THE AMERICAN MINERAIOGIST, VOL. 56, MAY JT]NE, 1971

INTERVALENCE ELECTRON TRANSFER EFFECTS IN THE

SPECTRA OF THE MELANITE GARNETS

RavuoNo K. Moonor aNn Wrr-rrau B. Wurrn, Mater'ial,s Research

Laboratory and, Department of Geochemistry and' Mineral,ogy, The Penk-

sylaanio State Uni.versity, (Jni'aersity Park, Pennsylttonia 16102

ABSTRACT

New data are presented on the opticai spectrum, the infrared spectrum, and the elec-

trical resistivity of the black high-titanium andradite garnets. An evaluation of all available

evidence leads to the conclusion that Tia+ substitutes on the octahedral site with concurrent

substitution of Fe3+ and AI3+ on the tetrahedral site. The titanium on the octahedral site is

mixed Ti3+ and Tia+ Ieading to several charge transfer processes which give these garnets

their characteristic color and optical spectrum.

INrnonucrroN

The black titanium-rich andradites of the melanite and schorlomite

varieties have been the subject of much recent attention. The problem is

the role of titanium in the crystal chemistry of these garnets. Among

the silicate garnets, most cationic substitution takes place on the cubal or

octahedral sites. The tetrahedral site is mainly occupied by silicon.

Discrepancies in careful chemical analyses can best be interpreted byassuming the substitution of some other ion, usually Ji++, qnto the

tetrahedral site. There are really two problems: The valence state of the

titanium and the sites onto which it substitutes in the structure. The

black or brown colors of these garnets also raise questions about the

electronic behavior of the titanium. Deep brown and black colors are

not easily explained by theousual crystal field spectra of transition metal

ions.The various substitutional possibilities are outlined below. Aluminum

is not specifically listed although it also occurs in most titanium garnets.

3+ 3+Cas(Fer- ,T i " ' ' ) S iaOrz

T

, L 1 r ? ! 4 +

Ca3(Fe, - ' Fer ' . uT i ;yT iu ' )S i3O1,

CasFez(Sia-,Ti,n+) o*

I-A3+ 3+ 4+

Cae(Fez- "T i " - ' ) (S i i -?T i r ' ) O12

II-A

ca, (r'el1,ri,'*.; lsir-.F"1*; o *II-B

1 Present address: Department of Natural Sciences, Radford Co1lege, Radford, Virginia.

II

826

SPECTRA OF GARNETS 827

Model f assumes substitution of pure Ti3+ on the octahedral sites andmodel I-A mixes the valence states by adjusting the valence state ofiron. Chemical analyses which show a clear deficiency in silica and theinfrared data both rule heavily against either models f or I-A. Model IIand model II-A are the most frequently discussed in the literature. IfereTia+ substitutes on the tetrahedral site with or without concurrent sub-stitution of some Ti3+ on the octahedral site. Model II-B is a less dis-cussed alternative which proposes the displacement of Fe3+ (or Al3+) tothe tetrahedral site and the substitution of Tia+ on the octahedral site.Still more complicated models could be constructed but these will sufficefor discussion. There are good chemical analyses which show quiteclearly that the SiOz content falls short of the required 6 Si per 24oxygens (Deer, Howie, and Zussman, 1962, and Howie and Woolley,1968). Microprobe traces across zoned crystals show the variation ofiron, titanium, and silicon with each other (Isaacs, 1968; Howie andWoolley, 1968; Gomes, 1969). Phase equilibria (Huckenholz, 1969) andchemical synthesis (Ito and Frondel, 1967) place limits on the extent oftitanium substitution.

Other lines of experimentation give direct information on the sitedistribution and valence state of the ions. Included here are the infrareddata of Tarte (1959, 1960,1965), and Howie and Woolley (1968). Elec-tronic spectra (Manning,1967, and Manning and Harris, 1970) shouldyield information on the valence state of the titanium but prove difficultto interpret. Mdssbauer measurements give further information on thesite distribution of the iron (Dowty and Mark, 1968).

