an infrared and raman study of carbonate glasses: implications for

Pergamon Geochimica et Cosmochunica Acta. Vol. 59. No. 5, 927-937. 1995

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An infrared and Raman study of carbonate glasses: Implications for the structure of carbonatite magmas

MATTHEW J. GENGE,* ADRIAN P. JONES, and G. DAVID PRICE

Research School of Geological and Geophysical Sciences, Birkbeck College and University College London, London WC 1 E 6BT. UK

(Received March 25, 1994; accepted in revisrdform November 2, 1994)

Abstract-Carbonatite magmas have been suggested as important agents of mantle metasomatism and yet, the structures of this important class of melt, which would be expected to control physical and chemical behaviour, have only been poorly constrained. The infrared and Raman spectra of carbonate glasses quenched from the systems La(OH),-Ca(OH),-CaC03-CaF,-BaS04 and MgC03-K2C03 at 1 kbar place constraints on the structures of these glasses and natural carbonatite magmas. The activity of the fundamental modes of the carbonate ion indicate that at least two structural populations of CO:- exist in carbonate glass structure, one of which, by virtue of the large vibrational splitting of its v) mode, is suggested to occupy a highly asymmetric site. The spectral activity of the O-H stretching region suggests that water exists both as molecular Hz0 and OH, interacting variably with carbonate ions and as metal complexes occupying relatively high symmetry sites in these glasses. The presence of bicarbonate groups, however, is prohibited by the absence of characteristic O-H stretching frequencies. It is suggested on the basis of the vibrational spectra that carbonate glass structures represent “flexible” frameworks constructed by the “bridging” of carbonate ions by strongly interacting metal cations and that the flexible framework is supported by framework modifying cations and molecular groups. The presence of at least two structural populations of CO:- in carbonate glasses implies a level of medium range order and the existence of extended structural units in carbonate melts and it is suggested that such groups represent metal-carbonate complexes. The possible effects of such complexes on the geochemical behaviour of elements in carbonatite melts is discussed and a general model by which variations in elemental solubility could be understood is proposed.

INTRODUCTION

Carbonatites are rare magmatic locks comprising more than 50% molar carbonate minerals; certain varieties contain the highest rare earth element (REE) contents of any igneous rock (up to 15000 ppm La: Cullers and Graf, 1984). There are, however, a little over 300 carbonatite occurrences worldwide, ranging in age from Archean to present (Woolley, 1989) and they are, hence, only of little significance in terms of global magmatism. Recent high-pressure experimental studies, however, have demonstrated that carbonate melts, generated as the first partial melting products of carbonated peridotites, are capable of metasomatising overlying mantle (Green and Wallace, 1988; Thibault et al., 1992; Dalton and Wood, 1993 ) The chemical signatures predicted for this carbonatite metasomatism have now been recognised in subcon- tinental and suboceanic mantle samples worldwide (Yaxley et al., 1991; Dautria et al., 1992; Rudnick et al., 1993; Ionov et al., 1993; Hauri et al., 1993), establishing carbonatite melts as important agents in the geochemical evolution of the lithospheric mantle.

The structure of this important class of melt, which must ultimately control both the chemical behaviour of its constit- uent elements and its macroscopic physical behaviour, have only been poorly constrained. The lack of structural resolution for carbonate melts is the direct result of technical difficulties in their analysis. Carbonatite melts cannot easily be studied directly at pressure and temperature, due to both their extreme

* Author to whom correspondence should be addressed.

921

reactivity to available pressure-cell window materials and the poor X-ray scattering response of C. The existence of a com- paratively restricted range of carbonate glasses provides, therefore, the only experimental means of studying carbonate melt structure.

Vibrational spectroscopic studies of glasses have frequently been used in order to derive qualitative and quantitative constraints on the structures of geologically relevant melt phases and have proved effective in the study of both water and COz speciation in natural and synthetic silicate melts (Stolper, 1982; Fine and Stolper, 1986; Mysen and Virgo, 1980). The similarity between glass and melt structures has been demonstrated by previous vibrational studies, which suggest high levels of structural similitude (Seifert et al., 198 1 ) ; however, differences in structure are implied from the higher densities of amorphous materials and observations of changes in the speciation of protonic species in glasses quenched at different rates.

Carbonate glasses are only known to quench from melts in two systems: ( 1) La(OH),-Ca(OH),-CaCO?-CaF,- BaS04 (Jones and Wyllie, 1983 ) and (2) MgCO?-K2C07 (Data and Tuttle, 1964; Ragone et al., 1965) at 1 kbar. In both systems, glass formation is centered on the eutectic regions of phase space. The REE-bearing system demonstrates glass formation from O-40 wt% La(OH), and additionally quenches to glass at temperatures below the liquidus. Glass formation in the alkali system is restricted to supraliquidus temperature melts and compositions between 29-48 wt% MgCO?. Both systems produce vesicle-bearing glasses.

928 M. J. Genge, A. P. Jones, and G. D. Price

The REE-bearing system was used by Jones and Wyllie ( 1986) to demonst&ethat bastnaesite can precipitate directly from a carbonatite melt and represents a synthetic Mountain Pass carbonatite composition. Although the Mountain Pass carbonatite is in respects unique since it is the only many occurrence in which the REE-minerals are demonstratedly

magmatic, this system does represent a direct analogue of a

natural carbonatite The alkali-bearing system cannot magma. be related directly to natural composition, since although any

‘L’ahle I. Range of compositions of analyzed carbonate glasses in wt%.

