to a Zr-bearing, alkali
aiuminosiiicate melt, the findings of Cume and Zaleski are possibly the result of Cl affecthg
the activity of alkalis or -0.
3.6 Conclusion
The solution mechanism of Ti"' in alkali aluminosilicate melts is believed to be
primarily that of alkali titanosilicate melt complexes, with Ti in five-fold coordination.
Rarnan spectroscopy provides evidence to suggest that Cl can destabilize these structures to
sorne extent in favor of fully-polymerized Ti-tetrahedra, especidly if the availability of
network-modimng alkalis is limited. The cornpetition for aikalis between CI and non-
bridging oxygen atoms associated with Ti-bearuig structures suggested by the results of this
study would aIso be expected to increase the viscosity of these melts by forming fûlly-
polymerized Ti-teuahedra. These results also support the mode1 that NaCl melt
complexation may be responsible for viscosity increases in other Cl-bearing silicate melts
as well.
Introduction to Chapter 4
Since alkalis play an important role in tbe solution mechanism of Zr and other bigh-
field strength elements in perdkaline magmas, it is useful to understand the structural
environment of alcalis and how it changes, if at aii, as other compositional parameters
change. A good technique for such a study is nuclear magnetic resonance magic-angle
spinning (NMR MAS) as it provides a direct analysis of changes in local environment of the
nuclide of interest. In Chapter 4, =Na NMR MAS analyses are performed on a suite of Na-
alurninosilicate @asses with varying Si02 to determine any change in locai structure such as
coordination number or average Na-O bond length.
Chapter 4
23Na NMR MAS Analyses of Na-Aluminosilicate
Glasses with Varying SiO, Content
Robert A. Man, Don R. Baker, A.E- WilliamsJones
Department of Eadh and Pianetary Sciences, McGili University, 3450 University Street, Montreal, Québec, H3A 2947
4.1 Abstract
=Na NMR MAS analyses of a suite of Na-aluminosiiicate glasses with Na/N = 2 and
varyîng SiO2 content h a revealed a negative shift in peak position as the glass structure
becomes more polymerized, i.e. the average nurnber of non-bridging oxygen atoms per
tetrahedron (NBOm decreases. This trer,d is observed only for giasses with NBOm s 0.27.
For more polyrnerized glasses no change in chemical shifi is measured. Past studies have
indicated a strong correlation between increasing Na-O bond length and increasingiy more
negative isotropie chemical shift in a ~ a NMR spectra. This suggests that for Na-
aiuminosilicate glasses with NBOK s 0.27, Na-O bond lengths would be expected to
increase as polymerïzation increases.
4.2 Iatroduction
Interpretation of UNa NMR MAS spectra in glasses is complicated by the
quadrupolar nature of this nuclide. In general, researchers compare the spectra of giasses
with those of crystalline phases to determine approximations of coordination number and
bond length (Maekawa et al., 1997). It then becomes possible to compare the spectra of
different giasses in order to observe how glass suucture responds to changes in composition.
The most conunonly studied characteristic of NMR spectra of glasses is the isotropic
chemical shift, the shifi in peak position resulting from differences in the degree of magnetic
shielding of the nucleus by local electrons. Changes in local structure distort electronic
orbitals and thus, change the amount of magnetic shielding. More negative chemical shifts
are indicative of p a t e r shielding. Systematic studies of glasses using = ~ a NMR MAS
indicated that the chemical shift trends toward more negative values with increasing
Si/(Si+Al) and Na/(Na+K) for compositions in the system NaAISi,Og-KAISi,O,-SiO,,
indicative of p a t e r shielding of the nucleus (Oestrike et al., 1985; Maekawa et al., 1997).
A sirnilar trend has been reported for melts with v;isring degrees of polymerization (Gee et
al., 1997; Xue and Stebbins, 1993; Maekawa et al,, 1997). The results of these studies
indicate a trend toward more negative chernical shifts as the mean number of non-bridghg
oxygen atorns per tetrahedron (NBO/T) is decreased. This paper reports the results of a
series of single-pulse * ~ a NMR MAS measurernents of a suite of Na-alumhosilicate glasses
with Na/Al=2 and varying SiOt content in order to identiw any shifi in peak position and
relate such shifts to structural changes in the glass network.
4.3 Experimental Procedure
Glasses were prepared using reagent-grade SiO, gel, A1203 and Na2C0,. A staRii1g
composition of 2N%O*Al,03-3Si02 was prepared and decarbonated at 1 atm and 750°C for
24 hours, then fused at 1 atm and 1450°C for 1 hour in a Pt crucible- From this base glass,
6 samples were prepared having the composition 2Na,O-Ai@,-nSiO,, with n = 3,4,5.7.9
and 11. For each composition (including the base composition), the sample was fbsed a
second time at 1450°C for 1 hour. AU samples were examined opticaiiy to ensure that no
crystals were present.