Our purpose in the present paper is to present some new information onthe optical spectra and the electrical properties of the high-titaniumgarnets and to attempt an evaluation and consolidation of the previouswork. fn addition we make an assessment of the role of delocalized orpartially delocalized electrons in explaining the color and spectra of theseminerals.

ExpnnrunNrll

The sources for the three garnets examined are given in Table 1. The specimens were

checked for phase purity by X-ray and precise difiraction data were measured using an

external silicon standard. The lattice parameters were calculated using a least squares

method and a Nelson-Riley function. The o values are also tabulated in Table 1. Elemental

analyses were determined by emission spectrographic methods. (Table 2) Spectra on all

specimens were obtained from finely ground powders by a diffuse reflectance technique

using the Beckmann Dk-2A spectrophotometer. An MgO-coated integrating sphere was

used with MgO as a reference material. Spectra. of polished plate of single crystal by trans-

mission were obtained on the Cary Model 14 spectrophotometer. The temperature de-pendence of the spectra was determined using a special dewar ceII on the Cary at temper-

atures down to 70oK. Measurements at temDeratures above room temperature were made

828 RAYMOND K. MOORE AND WILLIAM B. WHITE

'Ieulr 1. Onrcru, TrraNrult CoNrnnr, .qNo Cnr,r, Eocr orHrcrr-Trr.tNrurr Germrrs

Sample No. OriginWeight percent

as TiO:r, (A)

An-Ti 15.20

An-Ti 15.45

An(Ti)-84

Ivaarite,FinlandMagnet Cove,ArkansasSan Beni to Co.,California

1 5 . 2 0

15 .45

6 .60

t2 .298

12. t0 l

by heating the specimen with a soldering gun-thermocouple attachment. AII high-temper-ature measurements were made in triplicate to insure reproducibility.

Infrared spectra were measured on powdered samples embedded in KBr using a Perkin-Elmer Model 621 spectrophotometer. Some measurements were made to very low fre-quencies using powder smears on polyethylene plates on a Beckmann IR-l1 spectro-photometer.

The electrical resistivity of a single crystal was measured using a two-point probe

arrangement and a standard conductivity bridge.

TrrB Oprrcar SPECTRUM

Diffuse reflectance spectra are shown in Figure 1 and the single crystalspectrum of the melanite is shown in Figure 2.

Four features appear in these spectra: (i) An intense broad band

Ttr:rr. 2. Cnnnrca.r, ANAr,vsrs or SpncrupN ANTr-84

Component Weight percent

Sio:Al2o3FerosCrzOaFeOMeoNInOCaOTio,NaroKzO

3 s . 52 . 7

19.08< 0 . 0 5

2 . 2 21 . 0 00.09

34. 56 . 6 00 . 1 10 . 0 0

Total 1 0 1 . 8

IreO by tritrametric analysisAil other data by emission spectrography

SPECTRA OF GARNETS 829

S oz< oCDE ^o -qo 0

o

o

o

Frc. 1. Diffuse reflectance spectra of titanium-andradite garnets.

occurs in the near infrared at 5280 cm-l. It appears as a broad doublet in

the reflectance spectra but only as a single peak in the single crystal

transmission spectrum. (ii) All features are superimposed on a very rapidrise in absorption intensity in the blue end of the visible, presumably dueto some very intense absorption feature below the observable range. It

is this background absorption that gives the garnets their yellow-black

s o o r o o o 2 0 0 0

UJoz

dtax.o(t,lll

o. 7

t .

o . 9

o .

o 4

o . 3

o 2

o . l

5 o o t o o o t 5 0 0 2 0 0 0 2 5 0 0

W A V E L E N G T H ( n o n o m e l o r s )

t 5 0 0

W A V E L E N G T H ( n o n o m e l e r s )

Frc. 2. Transmission spectrum of rnelanite garnet single crystal. Crystal

thickness:0.24 mm.

l y ' A n ( T i ) - 8 4 *

A n - T i l 5 2 O

E

It l

- i

o@

I

! 2 4 t l

A n ( T i ) - 8 4 *

2 5 0 0

830 RAYMOND K. MOORE AND WILLIAM B. WHITE

colors. (iii) There is a sharp band in the single crystal spectrum super-imposed on the intense absorption curve at 22,OOO cm-l. (iv) There is abroad shoulder at 19,000-20,000 cm-r. The minimnm absorbance occursin the near infrared. at 1.2 to 1.4 microns. The positions of these bandsagree well with those reported by Manning (1967, 1969).