Samp I,a(OH), CaCO, Ca(OH), CaF, RaSO, M&O, K,CO, N,r “- EhLal6 16.48 33.38 17.28 22.50 10.32 - - IchLa20 20.00 31.98 16.55 21.55 9.88 - _ EhLa25 25.00 29.98 15.52 20.21 9.27 - - EhLa30 30.00 27.98 14.48 18.86 8.65 - - EhLa35 35.00 25.98 13.44 17.51 8.03 - - AHR2/3 - - - - - 28.91 71.09 AHR3/4 - - - - - 33.29 66.76 AHRl - - - - - 37.89 62.11 AHRY4 - 42.71 57.29 AHR3/2 - 47.78 52.22

Mg-rich carbonatite varieties occur they generally demonstrate low alkali contents and alkali-rich varieties are Na- rather than K-dominated (Woolley and Kempe, 1989). High-pressure melting experiments on carbonated peridotites, however, demonstrate that carbonate partial melts become increasingly Mg-rich with increasing pressure (Wallace and Green, 1988)) and since phlogopite is the predominant alkali- bearing phase at pressures higher than 30 kbar, it may not be unreasonable to assume that such melts are K-dominated. The

curred during quenching, due to the thermal “inertia” of the sample holder.

The resulting run products were a series of vesicle-bearing carbonate glasses of which the REE-bearing varieties may be yellow- orange, with colour generally increasing as eutectic compositions and temperatures are approached. Run products in the REE-bearing sys- tern contain euhedral liquidus crystals accumulated into capsule ba- ses. The presence of euhedral liquidus crystals of maximum dimen- sions of I mm and well-developed crystal and melt droplet fraction- ation indicates textural equilibration.

Analytical Technique

Infrared and Raman spectra of the synthetic carbonate glasses were obtained using the Bruker IFS-45 and IFS-85 Fourier transform mi- crospectrometers, respectively, with spot sizes of 200 pm; only sam- riles considered crvstal-free under an ootical microscone were ana- iysed. Samples containing microscopic’crystals proved easily identifiable, due to the intensity of crystalline peaks. Atmospheric background was removed by correction of spectra with a premea- sured reference analysis; as purging of the sample chamber was not possible, background removal was considered effective if the atmospheric CO2 doublet at 2430 cm ’ did not appear in corrected spectra.

Samples for absorption analysis were prepared as polished sections

alkali-bearing glass forming system may therefore be an appropriate analogue of mantle derived carbonatite melts.

The formation of glasses in these systems is problematic, since carbonates as ionic materials do not specifically satisfy the requirements for glass formation inasmuch as they do not demonstrate polymerisation and are, hence, unable to form the rigid three-dimensional network structures demonstrated by m&t glass-forming materials. Additionally, the low vis- cosities and densities of carbonate melts (Janz et al.. 1979; Dawson et al., 1990) and the high mobilities of their component species (Spedding and Mills, 1965 ) are unfavourable to glass formation since they facilitate spontaneous crystalli- sation. The lack of glass formation in most simple carbonate systems, even at the extremely high quench rates provided by shock quenching, relates essentially to the low activation energies of nucleation and crystal growth, and it is notable that glass formation only occurs in systems with unusually low eutectic temperatures.

EXPERIMENTAL METHODS

of 50-300 pm thickness and mounted on the sides of glass slides such that the beam path lies through sample only. Reflection samples were prepared as polished sections or glass shards and Raman samples & polished sections placed directly on the reflective mirror to reduce scattering by nonperpendicular surfaces to the incident beam. Raman spectra were excited by a Nd:YAG laser and measured using a Ge detector.

Sample mixtures were prepared from analytic grade chemicals ( >99.998 wt%), with the exception of MgCO?, which was prepared by carbonation of MgO. All samples were stringently dried prior to preparation at 110°C and stored in either a desiccator or drying oven, although MgCO? required drying at 200°C to ensure dehydration of the Mg carbonate hydrates. All synthetic mixtures were prepared to an accuracy of 0.0 I wt% in 20 g batches and repeatedly mixed under acetone with a mortar and pestle. Sample compositions are shown in Table I.

Reflection spectra were obtained, since both the mechanical insta- bility of these glasses and the extreme hygroscopic nature of the MgCO,-KzCOI glasses prevented sectioning below 50 pm thickness, at which the most intense peak maxima may not be resolved. Addi- tionally, analyses were restricted to the mid-infrared wave range between 4000 and 500 cm ’ since significant bands could not be resolved in either the far or near-infrared regions.

EXPERIMENTAL RESULTS All glasses synthesised in this study were quenched from melts at

I kbar pressure in order to reduce dissociation of the melt phase and run times were varied between 12 and 24 h, depending on run temperature, to ensure equilibration; chemical homogeneity was checked by microprobe analysis. Externally heated hydrothermal rapid- quench vessels were utilised for experimental runs which allow quench rates of 400°C SV’ to be attained. Rapid quench rates were facilitated by moving the sample from the hot zone to a water-cooled cold zone at pressure using an external ring magnet and an internal sample holder with a magnetic weight. All samples were encapsu- lated in welded thin walled gold capsules of 2.5 mm diameter to maximise heat loss during quenching and triply distilled ionised water was used as the pressure medium to reduce contamination by diffusion of impurities across capsule walls. Although quenching was essentially isobaric, small increases in pressure of ca 0.1 kbar oc-

Infrared Spectra

Figure I shows the background subtracted infrared absorp tion spectra of both La(OH),-Ca(OH),-CaC03-CaF2- BaS04 and MgCOJ-K,CO, glasses; the peak frequencies and band assignments are listed in Table 2. The spectra demonstrate three distinct regions: (1) the water stretching region between 3700 and 2500 cm-‘, (2) the carbonate internal mode region between 1800 and 500 cm- ’ , and ( 3 ) the central region with a number of distinct, well-resolved modes between 3000 and 1700 cm-‘, which are partially superimposed on the water and carbonate bands.