= ~ a NMR MAS spectra were acquired at a magnetic field strength of 7.1 T for a
Larmor frequency (vJ of 79.25 MHz with a spin rate of 5.5 kHz. Utilizing a Chemagnetics
CMX-300 instrument, analyses comprised 1000 acquisitions using a pulse width of 0.5 ps
and a recycle time of 200 m. The s h i f t in peak position measured is reported in parts per
miilion difierence in the nuclear resonance fiequency from that of solid NaCI. Typical error
in peak position is estimated to be about 0.1 ppm.
4.4 Resulr
The =Na NMR MAS spectra for ai i samples are presented in Fig 4.1. Each spectrum
consists of a single broad peak and spinning sidebands which overlap the main peak. The
peak positions. provided for each spectmm, become more negative as the S i 4 content in the
g l a s increases and the average number of non-bndging oxygen atoms per tetrahedron
(NBO/T) decreases. In Fig. 4.2, shifts in peak position are plotted as a hinction of (N'Born.
indicating that the trend is not continuous. Rather. the degee of shielding becomes constant
above NBOlI' - 0.27. indicating that continued depolymerization exerts less effect on the net
magnetization of the nucleus.
Fig. 4.1. =Na NMR MAS spectra of six Na-aluminosilicate glasses with constant Na/N and
vaqing SiO, content. The chernical shift relative to crystalline NaCl is indicated for each
spectrum.
O 400 PP*
Fig. 4.2. Plot of peak position as a hnction of the average number of non-bndging oxygen
atoms per tetrahedron (NBO/T). A negative trend in peak position is observed for glasses
with NBO/T 5 0.27. No shifi occurs for less polymerked glasses.
4.5 Discussion
The trend towards more negative chemical shifts in the NMR MAS spectra of =Na
with decreasing NBO/T results from greater s hielding of the nucleus from the appiied
magnetic field by the local electrons which generate a magnetic field of their own. The
magnitude of this shielding is a function of local structure as different coordination
geometries distoa electronic orbitals in different ways. which, in turn, affects the magnetic
field generated by the electmns in these orbitais as felt by the nucleus (Stebbins, 1995). " ~ a
NMR MAS spectra of Na-silicate and duminosiiicate crystdine phases (Maekawa er al.,
1997) have shown that the average Na-O distance is the most important factor affecting the
chemical shifi. Table 4.1 presents =Na chemical shifrs for several Na-silicate minerais as
weli as coordination numbers and mean Na-O distance. These data cleariy indicate that such
a relationship exists.
It should be noted, however, that the chemical shift does not change in a h e a r
fashion with increasing bond lenad. A plot of mean Na-O distance vs. chernical shift (Fig.
4.3) for several crystaliine Na-silicates is approximately iinear and negative in dope up to
a Na-O bond length of about 2.7 A. Beyond this point, the dope decreases slightly.
indicating that increasing bond length becomes Iess effective as a means of increasing the
degree of magnetic shielding.
The correlation between increasing polymensation and greater shielding of the
nucleus observed in this study for @asses has also been reported for melts. ResuLts from
severai studies of Na-silicate and aluminosilicate melts are piotted in Fig. 4.4. Above NBOn
= 1, the chemicai shifts become more positive very pdualiy, suggesting little change in the
local environment of Na. For more polymerized melts, the trend is still that of greater
shielding with decreasing NBOfT, but the slope is steeper. The implication is that as the melt
approaches full polymerization. the Na-O bond length becomes longer.
The implications of the trend in Fig. 4.4 for melts are somewhat counter-intuitive
phase ai (C0n.r Original C.N. (Na) d (W) NMR Structura mndud rdemna nfnma
l
NqBaSi,O, 1 Na@)
jadeite NaAtSi,O,
nep heline N~~(N&)ddAISiO,
Na site K site
albite NaAISi,O,
microciine
~ ~ ~ 1 7 U A ~ S ~ 3 O 8 sodalite Na.AI,Si,O,,CI anhydrous sodalite NaJAISiO J, natrolite NaJI,Si,O,,-2H,O tourmaline
NaMg&i,(B03,Si,O,,(OH), N a,LlYS &O,, Na(1) -1 7.95r0.5 -1 7.95
Na(2) -5.4=1 -5-4 narsarsukite
1 M NaCl 1 M NaCl
1 M NaCl 1 M NaCI 1M NaCl 1 M NaCl 1 M NaCl 1 M NaCl
1 M NaCl 1 M NaCl
1 M NaCl 1 M NaCl
1 M NaCl 3M NaCl 3M NaCl
1 M NaCl
1M NaCl
solid NaCl
1M NaCI 4+2H20
I ~k~ i s i . ~ . , -4.01 -4.0 1M NaCl 7 2-536 21 'Chernical shifts relative to IM NaCI standard; 0.2 ppm added to resutts using 3M NaCI (Templeman and van Geet, 1972); 7.3 pprn added to results using solid NaCl (Kundla et al, 198 1) bDat.a are simulations o f spectm rneasured at 200°C
Table 4.1. =Na chemicai shi f ts and structural data for c r y s h e Na-silicate phases. Coordination numbers indicated for Na represent bonding with oxygen atoms only. in some instances, Na may ais0 bond with other Ligands as noted. References are as follows: 1. Maekawa et al. (1997); 2. Xue and Stebbins, (1993); 3. Stebbins et al. (1989); 4. Phillips er al. (1988); 5. Kirkpatrick et ai. (1985); 6. Kundla et al. (198 1); 7. Buhl et al. (1988); 8. Pant and Cruickshank, (1968); 9. Pant (1968); 10. McDonald and Cruickshank, (1967); 11. Gunawardane et al. (1973); 12. Prewitt and Burnharn, (1966); 13. Daliase and Thomas, (1978); 14. Harlow and Brown, (1980); 15. Da1 Negro et al. (1978; 1980); 16. Barth (1932); 17. Felsche et al. (1986); 18. Pechar et al. (1983); 19. Buerger et al. (1962); 20. Gunawardane et al. ( 1982); 2 1. Peacor and Buerger, (1962)
Fig. 4.3. Mean Na-O distance vs. = ~ a chemicd shifi for Na-bearing silicate crystabe
compounds. Al1 chemical shifts indicated are relative to 1M NaCl solution, either directly
measured or corrected.