The intensities of all absorption features are large and obliterate themuch weaker spectra of Fe3+ and Fe2+. No trace appears of the strongestband of Fe2+ in garnet at 7200 cm-l. although this band falls into themost transparent portion of the melanite spectrum. There is also a roughproportionality between the intensity of the absorption features and thetitanium concentration. Formal Beer's law plots, however, cannot bedrawn because of the difficulty in measuring absolute absorbance fromdiffuse reflectance spectra and because of the uncertainty in valencestate and site distributions.

The temperature dependence of the near infrared band is shown inFigure 3. At low temperatures, the band is clearly resolved into twobands with an unequal temperature response. The behavior of thesebands is peculiar if the bands are to be assigned to crystal field transi-tions. The frequency increases with increasing temperature (Fig. 4) andthe intensity decreases (Fig.5). This is opposite to expectations for

\ ' " \ .' \ . . ' . \

ri.',, t\

'i

\ sz?"K

) 4 3 4 o K! 3 7 5 0 K

1 2 0 0 t 4 0 0 t 6 0 0 t 8 0 0 2 0 0 0 2 z o o 2 4 0 0 2 6 0 0W A V E L E N G T H ( n o n o m e t e r s )

Frc. 3. Effect of temperature on the near-infrared band,

o .7

lrloz 0.6

ctEo o .5o@

o.4UJ

r- o.sJUJt o.2

SPECTRA OF GARNETS

Frc. 4. Variation of frequency of near-infrared band with temperature

831

I

Eo

ozlrjlotJEL

crystal field bands. Crystal field bandshigher frequencies at Iow temperatureincreasing the crystal field. Intensitiesdecrease as temperature is lowered. To

would be expected to shift tobecause of lattice contractionof crystal field bands usuallythe contrary, it must be noted

Eo

(t

' 200 300 400

, T E M P E R A T U R E ( O K , )

Frc. 5. Variation of intensity of near-infrared band with temperature.

5 t o o

TEMPERATURE ( 'K )

832 RAYMOND K. MOORE AND WILLIAM B. WEITE

that the half-width of these bands and their intensity are about normalfor crystal field bands.

The strong absorption edge in the blue is most likely due to the wingof an intense liganrl--+metal charge transfer absorption in the ultra-violet. This edge is not present when titanium is absent. It is likewise notpresent in compounds where the titanium is completely oxidized. Themost probable assignment is an O2--+Ti3+ charge transfer with 02----+(Fea+)w not excluded.

The broad shoulder near 19,000 cm-l was assigned by Manning andHarris (1969) to the 2T2n--+2Eo oI Tit+ on the octahedral sites. The sharpband at 22,000 cm-l was said to arise from the6As---+4,!rn aEo transitionof ferric iron. Since all other characteristic Fe3+ bands observed ingrossularite and andradite are swamped out, some mechanism has tobe invoked for the enhanced intensity of this particular transition.

Tns INlnenED SPECTRUM

Much of the argument about the site distribution of titanium inmelanites and schorlamites has involved the interpretation of the infra-red spectra. Tarte (1959, 1960) first published the spectra of a largenumber of silicate garnets and showed that their spectra were all verysimilar except for the titanium-rich garnets. The titanium garnets ex-hibited bands in the 650 cm-r region that were not present when tita-nium was absent. Tarte interpreted these bands to be the vibrations ofTiOa tetrahedra. Howie and Woolley (1968) presented fR spectra ofa suite of very carefullv characterized garnets and came to essentially thesame conclusion.