Carbonate glasses and carbonatite magmas 929

REE-bearing glass at 3 100 cm ’ The presence of O-H stretching modes in the absorption spectra of MgCO,-K2C01 glass, despite the stringent procedures conducted to prevent contamination by atmospheric water emphasises the hygroscopic nature of these compositions.

The low wavenumber absorption bands in the region 1 SOO- 500 cm ’ relate mainly to the internal modes of the carbonate ion. In the absorption spectra of the La(OH)I-Ca(OH)2- CaCO?-CaFz-BaS04 glasses, three broad off-scale bands occur with a low intensity band at 999 cm ’ , which may represent the V, symmetric stretch of the CO; group. The absorption spectra of the MgCOI-K,CO, glasses is far better resolved and individual peak maxima may be identified. The band at 1060 cm ’ and a shoulder at 1075 cm ’ may relate to two components of the V, symmetric stretch of CO:- , due to its similar frequencies to the V, mode of calcite and aragonite at 1099 and 1085 cm ’ , respectively (Huang and Kerr, 1960). The bands at 690 and 724 cm~ ’ may relate to the L/~ out of

Table 2. Infra-red absorption, reflectance and gman frequencies with hand assignments from carbonate glasses in cm includmg data from a previous stud form of the pea 3;

by Sharma and Slmons (1980). Symbols relate to the s-shoulder, b-broad, i-intense, w-weak.

a

b

La(OH),-Ca(OH),-CaCO,-CaF2_RaSO,

Reflectance Absorption Assignment

725 (H) 0.i v, out of plane

777 (H)

I 4000

I I I I 3000 2000 1000

Wavenumber cm-’

F~ti. I. Background subtracted infrared absorption spectra of (a) Mgc 01-KLC02 glass and (b) La(OH)1-Ca(OH),-CaCOI-CaF2- BaSO, glass. Sample thickness 100 pm.

870 (\v)

999 (i)

1123 (w)

;$j): 1;’

1452 (s) 1508 (w) 1570 (b)

1945 (i)

2211 (i)

2521 (i)

2926 (i)

3100 (b)

809 (w) 873 (i)

1022 (w) 999 (i)

CO,‘- v2 in plane bend

CO,’ v, stretch

SO,” stretch

CO,’ v, stretch

The high wavenumber absorption mode represents a broad asymmetrical composite peak and occurs in the wave range of the O-H stretch of water and hydroxyl groups in many crystalline carbonate hydrates and water-bearing silicate glasses. Similar asymmetric peaks occur in the absorption spectra of the hydrous Mg carbonates nesquehonite MgCOI.3H,0, artinite MgC03’Mg(OH)?.3Hz0, and hy- dromagnesite 4MgC0,.Mg(OH)2.4H,0 and have been interpreted as the O-H stretches of both OH and Hz0 groups, with variable interaction to the carbonate ion. Sharp, discrete peaks superimposed on these broad bands in the spectra of the hydrous Mg carbonates have been interpreted as the O-H stretches of discrete hydroxyl sites, but are absent from glass spectra (Moenke, 1962; White 197 1). Similar broad asymmetric peaks have been identified in the infrared spectra of H,O-bearing silicate glasses with peak maximum of ca 3550 cm-’ and interpreted as O-H stretches of OH and Hz0 groups with a continuum of H bond lengths to the Si04 tetrahedra (Stolper, 1982). It is clear, therefore, that the infrared activity in this region cannot be used to specify the speciation of water in these glasses. The absence of discrete peaks, however, suggests that Hz0 and OH groups lack well-defined sites. The intensity maximum of the asymmetric O-H stretching band in the La(OH)?-Ca( OH),-CaCO,-CaF?-BaS04 glasses occurs between 3550 and 3600 cm ’ and suggests a level of H- bonding similar to that of H,O-silicate glasses, rather than hydrous crystalline carbonates, which have lower intensity maxima of 3430, 302 1, and 30 11 cm-‘, suggesting longer OH-CO? bonds. The O-H stretching modes in the MgCOT- K,COq glass spectra have lower peak frequencies than the

1346 (i) 1445 (i)

1563 (i)

1777 (i) 1770 (i) 2v: or vI+v,

2111 (w) 2130 (w)

2500 (w) 2504 (i) 2540 (w) 2540 (i)

2916 (WI 2920 (i) 2v

O-H stretch

O-H stretch 3550 Ih) 3550 (b)

M&O,-K,CO, Raman

Present Sharma Retlectance Absorption

621 (w) 690 (w) 724 (w) 804 (i)

872 (H)

Assignment

684 (i) 804 (w)

CO,’ v, out of plane hend

822 U$ v2 in plane

CO,? v, stretch 10.53 (i) 1058 1072 (i) 1079

1042 (i) 1080 (s J

1060 (i) 1075 (s)

1387 (w) ‘Iii; 1447 (i)

1525 (w) 1528

1398 (s) 1430 (i) 1470 (s)

CO,’ v., stretch

2v, or v,+v,

v,+\‘,

3100 (h) O-H stretch

930 M. J. Genge, A. P Iones, and G. D. Price

plane bend of CO:-, due to similarities to those of calcite at 712 and 724 cm-‘, and an intense band at 872 cm-’ the vz in plane bend of the CO:- group close to its position in calcite of 876 cm-‘.