2.20 2-40 2.60 2.80 3-00 3.20
Mean Na-O distance (A)
Fig. 4.4 Plot of =Na chernicd shift vs. number of non-bndging oxygen atoms per
tetrahedron (NBO/T) for several Na-silicate and Na-duminosilicate meIts.
Na-silicate melts (Stebbins et al., 1985) Na-silicate melts (Maekawa et a1.,1997)
O Na-sil icate melts (Stebbins et a1.,1985) A Na&-silicate melts (Stebbins and Faman, 1992) 0 Na,Al-silicate melts (Maekawa et al., 1997)
Since the "Na chemical shifl becomes more negative as the SiO2 content is increased and the
melt structure becomes more polymerized, the mean Na-O distance would appear to be
increasing. Yet, data from Riebling (1966) indicate that for any constant Na/AI ratio. the
molar volume of a melt decreases with increasing mole fraction of SiO,. Unfortunately. Na-
O bond lena& have only been measured for a few glasses and melts. The few results which
have been reported are summarized in Table 4.2. Clearly, these Limited data cannot provide
much insight into the trend observed in Fig. 4.2. Indeed, the only snidy which compared
glasses of vruying composition (McKeown et aL, 1985) indicated virh~dly no change in Na-
O bond lene&.
It should be noted that the shifts in peak position of the glasses andyzed in this study
are si,pificantly more negative than those of the melts in Fig. 4.4, even taking into account
a correction of +7.3 ppm for the soiid NaCl standard (Kundla et al. 1981) used in this study.
This difference results from second order quadnipolar shifts in the specua of the glasses.
The measured shift. 6,,, of a quadrupolar nucleus such as =Na is actuaily the sum of the
isotropic chemical shift, 4,. and the second order quadrupolar coupiing, 6,. The latter can
be determined by using the equation for the isotropic average of the second order
quadmpolar coupling (Youngman and Zwanziger, 1994):
With the quadrupolar coupling constant, QCC = 3 MHz and the asymrnetry parameter, qQ
= O (Xue and Stebbins, 1993; George and Stebbins, 1996), and a Larmor fi-equency (vJ of
79.25 MHz, the contribution of quadmpolar effects is about 35 ppm. When this contribution
is added, gNa chemical shifts measured in this study average about 15 to 20 ppm higher than
those of compositionaiiy-simïiar rnelts (Maekawa et al.. 1997; Xue and Stebbins, 1993;
87
Composition T s Method A N . (Nap r(Na-O) Reïerence
298 298 1 273 298 298
NaAlSiO, 298 Na&4I,,Si,O, 298 Na, ,AI,,Si,,,O, 298
J I 298
X-ray rdf neutron rdf X-ray rdf
Na-EXAFS Na-EXAFS Na-EXAFS Na-EXAFS Na -WFS Na-EXAFS
Na,CaSi,O.- 298 Na-EXAFS 'Coordination numbers rounded off to nearest integer.
Table 4.2. Na coordination numbers and Na-O bond lengths for NaAl-silicate @asses.
References cited are as foiiows: 1. Waseda and Suito, (1977); 2. Greaves et al., (1981); 3. McKeown et al., (1985).
Stebbins et al., 1985). A similar shifi to more positive 6- was noted for Na-silicate glasses
at ambient temperature as compared to melts of the same composition by George and
Stebbins (1996) for both static and MAS NMR. They observed that the chemical shift
became increasingiy more negative as the glasses were heated, which they interpreted to
result from thermal vibration of Na within its coordination sheli, or alternatively, increases
in the size of the Na site. Quadrupolar effects become unimportant for melts as al1
transitions become fully averaged.