The regions of the spectra of interest for samples An-93, AnTi-S4, andAn-Ti15.45 are presented in Figure 6. Bands appear at 640 cm-l and670 cm-l for An-Til5.45 and at 670 and 738 cm-l for AnTi-S4, which arenot present in most garnets. In addition, spectra of two aluminum-richgrossularites have an extra band at approximately 750 cm-l and auvarovite has two extra bands at 675 and 748 cm-l (Moore, White, andLong 1971). The second feature of note is the broadening of the high-frequencv tetrahedral bands. The spectrum of sample An-Ti15.20 isnearly identical to that of An-Ti15.45. The bands labeled A, B, C and Din Figure 6 were established as due to stretching vibrations of the SiOatetrahedra. Likewise the low-frequency bands labeled F-L are present inall ttnormal" garnet structures.

Hurrell et al., (1968) and Moore, White, and Long (1971) have made atheoretical analysis of the garnet structure and have shown that 17modes are permitted in the infrared. Moore, White, and Long (1971)found all of these can be accounted for in the usual spectra of pyralspites

SPECTRA OF GARNETS

BI

n

t7 3 5

FI{/i

!I

JI A n - 9 3

A nT i - 84 *

6 7 0

A n - T i l 5 4 5

'?"*ror'r", i .r , l l , l , , 4oo 3oo 2oo

Fro. 6. Infrared spectra of high-titanium garnet in comparison with andradite. Bandsmarked by letters are the "normal" IR bands of the garnet structure. Bands with labeledfrequencies are the "extra" bands.

and ugrandites. The extra bands that appear in Figure 6 are in violationof the garnet space group. The frequency range of these bands leaveslittle doubt that they arise mainly from the motions of the tetrahedralcations. The evidence that the bands are from TiOa tetrahedra seemsmuch more equivocal.

The frequencies of the extra bands and the SiOa band of A, B, C and Dare plotted as a function of the lattice parameter in Figure 7 (solidcircles). The frequencies of the tetrahedral high-frequeocy bands ofyttrium aluminum garnet and yttrium iron garnet (McDevitt, 1969)are also plotted on this figure as open circles. Added for further informa-tion are the extra bands from high aluminum grossularite (Gr-92) and auvarovite (Uv-44).

The extra bands for the high-aluminum garnets Gr-92, Gr-928, Ilv-44and AnTi-84 do occur'in the region of the AlOr bands of YAG, while theextra bands of An-Ti15.45 (high-iron and high-titanium garnet) occurin the region of the FeO+ YIG bands. No analysis of this latter samplewas made for Fe2O3, but a sample from Magnet Cove, Arizona (thelocation of An-Ti15.45) was analyzed. by Erickson and Blade in 1963 asreported by Huckenholtz (1968). An FezOa content of 18.32 wt percentand a TiOz content of 15.51 wt percent was fonnd as compared to thatreported for sample An-Ti15.45 of 15.45 wt percent TiO2. It is probable

833

Uoz

FF

=az

E.

834 RAYMOND K, MOORE AND WILLIAM B. WIIITL.

I

Ozu

ouEL

9 0

8 0

? o

"Jirr, l ' r 'oro*l; :rrf i""., , l ' , ' " '2 20 '2 30

Frc. 7. Frequency versus lattice parameter plot for the extra infrared bands.

that the Fe2O3 content of An-Ti15.45 is very close to the value reportedby Erickson and Blade for their sample (18.32 wt percent). The Gr-92and Gr-92B samples only exhibit one extra band and their SiOa bandsA, B, C, and D are in good linear relationship with the remainder of thenatural sil icate garnets (Fig. 7). The Uv-44, AnTi-S4, and An-Ti15.45samples have two extra bands and their SiOa bands deviate considerablyfrom the linear relation for the same bands established by the othergarnets.

The evidence presented implies stronely that both Fe3+ and Al3+substitute on the tetrahedral site and give rise to the extra bands in thespectrum. The comparison is directly with known AlOa and FeO+ tetra-hedra in the garnet structure and should be valid. Certainly this in-terpretation is no more arbitrary than was Tarte's assignment of thesebands to TiOr tetrahedra.