A number of bands in the absorption spectra of both types of carbonate glass are poorly resolved, due their high intensity and the necessarily large section thickness. However, en- hanced resolution may be derived from infrared reflectance spectra, although inherent divergences in frequency from absorption values must be expected. In the reflectance spectra of La(OH)j-Ca(OH)2-CaC03-CaF2-BaS0, glasses (Fig. 2), the high wavenumber band may be resolved into three components which may relate to a vibrationally split CO:- vj asymmetric stretching mode with frequencies 1346, 1445, and 1563 cm-‘, although it is notable that the high and low components are outside the range of frequencies demonstrated by crystalline carbonates. A less intense band at 873 cm-’ prob- ably relates to the v2 mode of carbonate and is very close to the band observed in the MgC07-K2C03 glasses and the poorly resolved bands at 725 cm-’ and 777 cm-’ may relate to the vq CO:- mode. The poor resolution of both the low wavenumber region below 800 cm-’ and the intense band between 1200 and 1100 cm-’ in the La(OH),-Ca(OH),- CaC03-CaF*-BaS04 glasses may well relate to the internal modes of the SO:- ion and it is notable that the vX modes of CaSO, at 1149, 1126, and 1095 cm-’ and BaSO, at 1179, 1147, and 1081 cm-’ lie in this region (Ramdas, 1954; Hezel and Ross, 1966). The reflectance spectra of MgCO,-K,CO? glasses (Fig. 3) also allow the CO:- vj mode to be resolved into three components at 1398, 1430, and 1470 cm-‘, other reflectance peaks corresponding fairly well with those observed in the absorption spectra described above, although divergences of up to 20 cm-’ in frequency occur in the intensity maxima of some modes.

The infrared activity of the CO: V’ mode and the vibrational splitting, suggesting loss of degeneracy, observed in the vj and vq modes, implies reduction of site symmetry of the carbonate ion in the glass with the loss of the threefold rotational axis. Such decreases in symmetry need not imply dis-

4600 3doo 2doo Wavenumber cm-’

do0

Frc. 2. Background subtracted infrared reflectance spectra of La(OH),-Ca(OHk-CaCO,-CaF,-BaSO, glass.

I 2000

I I

1500 1000 Wavenumber cm-’

1 500

FIG. 3. Background subtracted infrared reflectance spectra of the carbonate internal mode region of MgCO,-KZCO, glass.

tortion of the group itself, but may be attained by rearrange- ments of the coordinated metal cations as in crystalline aragonite structure carbonates.

The central wavenumber region between 3000 and 1700 cm-’ demonstrates a number of well-resolved bands in the absorption spectra of both glass systems. In the La( OH)7- Ca(OH),-CaC03-CaF-BaS04 glasses, four distinct peaks occur at 1770, 2130, 2504, and 2920 cm-‘. Two of these bands demonstrate vibrational splitting, the 2504 cm-’ band with another component at 2540 cm-‘, and in some spectra, the 2920 cm-’ band demonstrates three components at 2960, 2942, and 2420 cm-’ (Fig. 4a). The MgCO1-K2C03 glass spectra demonstrate only two distinct peaks, one of which occurs as a shoulder on the v3 CO:- mode and although of lower intensity correlate well with those of the REE-bearing glasses occurring at frequencies of 1745 cm-’ and a doublet with maxima at 2455 and 2560 cm-’ (Fig. 4b). The assignment of these bands is problematic, however, similar bands occur both in the infrared absorption spectra of crystalline calcite and aragonite structure carbonates (Fig. 5) and in COz- bearing forsteritic glasses (Fig. 6). In crystalline carbonates, such bands have been interpreted as overtones and combina- tions of the fundamental modes of the carbonate group and although assignments are tentative, the bands at ca 1770, 2130, 2520-2580, and 2979 cm-’ have been interpreted as 21+ or v’ + vo, V’ + v~, V, + v2, and 2v, complex modes, respectively (Ross and Goldsmith, 1964). A similar interpretation may also apply to near identical modes in CO*-bearing forsteritic silicate glasses (J. R. Holloway, unpubl. data).


3doo 25bo 20’00 Wavenumber cm-’

FIG. 4. Background subtracted infrared absorption spectra of the complex modes of (a) MgCO,-K&O? and (b) La(OH),-Ca(OH)Z- CaCOi-CaF-BaSOd glasses.

Raman Spectra

The Raman spectra from both glass systems correspond well with both the infrared activity demonstrating similar peak frequencies and with a previous Raman study of MgCO1- K2C03 glasses (Sharma and Simons, 1980). The most notable divergence from the infrared spectra are the activity of the O-H stretches in the La(OH)~-Ca(OH)2-CaC03-CaF2- BaSO, glasses, in which the broad asymmetric peak is resolved as two populations, of which only the lower frequency component with a peak maxima at 3100 cm-’ is Raman active (Fig. 7 ). It is notable that this lower frequency population corresponds well with the O-H stretches of the MgC07- K,CO? glasses and it is perhaps significant that this lower frequency O-H stretching population has a similar intensity maximum to the crystalline carbonate hydrates.