4.6 Conclusions
The results of this snidy agree with those of several previous NMR studies of
alkali silicate and duminosilicate glasses and melts confinning that there is a trend toward
greater shïelding of the nucleus with increasing polymenzation. This study, however, has
reveaied that for aikali duminosilicate glasses, that the trend appiies ody to compositions
with NBO/Ts 0.27. For less polymerized glasses, the zNa chemical shîft and, by inference,
the average Na-O bond length remain approximately constant,
Chapter 5
Controls of Alkali Zirconosilicate Crystallization
in the System: Na20-K20-AI,O,-Si0,-Zr02-H20*F
at Temperatures between 575 and .ïOO O C
5.1 Introduction
Although zircon is the most common zirconium-bearing minerai, there are a
bewildering array of alkali and aikaiine-earth Wconosilicates associated with felsic,
perdkaline rocks. Nearly two dozen of these minerals have been reported to date (Marr and
Wood, 1992; Birkett et aL, 1992). Among the most widely reported of these are elpidite
(Na@i,O,,*SH,O), catapleiite (Na&Si,0~2H20), vlasovite (Na2ZrSi JO, ,), wadeite
(KJr!Si,O,), dalyite (&2rS&O,,) and eudialyte (Ng Ca ZrSi, q, (OEQ ). The potential exists
for using these minerals as petrogenetic indicators, but only M t e d experimental data exist
on the physico-chernical conditions of crystaiiization of these minerals.
An important phase equilibrium study in the N@-Si0,-m-H20&1 system by
Cume and Zaleski (1985) constrained temperature of the reaction: elpidite = vlasovite + quartz + H20 for pressures of 0.5 to 2.0 kbars. In the same study, the authors reported that
parakeldyshite (Na,ZrSi,O,) + quartz was produced in synthesis experiments at temperatures
less than 700°C. when NaCl was used instead of N a m , as the source of sodium in the
starting materids. The authors attributed this to an effect of Cl- on the activity of silica or
H20. Whiie these were syntheses, not reversed experiments, and therefore, the assemblage
observed mayhave been metastable, the results suggest that halogens may- play a significant
role in the speciation of zirconosilicate minerals.
The aim of the present snidy is to define the role of halogens in the speciation of
akali zirconosiLicate complexes in feisic, perdkaline melts. was chosen for study
as it is one of the most important halogens in geological systems. Furthemore, the
complexation behaviour of fluorine in sodium aluminosilicate melts has been weU-
characterized (Mysen and Virgo, 1985a,b), thus allowing more meaningful interpretation of
results.
5.2 Experimental Procedure
The starting matenals were @asses with compositions as presented in Table 5.1. The
glasses were prepared from reagent-grade SiO,, Ai,O,, Nago , , &CO, and NaF powders.
The mixtures were decarbonated at 750°C in air for 48 &ours, ground. to a fine powder and
melted at 1550°C in air for one hour. The resulting glass was ground and fused a second
time. After the glasses were prepared, they were inspected optically for the presence of
crystals. Those samples with more than approxirnately one percent crystailine material were
discarded. ï h e samples were analyzed using an electron microprobe to determine the degree
of homogeneity and aikaii loss. Zirconium was added to the glasses as reagent-grade 21-0,.
Initial experiments involved the addition of 1.0 W.% Zr& to each composition. The glasses
were then fused a third time, analyzed by eiectron microprobe, ground and stored in a dryhg
oven until used.
Experhents were performed using cold-sed vessels (Tuttle, 1949). The cold-seal
apparatus in the Department of Earth and Planetary Sciences at McGill utilizes argon as a
pressure medium and can be employed up to pressures of 1.5 kbar. Pressure is monitored
using Astra Bourdon-tube gauges calibrated against a large dia1 Heise gauge which was
calibrated at the factory. Pressure measurements are considered accurate to +cO bars.
An Omega mode1 650 themocouple themorneter with chromel-alumel
thermocouples was used to determine temperature near the sample to within H C . The
thermocouples were calibrated against the melting points of H20, Sb, Bi and NaCl.
The experiments involved sealing 0.010 g of sample and 2.0 to 2.5 pL of distilled
water in a gold capsule. The capsules were welded and the weid was inspected opticaiiy. The
capsules were heated at 1 10°C for 15 minutes and reweighed to detect any fluid loss. The
capsules that underwent no mass loss were placed in the cold-seal vessels dong with a
ceramic filler rod. The experiments were run for one week at 1 .O kbar water pressure and
tempentures indicated in Table 5.2.