It would be useful to compare the extra bands with spectra of com-pounds containing isolated TiO+ tetrahedra. Such compounds are rare.Ilowever, numerous spectra of germanium garnets have been reported,mainly by Tarte (1965). The principal stretching region of the GeOatetrahedra is 680-780 cm-I with the strongest bands at 720 730 cm-l.Since Gea+ has the same charge and is heavier than Tia+, one would ex-pect the principal stretches of TiOa tetrahedra to occur near the fre-quencies of GeOa or slightly higher. It therefore seems less reasonable toassign the bands at 640-680 cm-l to TiO+ tetrahedra than it does toassign them to AIO+ or FeOn tetrahedra which because of one less ioniccharge have somewhat lower stretching frequencies.

SPb)CTRA OF GA]WDTS

Some comment must be made about the fact that the extra bandsoccur at all. Theoretically, the substitution of any heavy ion on the sili-con site should simply cause the frequencies to shift to lower values.Experimentally, however, some cases are found in which the spectrumof the substituting group appears superimposed on the unperturbedspectrum of the host material. Some specific examples where bands occurin addition to those permitted by the space group are the spinel seriesNizGeO+-NizSiOa (Tarte and Ringwood, 1962), the mimetite-pyro-morphite series (Adler, 1968), and in the substitution of silicon into thetetrahedral site of yttrium iron garnet (Wickersheim, 1960).

The origin of extra bands in the spectra of simple systems with therock-salt, fluorite, and sphalerite structure is currently a subject of muchdebate in the physics literature. Several models all based on latticedynamics, have been proposed. [For three points of view see the papers

by Verleur and Barker (1967), Lucovsky and Burstein (1968), and Changand Mitra (1963).] More complicated structures such as the spinels andgarnets have the additional confusion that ordering of cations can alsoproduce extra bands in the infrared spectrum. White and DeAngelis(1967), for example, could explain the multimode behavior of the

NirSiO4-NirGeOn spinels by invoking a 1:1 ordering on the tetrahedralsites-although no crystallographic evidence has been forthcoming toprove or reject this explanation.

Tse Er,ncrnrcar- Pnoprnrrns

If electron transfer processes are responsible for the dark colors in themelanite and schorlomite garnets, there should be corresponding changesin the electrical properties. The electrical conductivity of a low-titaniumandradite (Titanium:0.01 wt percent) and of the intermediate titanium-rich andradite, AnTi-84 were measured using a two-point probe andbridge. The room-temperature resistivit ies were 6.7X1011 and 1.3X108ohm-cm respectively. Unfortunately, the crystals of the more titanium-rich garnets were too small for measurement. These data do show thatthe andradites are insulators and become semiconducting as the titaniumconcentration increases. The resistivity of AnTi-S4 was measured as afunction of temperature and the data are plotted in Figure 8. From theslope of the line in Figure 8, the activation energy for charge transferacross the energy barrier can be calculated. The value is 0.47 electronvolts or 3800 cm-l. This corresponds rather well to the onset of ab-sorbance in the near infrared band.

DrscussroN

The Role oJ Titanium in the Garnet Structure. The various lines of evidencecited can now be pulled together to explain the crystal chemistry of the

835

836 RAYMOND K. MOORE AND WILLIAM B, WHITE

0 . 2 0 0 0 . 2 5 0 0 . 3 0 0 0 . 4 0 0

l , , ^ r' /T (x lo - ' )

Frc. 8. Variation in the electrical conductivity of AnTi-84 with temperature.

The slope gives the activation energy for conduction.

o.o

- t . o

? ' 2 .cor,

qo

C- 3 .

- 4 . ,"6)=(9(-Jw h e r e ' E g : O . 4 7 e v .

SPECTRA OF GARNETS 837

titanium garnets. The evidence that some ion other than sil icon sub-

stitutes on the tetrahedral site seems overwhelming. The chemical

analyses and infrared spectra both require this substitution so all models

of Type I can be ruled out. The real argument is between models II and

II-A and model II-B. We shall argue the model II-B is the closer approxi-

mation.The only really substantial evidence for 4-coordinated Tia+ has been

Tarte's infrared spectra. It was shown in a previous section that the

extra bands could be equally well explained by 4-coordinated Al3+ and

Fe3+ and indeed the variation in these bands from one specimen to

another suggests that the proportions of tetrahedral substitutions varies.