The Raman activity of the carbonate internal modes is similar to the infrared activity, although the V, band is the most intense. The presence of a less intense vj in the Raman spectra further supports the loss of degeneracy of this mode, as suggested from the infrared spectra and similarly, is vibrationally split into three components at 1300, 1437, and 1563 cm-’ in the La(OH)3-Ca(OH)2-CaCOX-CaF2-BaSO~ glass spectra, although another low intensity band at 1508 cm-’ is also observed in this region, and 1387, 1447, and 1525 cm-’ in the MgCO?-K2C07 glass spectra (Fig. 8). The vq and v2

bending modes also demonstrate vibrational splitting consistent with that derived from the infrared spectra.

A broad, low-intensity peak in the REE-bearing glass spectra is also observed at 1123 cm ’ which, as discussed above, may represent a stretching mode of the SOS group.

DISCUSSION

Carbonate Glass Structure

Carbonate melts are not expected to quench to glasses since they do not satisfy the requirements for stable glass structures. Most glasses demonstrate three-dimensional, rigid network structures, representing random arrangements of interlinked polyhedra, which only demonstrate significant order on the intramolecular scale. The carbonate ionic group is, however, theoretically unable to polymerise due to the incorporation of all bonding orbitals in intramolecularp7r- and c-bonding and, hence, cannot form network structures, sensu stricto. A random, three-dimensional framework structure could be constructed through the bridging of carbonate groups by ionically interacting metal cations; however, such a structure would not be rigid, since ionic bonding is nondirectional and would suggest that large open framework structures could not be supported. The densities and molar volumes of the carbonate

t I I I I 1 4000 3500 3000 2500 2000 1500

Wavenumber cm-’

FIG. 5. Background subtracted infrared absorption spectra of the complex modes of (a) aragonite, (b) calcite, and (c) magnesite. Band assignments after Ross and Goldsmith (1964).

932 M. J. Genge, A. P. Jones. and G. D. Price

1

I I I 3000 2000 1000

Wavenumber cm-’ FIG. 7. Raman spectra of La(OH),-Ca(OH)2-CaC04-CaF-

BaS04 glass. Note the infrared O-H stretching mode has been superimposed to highlight differences in activity.

4doo 35bo 3doo 2ioo 2doo Wavenumber cm-’

FIG. 6. Background subtracted infrared absorption spectra of the complex mode of CO,-bearing forsteritic silicate glasses (J. R. Hol- loway, unpubl. data).

glasses are, however, generally consistent with an open framework structure, with densities significantly less than those of crystalline carbonates at 2.8-3.25 g/cc and 2.298-2.09 g/cc for the REE-bearing and alkali-bearing carbonate glasses respectively. The flexible framework structure of carbonate glass could, however, be supported by species contained in the rings of the framework structure, such as K in the MgC03-K2C03 glasses, and possibly hydroxyl or water complexes in the La(OH),-Ca(OH),-CaCO1-CaF,-BaSO1 glasses; such a structural role for framework-modifying species may explain why simple unary carbonates such as CaC03, Na2C03, and MgCOl do not quench to glasses, since no such modifiers are available to stabilise a glass framework structure. The vibrational spectroscopy of these synthesised carbonate glasses constrains the structural role of both carbonate and hydroxyllwater groups.

An ideal glass with an entirely disordered structure would be expected to demonstrate a single structural population of any polyhedra occupying a low-symmetry site and giving rise to a single vibrational population of broad fundamental modes. However, the constraints of a three-dimensional framework of uniform polyhedra might be expected to impose longer range order on the glass structure giving rise to a number of discrete structural populations. In silicate glasses, for example, such populations relate to variably linked SiOT4 polyhedra and the presence of nonbonding oxygens. The vibrational spectra of carbonate glasses from both systems demonstrate at least three components of the z+ asymmetric stretching mode of the carbonate ion, which implies either three structural populations with degenerate V) modes or two populations, of which only one is degenerate. The presence of three degenerate v3 carbonate populations would appear unlikely, since all three populations would necessarily occupy high symmetry sites, such as &, demonstrating threefold rotational axes. Furthermore, this is not supported by the infrared and Raman activity of carbonate fundamentals in these

glasses, with at least a single population demonstrating infrared V, activity and all three components of the V) mode being Raman active. It is worth mentioning at this point, that although the v1 mode of the carbonate ion may demonstrate infrared activity in calcite structure carbonates without vj vibrational splitting due to anionic deformation (Goldsmith and Ross, 1966), there are no reports of complementary Raman activity of the L+ mode. Hence, each structural population of the carbonate ion would be expected to give rise to a vibrationally split v3 doublet.

The frequencies of the components of the uj mode are 1300, 1437, and 1563 cm-’ and 1387, 1447, and 1525 cm-’ in La( OH),-Ca( 0H)2-CaCO~-CaF2-BaS04 and MgC03- K2C03 glasses, respectively. It is notable that in the Raman spectra of the REE-rich carbonate glasses a doublet may be

1400 do0 do0 8bO 6bO

Wavenumber cm-’

FIG. 8. Raman spectra of the carbonate internal mode region of (a) MgCO,-K2C03 and (b) La(OH)3-Ca(OHh-CaC03-CaF2-BaS04 glasses. Spectra are not baseline corrected.