SiO, 54.70 66.37 77.67 53. 17 66.26 78.50
40, 22.97 17.17 1 1.88 24.3 1 17.51 11.70
N-O 1 1.38 8.18 5 -40 1 1.32 8.25 5.36
&O 12.46 9 -49 6.57 12.56 9.39 5-36
F - - 0.49 0.23 0.34 - O = F - - - -0.2 1 -0.10 -0.15
Tot al 10 1.5 1 101.21 10 1.52 101.64 10 1.54 101.1 1
Alk/Al I .40 1.38 1.35 1.33 1.36 1 .25
Na/K 1.39 1.3 1 1.25 1.37 1.34 1-52
Table 5.1: EIectron microprobe analyses of starting materials for fluorine-free and fluorine-bearing experiments. Results represent the average of ten andyses. Compositions 1, 2 and 3 correspond to a leucitite, alkali feldspar syenite and a grnite respectively.
After the experiments were compIeted, the reaction vesseIs were airquenched,
attaining subsolidus tempentures in under 30 seconds. The capsules were then removed h m
the vessels and the charges were exarnined. Since these experiments were performed
below the d q granite solidus, the presence of significant amounts of residuai glass in the run
products was taken as good evidence that water vapour did not escape' fiom the capsule
during the experiments. The capsules were also reweighed to detect any loss of volatiles.
Oxygen fugacity was not explicitiy controlled because no redox reactions were
invoIved in the experiments. However, fO, is expected to be within a log unit or two of the
Ni/NiO boundary as a result of the buffering effects of the reaction vessels which were made
of a nickel-bearing alloy (Chou, 1987).
The run products were malyzed using an electron microprobe with an accelerating
voltage of 15 kV and a beam current of 20 nA. Analyses of gisses was achieved by using
a wide beam (10 to 15 pm) to minimize sample damage. Albite (Na, Al, Si), orthoclase (K),
fluorite (F) and zircon (Zr) were used as standards,
5.3 Results
Wadeite (&ZrSi,O,) was obsewed in the run products of the haiogen-fiee
experiments for both the leucitite and the alkali feldspar syenite. The observed assemblages
are indicated in Table 5.2. The granitic composition yielded no zirconium-bearing minerais,
except for smdl aggregates of ZrO, grains, which are believed to be undissolved starting
material. Zircon was not observed in any of the run products.
Microprobe analysis of the feldspars, feIdspathoids and alkali zirconosilicates in the
run products of the fluorine-free experiments indicate very Little solid solution between Na
and K except at the highest temperatures. Even for the experiments at 700°C, the alkali
zirconosilicates and feldspathoids retain compositions close to that of ideal end-members.
Some representative mineral analyses are provided in Appendix 1.
590°C ne + Ksp + wd + glass
640°C ne + Ksp + wd + g l a s
ne + Ic + wd + glass
Ksp + wd + glass
Ksp + wd + glass
Ksp + qz + glass glas -
ne t Ksp + wd + glass
Ic+ Ksp+wd+@;tss
Ic + wd + glas
Ksp + pk* + &us
Ksp + glass
Ksp + glas
Ksp + qz + glas
1 3F 700°C Symbols used as foIlows: ne = nepheline; Ic = Ieucite; Ksp = K-feIdspar. qz = q u m ; wd = wadeite; pk = parakeldyshite; *(my be metastable)
Table 5.2: Observed assemblages in run products of expenments performed in this study. Al1 expenments were run for one week at 1 kbar pressure.
The addition of fluorine to the glasses resulted in the appearance of a Na-
zirconosïiicate phase in the aikali feldspar syenite composition, but only for the lowest
temperature expriment. This phase had the chernical composition of parakeldyshite
(N+ZrSizQ), but only appeared at a temperature si@cantly below that reported by Currie
and Zaleski (1985) for the appearance of parakeldyshite in their experiments. Furthemore,
this phase lacked rationai crystai boundaries and may have k e n metastable. Aside fiom this
one difference, the observed assemblages remain unchanged from the fl uorine-fkee run
products. As was the case with the fluorine-free experiments, most of the 2r0, remained
undissolved in the granitic melt. Zircon was not observed in any of the fluorine-bearing run
products.
Once again, significant solid solution between Na and K-rich end-members was not
evident at lower temperatures and not even in the high temperature experiments for the
zirconosiIicates and feldspathoids.
5.4 Discussion
Silica activity appears to have an effect on the solubiiity of Zr-bearing phases in
peralkdine melts. Analyses of the residud @ass from experimental run products of
composition IA at 640°C and 2A at 600°C (Appendix 2) indicate nearly an order of
magnitude increase in the solubility of wadeite in the more silica-nch melt. A similar trend
was observed in the solubility experiments reported in Chapter 2 for SiO, contents below
approximately 57 wt.%, although the degree of enrichment was more modest. Yet, Watson
(1979) reported that the soIubility of zircon in peralkaline grmitic mefts is insensitive to SiO,
content. Therefore. it appean that the solubility of wadeite and possibly other alkali
zirconosilicates is more sensitive to silica activity in melts than is that of Wcon.