AIso ruling against tetrahedral Tia+ are the crystal chemical argumentsrecently summarized by Hartmann (1969). Tetrahedral t itanium is a

crystal chemical rarity whereas tetrahedral aluminum and ferric iron

are common. Ito and Frondel (1967) found that they could drive Tia+

onto the tetrahedral sites in synthetic garnets in which the octahedralsite was blocked withZra+. They were not successful in their attempt to

synthesize CaeTiz(TiFe2)Ou. If Tia+ does not enter the larger FeOa

tetrahedra, its substitution into the smaller SiOa tetrahedra seems un-

likely.The smaller ionic size of Tia+ compared with Fe3+ has been cited as

evidence that Tia+ should be preferred on the tetrahedral site. It would

have been interesting to compare ionic sizes on the new Shannon-Prewitt(1969) ionic radius scale but unfortunately (Ti4+)rv is so rare that no

radius for it is listed.Evidence that Fe3+ occurs on the tetrahedral site is provided by the

Mcissbauer spectra of Dowty and Mark (1968) who find one type of Fe3+

in andradite but two kinds of Fe3+ in titanian andradites which they

assign to (Fer+;rv.The electronic spectra of t itanian andradites (Manning, 1967, Man-

ning and llarris, 1970) and work described above are evidence that some

titanium is on 6-fold sites and that of this, some is present as Ti3+. Be-

cause of the possible charge compensation mechanisms among Fe3+,ps2+, Ji3+, and Tia+ with resulting enhancement of band intensities by

interactions of various kinds, it is idifficult ito lnr" iband iintensities to

measure the concentration of Ti3+.In summary the wide variety of experiments that have been performed

all seem to indicate that the silica deficiency of the high-titanium garnets

is compensated by the substitution of Al3+ and Fe3+ driven onto the

tetrahedral sites by the concurrent substitution of Tia+ on the octahedralsites. The bulk of the titanium seems to go onto the octahedral sites with

some unknown portion of it as Tis+. Of the various measurements, de-tailed examination of the extra bands in the infrared spectrum would

838 RAYMOND K. MOORD AND WILLIAM B. WHITD

seem to give the most direct information on tetrahedral site substitutionsof specific suites of garnets. '

The Electronic Spectrum oJ Titaniwm and Charge TransJer Efects. Thecrystal chemical evidence suggests a mixture of Tia+, fir+, pe3+, and Fe2+all on the svmmetry-equivalent octahedral sites. In this situation thenumber of possibilities for charge transfer absorption is large and com-plex.

The yellow-to-black color of titanian andradite arises from an absorp-tion in the ultraviolet and no one has yet measured the band itself. Thebroad wing extending into the visible is characteristic of ligand--+metalcharge transfer. Since this absorption wing is at much higher frequencieswhen only iron is present, we can assume that titanium plays some role.

The sharp band at 22,700 cm-1 and the broad shoulder at 19,000 cm-lhave been assigned by Manning and Harris (1970) to the 6Aro-nAro,a-En transition of (Fea+)vt and the 'Tzn-'En transition of (T'i3+)vr re-spectively. We have nothing further to add to these assignments exceptto comment that the frequency is in good agreement with the high-intensity (Fer+;w band in andradite.

The strong double band near 5000 cm-l is difficult to assign. The bandshape and intensity favor a crystal field band and if so it is likely due toFe2+ on the tetrahedral sites, as suggested by Manning and Harris (1970).Further evidence for this point of view is the new Mrissbauer results ofDowty and Appleman (1970).

An alternative explanation is to assign the band to an intervalenceelectron transfer transition. Ti3+-+Ti4+ poses difficulties because of thelarge (-Jfi) separation of octahedral catlons. Another alternative wouldbe (Fez+)vtrr--+(Tir+;vr and many other combinations could be written.Charge transfer involvinq the tetrahedral cations seems less likely be-cause of the large differences in character between the two sites (Robinand Day, 1967). The intensity of the band would be expected to decreaseat low temperature if it were a crystal field band. An increase in intensitywould be expected, however, from an intervalence charge transfer bandbecause of the increased orbital overlap at shorter interatomic distances.The frequency of the band is in accord with frequencies observed formany titanium compounds, some of which are l isted in Table 3. Thefrequency of the Ti3+--+Ti4+ interaction is very variable and indeed inthe titanium oxides the band varies from 5,500 cm-1 in reduced rutilethrough intermediate values for the Magneli phase, TinOzn-1, series ofcompounds to 22,000 cm-L for pure Ti2O3 (Porter, 1965). The factorsthat control the frequency of this band are not yet known. The best evi-dence of the charge transfer origin of this band is the close agreementbetween the onset of absorption and the activation energy for conduc-tion. Several recent papers (Manning, 1969a, Manning, I969b, Faye and