A ,’

bo : v

“a”

FIG. 9. Schematic diagram of the carbonate sites in the acid carbonates (a) kalicinite (KHCO1) and (b) nacholite (NaHCOJ (after Nakamoto et al., 1965; Novak et al., 1963).

resolved at 1437 and 1452 cm-‘, representing vibrational splitting of 15 cm-‘, which is within the range of that observed for the vj mode of crystalline carbonates, which does not exceed 25 cm-’ (Fong and Nicol, 197 1) This component lies at similar frequencies to the z+ mode of crystalline calcite- type and aragonite-type carbonates (Huang and Kerr, 1960), which may suggest a relatively symmetric carbonate site with each carbonate oxygen incorporated in surrounding metal polyhedra. This site would necessarily lack the threefold rotational axis of the crystalline site, suggesting a CZvlh or C, site symmetry. Sharma and Simons ( 1980) in their previous Raman study of MgC03-K2C03 glasses suggested that the vibrational splitting of these three populations was obscured by the inherent poor resolution of glass spectra, which may suggest that the v3 component at 1447 cm-’ in the alkali- bearing glasses represents a similar site to that of the 1437- 1452 cm-’ band of the REE-bearing glasses.

The assignment of the high and low frequency v3 components to individual carbonate populations by Sharma and Si- mons ( 1980) was made since vibrational splitting of 137 cm-’ would be required if these represent a single population, higher than observed in any anhydrous crystalline carbonate. However, large frequency differences between the stretching modes of the acid carbonates nahcolite NaHCO, and kalicinite KHC03 of up to 360 cm-’ occur with peak frequencies of 1682 and 1367 cm-’ (Novak et al., 1963; Nakamoto et al., 1965). The high-frequency stretches and the large vibrational splitting observed in acid carbonates are attributable to the arrangement of the bicarbonate ion in the crystal structure, with variable H bonding to carbonate ions and the presence of a free carbonate oxygen without bonding to a H proton. The high-frequency stretch results from the localisation of the carbonate double bond across the free C=O bond (Fig. 9). The high-frequency v3 component and the large vibrational splitting in the carbonate glasses reported here cannot, however, be attributed to the presence of bicarbonate groups, since the characteristic low-frequency O-H stretching modes associated with this group in the acid carbonates in the range 2620-2460 cm-’ are not observed.

Similar high-frequency v3 stretches of 1610 cm-’ and vibrational splitting of 235 cm-’ are observed in the vibrational spectra of CO*-bearing NaAlSi206 glasses (Fine and Stolper, 1985) and based on similarities to the activity of CO:- in scapolite, have been interpreted as a response to anionic dis-

tortion. However, the compositional variation reported for splitting in CO,-bearing silicate glasses and the absence of significant increases in the frequency of the v3 mode of calcite structure carbonates demonstrating v, infrared activity suggest that an anionic distortion mechanism for such spectral phenomena may be inappropriate. The speciation of CO:- in silicate glasses may be as metal-carbonate complexes (Fine and Stolper, 1986; Mysen and Virgo, 1980) and hence, a mechanism similar to that for acid carbonates is proposed here, where nonbridging carbonate oxygens, that is, oxygens not coordinated to strongly interacting bridging metal cations, result in localisation of bonding electrons and disparate bond strengths (Fig. 10). It is interesting to note that lower frequencies of 1515 and 1440 cm-’ for the vJ mode are observed for C02-bearing CaA12Si208 glasses (Fine and Stolper, 1986). Conceptually, if the interacting metal cation causes redistribution of bonding electrons, we might expect the stretching frequency to be related to the electronegativity of the cation, increases in frequency occurring with increases in electronegativity However, incorporation of CO:- into CaA12Si208 glasses as CaC03 complexes rather than NaCO, complexes in the aluminosilicate glasses would result in carbonate groups with two free C-O bonds, across which elec- tronic charge is to be distributed, hence, lower vibrational splitting might be expected. The high-frequency stretch and the large vibrational splitting of the v1 mode in carbonate glasses may, therefore, suggest a second structural population of CO:-, in which at least one carbonate C-O bond is free, that is, not coordinated to a strongly interacting bridging metal cation. This nonbridging site would obviously be highly asymmetric and demonstrate very low site symmetries, such as C,.

In our model of carbonate glass structure the terms bridging and nonbridging refer to the general structural role of the species to which they are applied and have been used to empha- size similarities with network glass structures. Bridging species are, therefore, those which link the framework structure together, achieving a similar role to oxygens shared between SiO, tetrahedra in silicate melts, whereas nonbridging species are those not directly involved in the framework structure and exist due to the charge-balancing requirements of framework modifying species (Fig. 11). For carbonate glasses elements described as framework formers are C and 0 forming the

I 0

FIG. 10. Schematic diagram of the nonbridging carbonate ion site. Note, one carbonate oxygen is not coordinated to a bridging cation resulting in localisation of double bond. Ionic bonds shown as dashed lines.

934 M. J. Genge, A. P. Jones, and G. D. Price

a.

h

nb bun _O

so; ‘00 _ b _ .o

Q

FIG. I 1. Schematic diagram showing the similarities between silicate network and carbonate framework structures. Species are labelled in terms of structural role: (nb) nonbridging and in the carbonate structure (b) denotes bridging cations. Network modifiers are represented by solid circles

carbonate groups and those elements which interact strongly with the carbonate group such as Ca2+, Mg ‘+, and Sr *+ , likewise, framework modifiers are those species which have only weak interactions with CO:-, such as the alkalies, and molecular groups, such as metal hydrate/hydroxyl complexes.