The presence of wadeite as the saturating phase in the run products of experiments
using composition 2A is interesthg, as zircon was the saturating phase for aU experhents
with S i 4 above 55 W.% in Chapter 2. Since these latter experiments were ai l performed
at 800°C, this indicates that increasing the siiica activity depresses the upper thermal stabïiity
Iimit of wadeite in favour of zircon. This Iends further support to the conclusion from
Chapter 2 that peralkaline melts with high initial 210, content would be expected to
crystallize zircon rather than alkali zirconosilicates,
5.5 Conclusions
Severai concIosions may be drawn fiom the results of these experiments. First of aii,
the solubility of wadeite is greatly affected by silica activity in the melt, increasing
dramatically with the SiO, content of the melt. Secondly, wadeite is the saturathg Zr-
bearing phase across a wide range of thermai and chernical conditions and may be the fmt
alkali zirconosilicate to crystallize fiom many peralkaline melts in nature. Findiy, the upper
thermal stability Limit of wadeite is depressed as the silica content in the melt increases.
Therefore, hi$-silica, peralkaiine melts may crystallize zircon rather than an alkali
zirconosilicate, particularly if initial Z a content is hi@.
Chapter 6
Conclusions
6.1 Conclusions
The solubility of Zr-bearing xnherals has been previously reported to be Iargely
unsected by SiO, content in peralkaline grrinitic melts (Watson, 1979). However, the
results of this study indicate that for syenitic compositions, does become an important
control on the solubility of zircon and wadeite (K,ZrSi,O,). For -0-saturated peralkaline
melts at 800°C and 1 kbar, the solubiiity of zircon reaches a maximum of approximately 4
W.% Zr02 at a SiO, content of about 57 wt.%. For compositions with less silica, the
saturating phase becomes wadeite and the saturation Ievet drops sharply with decreasing
SiO,. Similar behaviour is observed in experiments with about 1 wt.% F, with two important
exceptions. The solubility reaches a maximum at about the same SiO, as that of the F-free
experiments, but the satuntion concentration of ZrO, is approximately 3.5 wt.%. Aiso, the
saturating phase for the Iower siiica compositions is ZrO, nther than wadeite. Experiments
with the same compositions at 650°C do crystdlize wadeite, however, indicating that the
addition of F Iowers the upper thermal stability limit of wadeite. The addition of small
amounts of Cl (less than 0.5 wt.%) to perdkaline granitic and syenitic melts lowers the
solubility of Zr-bearing minerais and no peak appears even though the transition from zircon
saturation to ZrOz saturation SUU occurs at about the sarne SiO, content. The saturation
concentration for al1 Cl-bearing melts in this study was about 2 to 2.2 wt-% ZrO,.
Raman spectra of aikali duminosilicate and Zr-bearing akali duminosilicate @asses
show no detectable change as small concentrations of CI are added. However, some
differences are detected in the spectra of Ti-bearing alkali aluminosilicate glasses with and
without added Cl. The spectrum of the Cl-free glas contains a strong, asymmetric peak at
900 cm-' which is associated with Iq~i-O stretching vibrations. This peak is shifted to higher
frequency and becomes more symmetric with the addition of 0.3 wt.% Cl. Deconvolution
of the high-frequency region in each spectra suggests that the observed differences are the
result of a contribution from a peak at 945 cm-'. This peak is believed to be the result of Ti-
O vibrations in fuiiy-polymerized titarate tetrahedra. It is proposed that the addition of Cl
destabilizes five-fold coordinated Ti in favour of tetnhedral coordination as a result of
cornpetition between Cl and titanate groups for alkali ions.
NMR MAS analyses of a suite of Na-aluminosilicate @asses with NalAl= 2 and
varying SiOz content have revealed a trend toward more negative chemicai shifts (greater
shielding of the nucleus) as the average number of non-bndging oxygen atorns per
tetrahedron (NBOm decreases. This trend is observed only for glasses with NBO/T a 0.27.
For more polymerized glasses no change in chernicd shifi is evident. Negative =Na chernical
shifts for crystaiiine Na-silicates are indicative of p a t e r Na-O bond length (Maekawa et al.
1997). This suggests that for Na-aluminosilicate glasses with NBOlT i 0.27, Na-O bond
lengths increase as full polymerization is approached.
6.2 Contributions to Knowledge
1. The solubilities of Zr-bearing rninerals have been detemiined for syenitic,
peraikaline melts. Prior to the completion of this thesis, th is data was only available for
grmitic compositions despite the fact that many Zr-rïch plutons such as Mont S t Hilaire and
Lovozero are syenitic.
2. It has k e n demonstrated that for H20-satunted. perdkaline melts, the addition of
F has either little effect or a detrimental effect on the solubility of Zr-bearing minerais
depending on bulk composition. This indicates that F affects the dissolution mechanism of
Zr in peraikdine melts differently than in metduminous melts since previous work has
indicated that F enhances the solubility of zircon in metaluminous, granitic melts (Keppler,
1993).
3. It has also k e n demonstrated that Cl has a detrimental effect on the solubility of
Zr-bearing minerals regardless of melt composition.