SPECTRA OF GARNETS

'l',c.nre 3. OesnnvetroNs ol Noen-Iwrnanno Oprrclr- AlsonprroN

rN Tt3+-CoNrlrxrNc Oxrlns

839

Cornpound Near-ir band (cm-l) Reference

Reduced rutileTiO:-,Magneii phasesTi"On-r

SrLiOr-,

SrSml-"Ii,O3

6700

12 ,000-15 ,000depending on rz

4500

6500-12 ,000depending on *

Johnson el ol.,(1e68)I'orter (1965)

McCarLhy et al.,(1e69b)

McCarthy et oL,(1969a)

Harris, 1969) have assigned bands in the neighborhood of 20,000-22,000 cm-l in various micas, in andalusite, and in tourmaline to aTi3+--+Ti4+ electron transfer process.

It would thus appear that the optical and electrical properties of thetitanian garnets are dominated by charge transfer effects. These arisebecause of the mixture of valence states of ions that occupy the octa-hedral and perhaps cubal sites. The possibility of tetrahedral Fe2+cannot be ruled out on the basis of the electronic spectra.

AcKNowLEDGMENT

The work was supported in part by the National Science Foundation under Grant GA1432.We thank Professor L. E. Cross for the loan of the resistivitv bridse.

RnlrneNcrs

Aornn, H. H. (1968) Infrared spectra of phosphate minerals: splitting and frequency

shilts associated with substitution of POa3 for AsOa3 in mimetite (Pbs(AsOrhCl).

Amer. M iner atr. 53. 17 4A-fi M.Crlwc, I Ir , aNo S. S. Mrrre (1968) Applications of a modified random-eLement-isodis-

placement model to long-wavelength optic phonons of mlxed crystals. Phys. Reu. 172,

92+933.Denn, W. A., R. A. Ilowm, a.No J. Zussuex (1962) Roch F-ormi,ng Minerutrs, Vol. 1, John

Wiley, New York, p. 90 91.Dorvrv, E., eNr R. M,q.m (1968) Application of M6ssbauer and in{rared spectroscopy to

the crystal chemistry of natural titanian garnet. Geol. Soc. Amer. Spec. Pap. 121' 80.

Dorvrv, E., .lNo I). E. Arprrnre.r (1970) X-ray refinement and M6ssbauer study of Ti-Zr

garnets : tetrahedral Fe++? Geol. S oc Amer. Ab slr. Pr o gr am s, P t. 7, p. 541.

Favn, G. H., er.ro D. C. Hannrs (1969) On the origin of colour and pleochroism in andalusitefrom Brazil. Can. l{ineral,.10, 47 56.

Gouns, C. oa B. (1969) Electron microprobe analysis of zoned melanites Amer. Mineral.54,1654-t661.

Henruex, P (1969) Can Tia+ replace Sia+ in silicates? MineraL. Mag.37r 36G369.Howrn, R. A., eNo A. R. Woor.r.r:v (1968) The role of titanium and the effect of TiOz on

cell-size, refractive index, and specific gravity in the andradite-melanite-schorlomiteseries. Mineral,. M ag. 36, 7 7 5-790.

Hucxrxnor.z, H. G. (1969) Synthesis and stabitity of Ti-andradite. Amer. f . Sci.267-A'209-232.