The other internal modes of the carbonate ion in these glasses generally support the conclusions derived from the better resolved vj mode, in that the v4 and v2 modes occur at similar frequencies to anhydrous calcite or aragonite structure carbonates and the v4 mode demonstrates vibrational splitting relating to the loss of the threefold rotational axis. Addition- ally, both the vi and v2 modes of the MgCO?-K2C03 glasses demonstrate doublets supporting the presence of two discrete structural populations, although such activity may be obscured in the La(OH)3-Ca(OH)2-CaCOi-CaF2-BaS04 glasses due to the presence of SO;’ internal modes in this region.

The bands in the intermediate region of the mid-IR were interpreted above as complex modes of CO:- fundamentals. It is notable, however, that they demonstrate a closer similarity to the overtone and combination bands observed in the infrared spectra of COT-bearing forsteritic glasses than crystalline carbonates. The speciation of CO:- ions as highly asymmetric metal-carbonate complexes in these CO,-bearing silicate glasses may, therefore, suggest that these complex modes relate exclusively to the nonbridging, rather than the more symmetric bridging structural population.

The stretching modes of proton groups are inherently complex, due to the variety of possible donor-acceptor interactions with electropositive ions. The interpretation of these modes in glasses by comparison with crystalline materials is further complicated by the poor structural definition of many crystalline hydroxyls and hydrates. The stretching modes of water and hydroxyl groups in these glasses are represented by intense asymmetrical composite peaks in both infrared and Ra- man spectra. In the infrared spectra of the REE-bearing glasses, the composite peak relates to at least two components, of which only the lower wavenumber component is Raman

active. In the MgC03-K2C01 glasses, the composite peak consists only of this Raman-active component.

The stretching frequencies of both free OH- and Hz0 molecules occur in the range 3700-3500 cm-’ (Sverdov et al., 1970) ; however, both these groups demonstrate infrared and Raman activity. Similar frequency ranges are demonstrated in compounds in which interactions between electropositive ions and hydroxyl or water groups are restricted to electrostatic interactions with the oxygen atom of the proton group, such as the alkaline earth hydroxyls and certain hydrates, such as Mg(OH)* and Ca0.H20. However, stretching modes in most of these compounds are also infrared and Raman active. The high wavenumber O-H stretch in the REE-bearing glasses must, therefore, represent some metal hydroxyl or hydrate complex in the glass in which interaction is mainly electrostatic and in which the proton group site is of an appropriate symmetry, such that spectral activity is restricted to the infrared region. Such a speciation, which demonstrates a specific symmetry requirement would necessarily be isolated, that is, not directly involved in the glass framework, possibly restricted to interstitial areas in framework rings and hence, may fill a framework supporting role as discussed earlier. It is significant that this high frequency component is absent from the MgCO1-K,CO? glass spectra as the absence of metal-hydroxyl or -hydrate complexes might be expected, due to the lower accidental water content.

The lower frequency component of the O-H stretching peak has an intensity maximum of 3100 cm-’ and is both infrared and Raman active. Similar O-H stretching modes are observed in the Mg carbonate basic hydrates (White, 1971) and Na carbonate hydrates (Buijs and Schutte, 196 1). In the crystalline Iiydrates, these stretching modes are attributed to both OH and H20 groups, which variably interact with each other or CO:- groups via H bonding. The absence of distinct peaks in the carbonate glass spectra implies less constrained sites for such groups than in the crystalline carbonates, in which chain-like structures have been suggested; it does, however, seem likely that there may be at least limited interaction be-


tween CO: groups and OH/H*O, although as discussed above, it is unlikely that bicarbonate groups occur. The low- frequency O-H stretching population may, therefore, represent hydroxyl groups which interact with the glass forming framework and adopt a limited structure forming role.

Carbonate Melt Structure

has been suggested under the oxidising conditions of the an- ode (Lu and Selman, 1989). Reactions of this form may be expected to have a complex effect on the level of complexation in the carbonate melt. Utilising the constraints provided by the above reactions and solubility data derived from previous experimental studies, it is possible to relate general variations in solubility to the level of metal-carbonate complexation.

Glass structures are closely related to the melt structures The solubilities of elements which preferentially form from which they are quenched, although certain fundamental metal-carbonate complexes might be expected to increase differences undoubtedly arise during the glass transition. Dif- with increasing complexation of the carbonatite melt. Exper- ferences between glass and melt structure occur, since the imental solubility data on NiO in alkali carbonate melts, for glass transition is a kinetic phenomenon occurring over a example, suggests its preferential incorporation as carbonate range of temperatures and is hence, unlikely to preserve an complexes in the melt and demonstrates increasing NiO sol- isothermal sample of melt structure; elements of glass struc- ubility with increasing partial pressure of CO? (Orfield and ture derived during quenching as a function of the activation Shores, 1988, 1989). However, reductions in NiO solubility energies of liquid rearrangement processes. It is also likely with small alkaline earth contents also implies that competi- that carbonate glass structures are further separated from their tion between metal cations for carbonate groups will also be melt structures than polymerisable materials in which semi- an important factor in solubility of such elements (Doyon et rigid networks occur. al., 1987).