4. Raman spectra of Ti-bearing alkali aluminosilicate glasses have indicated that
smaii amounts of Cl result in a signifcant change in a peak which has k e n associated with
Ti-O "stretching" vibrations. A mode1 has been proposed suggesting that cornpetition
between alkalis and five-fold coordinated Ti results in some Ti becoming tetrahedrally-
coordinated. This has impIications for the stability of &di titanosilicate minerals such as
narsarsukite [N+(Ti,Fe)Si,O,,(OH,F)j and aenigmatite W F ~ ; ~ T ~ S & O ~ ] wwhh may
crystallïze from an alkali titanosilicate melt cornplex based on five-fold coordinated Ti.
5. U ~ a NMR MAS spectra of Na-aiuminosilicate glasses have indicated that for
compositions with NBO/T < 0.27, the chernicd shift becomes increasingly negative with
increasing polymerization, implying that Na-O bond length increases. Although previous
studies of Na-bearing crystalline phases have established the relationship between Na-O
bond length and chemical shift, this study is the first to systematically measure chemical shift
for highly-polymerized alkali aluminosilicate glasses, which more closely approximate
naturai melts.
6.3 Recommendations for Future Woïk
Much more phase equiiibria work must be done before the alkali zirconosilkate
minerais will be usehl as petrogenetic indicaton. Literally dozens of possible reactions
remain to be constrained. Additionally, calorimeuic studies of some of the more common
alkali zirconosilicates could provide valuable thermodynamic data which would allow
modelling of some kineticdly-unfavounble reactions.
The role of CI in granitic and syenitic melts needs to be explored more thoroughly.
Although it is known that CI affects the physical properties of silicate melts very differently
than the other common halogen F, theories on its solution mechanism are purely speculaîive.
Spectroscopie studies usuig X-ray absorption techniques or U ~ a NMR MAS studies of Cl-
bearing Na-silicate basses rnay provide some insight. Furthemore, while the solubility of
Cl in granitic melts has k e n studied (Webster, 1992), it has not been determined for silica-
undersaturated melts. Given that many nepheline syenites such as that at Mont St. Hilaire
have elevated Cl concentrations (i.e. normative sodalite), a systematic study is called for.
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Appendix 1
Appendix 1 : Represenrritive elecuon microprobe andyses of crystalline phases appearing in run products of phase equilibria experhents described in Chapter 5. -Mineral abbreviations are 3s follows: ne: nepheline; fp: dkaIi feldsprir, wd: wadeite: Ic: Ieucite: pk: pankeIdyshite. Analyses have been recalculated to the appropriate number of oxygens in the formuia unit of each mineral.
nelA/610 ne lA164O f'p l M 0 wdlM640 w d l M 0 0 f'p3A/600 fp3A/600 fp3N550 Si 1 .O3 1 -04 3.0 t 3 .O2 3.03 3 .O2 3.03 3
wdl F/665 fp 1 FI665 Ic 1 F/665 lc 1 F/665 fptFl575 p WFl575 pk2 F/575 fp2Fl595 fp2Fl595 Si 3.1 3 2.0 1 2-06 3 .O2 2.03 2.07 3 .O2 3 .O3
Appendix 2
Appendix 3: Electron microprobe anaiyses o f some residual glasses from run products of experiments described in Chapter 5. Totds do not indude loss on ignition (LOI) resultinp from dissoived H20.
Appendix 3
Appendix 3: Electron microprobe analyses of residual glass for al1 experiments reported in
Chapter 2. Totals do not include dissolved H,O.