840 RAYMOND K. MOORE AND WILLIAM B. WHITE

Hunrull, J. P., S. P. S. Ponro, I. F. Cn,l'Nc, S. S. Mrrnn, exo R. P. Bauu.lN (1968)

Optical phonons of yttrium aluminum garnet. Phys. Rea.173,851-856.Iseacs, T. (1968) Titanium substitutions in andradite. Chem. Geol.3,219-222.Iro, J., eNo C Fnoxorr, (1967) Synthetic zirconium and titanium garnets. Amer. Mineratr.

s2,773-781.

JonrvsoN, O. W., W. D. OnlsnN, eNt P. I. Krmcstunv, Jn. (1968) Defects in rutile. IIIOptical and electrical properties of impurities and charge carriers Phys. Reo. 175,tl02-tro9.

Lucovsrv, G., M. Bnonsrv, ANo E. Bunsrern (1968) Optical lattice mode of mixed polarcrystals. In R. F. Wallis, editor, Proc. ConJ. on Localized Excitations in Soli.d.s, PlenumPress, N.Y., p. 592 601.

MINNTNG, P. G. (1967) The optical absorption spectra of some andradites and the identi-fication of the 6Ar+aAr aE(G) transition in octahedrally bonded Fel+. Can. f . Earl,hSci. 4,1039-1047.

- (1969a) An optical absorption study of the origin of colour and pleochroism inpink and brown tourmalines. Can. MineyaL. g,678-690.

-- (1969b) On the origin of colour and pleochroism of astrophyllite and brown clinto-nite. Can. Mi.nerol,. 9, 663-677.

- (1970) Optical absorption and electron microprobe studies of some high Ti-andra-diIes. Can. Mineral. 10,260.

McClnrnv, G. J., W. B. Wrrrrr, .q,No R. Rov (1969a) Samarium titanates, strontiumsamarium titanates and the question of the existence of divalent samarium in oxides.Inorg. Chem.8, 1236 1239.

--t AND - (1969b) Phase equlibria in the 1375"C isotherm of the systemSr-Ti-O. J. Amer. Ceram. 50c.52,463-467.

McDnvnr, N. T. (1969) Infrared lattice spectra of rare-earth aluminum, gallium, and irongarnets. J. Opt. Slc. Amer.59, 1240-1244.

Moonr, R. K., W. B. Wrurn, ,r,un T. V. LoNc (1971) Vibrational spectra of the commonsilicates. I. The garnets. Amer. Mineratr.56, 54-71.

Ponrnn, V. R. (1965) Stu,iLies in the Titanium-Otcygen System and. the DeJect Notrure oJRutile. Ph.D. Diss., The Pennsylvaina State University, 99 pp.

I{onrn, M. B., aNn P. Dav (1967) Mlxed valence chemistry-a survey and classification.Ad.a. I nor g. Radiochem. 10, 247-422.

Su,lnNoN, R. D., ano C. T. Pnrwrrr (1969) Efiective ionic radii in oxides and fluorides.A ct a Cr y s t all o gr. 825, 925-9 46.

T.ttrn, P. (1959) Recherches sur le spectre infrarouge des silicates: II. Determination durole structural du titan dans certains silicates. Silicates Ind.ust.25, l7l-175.

Tenrn, P. (1960) Infrared spectrum of garnets. Nalure,1861 234.A. E. Ringwood (1962) Infrared spectrum of the spinels NigSiO+ Ni2GeOa, and

their solid solutions. N ature, 193, 97 1-97 2.-- (1965) Etude experimentale et interpretation du spectre infrarouge des silicates et

des germanates. Application a des problems structuraux relatits a I'etat solide. Aca.d.Roy. Soc. BeIg. Mem.35, 103-119.

\-nnr,nut, H. A., eNo A. S. Bmxrn, Jn. (1966) Infrared lattice vibrations in GaAsrPl-a -alloys. Phys. Ret. 149,715-729.

Wnrrr, W. B., ewo B. A. DnAlrcolrs (1967) Interpretation of the vibrational spectra ofspinels. S pectrrochim. Acla, 23A. 985-995.

Wrcxnnsrmrar, K. A. (1961) Optical and infrared spectra of ferrites and ga--nets J. APfl.Phys 32,2055.

M anuscript receited, August 24, 1969; aceeptd for publicotion, February 17 , 197 L