The presence of two structural populations of CO:- in these carbonate glasses implies a level of medium range order in the melt phase, which would not be possible if carbonate melt structure consisted merely of separate carbonate and metal ions, as suggested by simple ionic salt models. The presence of two structural sites, therefore, implies the existence of extended structural units in carbonate melts, which since the CO:- group is unable to polymerise, must relate to ionic metal-carbonate complexes.

The level of metal-carbonate complexation in the carbonate melt would be controlled by genera1 reactions such as

MC07 = CO:- + M”, (I)

and clearly would be dependent on the activity of the carbonate ion as controlled by the dissociation of CO:- and hence, a function of both fOz and the partial pressure of CO2 (reaction 2).

The solubility of species in competition with CO:- for metal cations, for example P, which in silicate melts is sta- bilised by divalent cations (Ryerson and Hess, 1980). are likely to be reduced by increasing metal-carbonate complexation. Experimental studies on P solubility in CaC07 melts (Baker and Wyllie, 1992) demonstrate that solubility is reduced with increasing partial pressure of COz and decreasing temperature consistent with increased metal-carbonate complexation. In carbonatite magmas with low contents of divalent cations, such as natrocarbonatite melts, metal-carbonate complexation could effectively remove all available divalent cations and it is therefore significant that the natrocarbonatites of Oldoinyo Lengai, Tanzania have particularly low P contents and are apatite-free (Dawson, 1989).

co:- ~coz+O’-. (2)

The dissociation of the carbonate ion in the melt would be expected to control variations in structure with temperature and pressure. Dissociation of the carbonate ion increases with increasing temperature and decreasing pressure and leads to reductions in the activity of CO:- and hence, lower levels of metal-carbonate complexation would be expected.

The speciation of water in the La(OH),-Ca(OH)2- CaCO,-CaF,-BaSO, glasses is suggested above to be as both metal hydroxyl and hydrate complexes and isolated molecules which interact with carbonate ions. It would seem likely, therefore, that a certain level of association between metals and OH/H20 occurs in the melt phase as soluble metal hydroxyl or hydrate complexes. Likewise, the absence of bicarbonate groups from these glasses precludes such speciation in the melt, although this is likely to be a function of oxygen fugacity. Interaction between dissolved water molecules and carbonate ions have been suggested previously from experimental studies of molten carbonate fuel cells in which the genera1 coupled reaction

The effects of metal-carbonate complexation on P solubility as described above provides a useful example of how carbonatite melt structure could be used to constrain the geochemical behaviour of carbonatite magmas under geologically relevant conditions. Baker and Wyllie ( 1992) have suggested that small apatite contents in the source regions of mantle- derived carbonatite magmas can have appreciable effects on the REE contents of near solidus melts. Therefore, if P solubility is reduced in alkali-rich carbonatite melts by effective removal of free metal cations by carbonate complexation, then we would expect the REE content of near solidus mantle derived alkali-rich carbonatites to be significantly lower than alkali-poor varieties. Such considerations would be expected to have a major effect on the efficiency of such carbonatite magmas for enrichment of the overlying mantle in LREEs.

Vapour Phase Evolution

CO:~~ + HZ0 = CO, + 20H- (3)

The absence of the molecular CO* v1 stretching mode from the infrared spectra of both glasses as observed in COz-bearing silicate melts in the region 2460-2340 cm-’ suggests low concentrations of CO, in the melt phase. Comparison with silicate glasses where the CO, vi mode is identifiable at concentrations greater than 1 Oppm (Fine and Stolper, 1986) suggests that for the carbonate systems studied here, CO1, solubility is very low; CO, produced by dissociation of carbonate

Y36 M. J. Genge, A. P. Jones, and G. D. Price

being partitioned into the vapour phase. If such low solubilities for CO? are typical of carbonatite magmas, then the evolution of the CO? component of the vapour phase will be a function of the production of CO, by dissociation rather than solubility. The lower values of the dissociation constant of carbonate at higher pressures, therefore, would be expected to prohibit the development of a CO,-dominated vapour phase in carbonatitic systems, implying that vapours coexisting with an H,O-bearing carbonatite magma at high pressure will be HzO-dominated. Such a suggestion is consistent with observations of the form of fenitisation associated with carbonatite complexes in which carbonate-rich potassic fenitisation has been suggested to occur only at shallow crustal levels (Le Bas, 1977).

CONCLUSION

Vibrational spectroscopy provides evidence for the presence of molecular metal-carbonate groups as the main structural forming units of carbonate glasses and suggests that glass structure is constructed from flexible metal-carbonate frameworks supported by framework modifying species re- siding in interstitial rings. The occurrence of such large molecular metal-carbonate groups in the glass implies their presence in the melt phase, which would be expected to be a major control of both the physical and chemical behaviour of the melt and hence, be critical to both the petrogenesis of carbonatites and the efhciency of carbonate melts as agents of mantle metasomatism.

Acknowlrdgments-We would like to thank Professor P. Wyllie for the use of his experimental equipment in the synthesis of samples funded by NSF grant EAR-90-17197 and Dr. E. Stolper for useful discussions on glass spectra. Also, we wish to acknowledge the help and advise offered by Drs. N. Ross and J. Milledge at UCL in the infrared and Raman analysis of samples and the constructive critisms provided by the referees. This work is paper ?&JO58 of the research school of geological and geophysical sciences.

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an infrared and raman study of carbonate glasses: implications for

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