b
SiO, Al203 Na,O K2O m. F =O
total
Si02 A1203 Na20 K2O Zr02 F =O
total
SiO, AI203 Na20 K2O ZrO, F =O
total
SiOz A I 2 0 3
NazO KzO 202 F =O
total
r
SiO, A1203
Na20 K2O ZQ F =O
total
Si02 A1203 Na20 K20 Zr02 F =O
total
Si02 Al203 Na20 KzO 2 0 2 F =O
total
Si02 A1203 Na20 K20 ZrO, F =O
total
RMG4 RMG4 RMG4 RMG4 RMG5 RMG5 RMG5 RMG5 RMGS RMG5 RMG5 RMGB' 54.30 54.14 54.02 54.05 55.01 55.09 54.90 54.88 54.87 55.22 54.90 55.61
95.61 9527 95.25 95.19 95.09 95.06 94.91 94.83 94.96 95.22 94.82 95.83 RMG5 RMG5 RMG5 RMGS RMG5 RMGS RMG6 RMG6 RMG6 RMG6 RMG6 RMG6 55.48 55.24 55.46 55.61 55.65 55.64 56.43 56.26 56.28 56.41 56.52 56.35
total
SiO2 AI203 Na,O K20 ZrO, F =O
total
SiO, A(2O3 Na20 K2O ZQ F =O r =O
total
RMG7 RMG7 RMG7 RMGlr RMGlr RMGlr RMGlr RMGlr RMGlr RMGlr RMG2r RMG2r 69.53 69.52 68.86 50.27 49.90 49.95 50.01 50.05 50,09 49.67 51 -47 51.44
93.69 93.89 93.75 94.34 93.68 93.67 95.61 95.52 94,83 95.42 94.87 95.21 RMG& RMG6c RMG6c RMGGc RMGGc RMG6c RMG6c RMG6c RMGGc RMG6c RMG6c RMG6c
57.01 56.84 57.03 56.79 56.65 56.79 56.74 56.97 56,72 56.64 56.96 56.49
C
SiO, A1203
Na20 K2O Zr02 CI =O
total
Si02 4 0 3
Na20 K*O Zr02 CI =O
total
Si02 A1203 Na,O K20 2 0 2 CI =O
total
Si02 A1203
Na20 K20 Zr02 CI =O
total
RMJ1 RMJl RMJl RMJ1 RMJ1 RMJl RMJ1 RMJ1 RMJ1 RMJ1 RMJ2 RMJ2 52.88 52.85 52.78 52.92 52.70 52.74 52.77 52.36 52.77 52.58 56.13 56.36
94.79 94.79 95.08 94.92 94.43 94.72 94.83 94.52 94,77 94.72 94.91 95.22 RMJ2 RMJP RMJ2 RMJP RMJP RMJ2 RMJP RMJP RMJ2 RMJ2 RMJ2 RMJP' 56.51 56.26 56.40 56.32 56.31 56.45 56.33 56.03 56,07 56.15 55.82 56.06
total
SiOz
95.10 95.09 94.68 94.85 95.02 94.98 95,Ol 94.74 94.97 95.18 94.93 95.20 RMHl RMH1 RMH1 RMH1 RMH1 RMH1 RMH1 RMH1 RMH1 RMHI RMH1 RMH1 51.61 51.80 51.82 51.73 51.66 51.42 51.43 51.13 51.50 51.79 5150 51.69
total
Si02
93.92 93.85 93.78 93.91 93.88 93.65 93.61 93.06 93.43 93.89 93.05 93.72 RMH1 RMH2 RMH2 RMH2 RMH2 RMH2 RMH2 RMH2 RMH2 RMH2 RMHP RMH2 51.49 52.25 52.01 52.19 52,30 52.21 52.15 52.01 52.35 52.42 52.39 52-16
total K
SiO2
ZrO, total
RMH3 RMH3 RMH3 RMH3 RMH3 RMH3 RMM RMH4 RMH4 RMH4 RMH4 RMH~' 55.12 54.44 54.53 54.73 54.67 54.93 54.71 56.87 56.65 56.77 57.00 56.51 17.90 18.07 18.24 18.20 18.06 17,94 18.10 16.50 16.55 16.36 16.56 16.63 8,89 8.83 8.86 8.87 8.90 8.70 8.93 9.04 9.04 8.95 9.07 8.89 9.42 9.43 9.40 9.36 9.36 9.45 9.46 8.45 8.43 8.34 8.33 8.35 3.34 3.72 3.29 3.34 3.33 3.39 3.37 4 .O2 4 ,O6 4.07 3.93 4.03
94.67 94.49 94.32 94.50 94.32 94.41 94.57 94.88 94.73 94.49 94.89 94.41 RMH4 RMH4 RMH4 RMH4 RMH4 RMH4 RMH4 RMH4 RMH4 RMHS RMHS RMHS 56.68 56.56 56,62 56.76 57.02 56.63 56.75 56.77 56,69 63.79 64.31 64 .O3 16.63 16.58 16.78 16.45 16.45 16.73 16.50 16.53 16.58 12.74 13.08 12.72 8.89 9.08 9.07 8.86 9.02 9.15 8.94 8.85 9.1 1 7.46 7.39 7.40 8.33 8.30 8.37 8.32 8.40 8.23 832 8.45 8.35 6.23 6.57 6.43 3.74 3.96 4.00 3.95 3.94 4.01 3.92 3.96 3.97 3.81 3.64 3.69
94.27 94.48 94.84 94.34 94.83 94.75 94.43 94.56 94.70 94.03 94.99 94.27 RMH5 RMH5 RMHS RMH5 RMH5 RMHS RMH5 RMH5 RMH5 RMH5 RMHS 63.96 64.38 64.25 64.11 64.37 64.01 6458 64.48 64.26 64,27 63.63 13.38 12.82 12.87 12.62 12.85 12.87 12.82 12.78 12.81 13.15 12.71 7.13 7.46 7.59 7.72 7.76 7.58 7.66 7.51 7.51 6.93 7.17 7.01 6.28 6.39 6.30 6.17 6.31 6.24 6.35 6,30 6.88 6.33 3.46 3.93 3.89 4.43 4.06 3.91 3.91 3.87 3.77 3.41 3.76
94.94 94.87 94.99 95.18 95.21 94.68 95.21 94.99 94,65 94.64 93.60 I