Raman study of crystalline polymorphs and glasses of spodumene compositionquenched from various pressures

Ameilcan Mineralogist, Volume 66, pages I18-126, I98I

SHtv K. SHanue' eNo BRUNo SrMoNs2

Geophysical Laboratory, Carnegie Institution of WashingtonWashington, D.C.20008

Abstract

Raman spectra are reported for the three known polymorphs of LiAlSiro., for glasses ofLiAlSi2O5 composition synthesized at 1500' and l750oC and over the pressure range 0.001-20kbar, and for a glass of Li'rAlSirou- composition synthesized at I atm and l500oC. Acomparison of the spectra of LiAlSirO.-I, -II, and -III indicates that the change in the coordi-nation of Al from four-fold (LiAlSiro6-II and -II! to six-fold (LiAlSiro5-I) results in majorchanges in the spectra. The observed number of Raman bands are compared with those pre-dicted on the basis offactor group analysis, and we propose that little or no deviation of thestructure of a-spodumene from the C2/c space group occurs.

The spectra of the glasses of LiAlSirOu composition synthesized over the pressure range0.001-20 kbar resemble thoge of LiAlSirO6 -II and -III, which indicates that Al3* is four-coordinated in the glasses over the pressure range investigated. The major spectral diferencesbetween the I atm and high-pressure glasses are explained in terms of differences in the sym-metry of the local ordering of the network structure and a systematic decrease in the T-O-Tbond angle with increasing pressure. The overall effect of pressure on the structure ofLiAlSirO5 melts is to increase the degree of local ordering and to change the local networkstructure from phase II- and phase III-like arrangements to a coesite-type structure.

Introduction

Sharma et al. (1979) recently investigated thestructure of melts of jadeite composition as a func-tion of pressure by measuring the Raman spectra ofthe jadeite melt quenched from pressures up to 40kbar, and demonstrated that Al3* remains tetrahe-drally coordinated throughout the pressure range in-vestigated. The results of that study led to a rejectionof a previous hypothesis (Watr, l9Z5) that a pressure-induced coordination change of Al3* from four- tosix-fold is responsible for the observed decrease inviscosity of melt of jadeite composition at elevatedpressures (Kushiro, 1976,1978; Kushiro et al.,1976).To aid in understanding the structure of aluminosili-cate melts at ambient and high pressures, the Ramanspectra of LiAlSirOu in both crystalline and glassystates have been measured. Spodumene composition

rPresent address: Hawaii Institute of Geophysics, University ofHawaii, 2525 Cortea Road, Honolulu, Hawaii 96822.

2Present address: Mineralogisch-Petrographisches Institut undMuseum der Universitdt Kiel, Olshausenstrassc 40-60, 2300 Kiel.West Germsny.

w3-MK/81/0r02-01t8$02.00 nE

was selected because it can be crystallized, by vary-ing pressure and temperature, in three crystallinepolymorphs (Munoz, 1967), which have aluminum infour-fold (LiAlSirO6-II and -III) and six-fold(LiAlSirO.-I) coordination. In the literature, thepolymorphs LiAlSirO6-I, -II, and -III are also re-ferred to as a-spodumene, B-spodumene, and 7-spodumene (or B-eucryptite), respectively. The latterconvention is not structurally appropriate for naminghigh-pressure-related polymorphs; therefore, the no-menclature introduced by Li (1968) is used in thispaper. By comparing the Raman spectra of di-fferentpolymorphs of LiAlSirOu the effect of Al in four- andsix-fold coordination on the spectra can be evaluated.Furthermore, a direct comparison of the Ramanspectra of glasses of spodumene composition pre-pared over the pressure range 0.001-20 kbar with thespectra of crystalline polymorphs can provide a bet-ter understanding of the local structure of theseglasses.

lnfrared spectra of LiAlSirO.-I and -II were stud-ied by Ignat'eva (1959). Murthy and Kirby (1962) re-ported the infrared spectra of solid solutions of

SHARMA AND SIMONS: RAMAN STUDY II9

LiAlsi,Ou-Il-silica and LiAISi,O.-III (B-eucryptite)- refractive indices and densities were corrected forsilica. thermal expansion to 25oC.

Measurements of the Raman spectra of differentpolymorphs of LiAlSirOu, of glasses of the composi-tion LiAlSirO. synthesized at 1500o and l750oC overthe pressure range 0.001-20 kbar, and of a glass ofLi,,AlSi2O.,, composition (LiAlSirOu + 0.25 LirO)synthesized at I atm are reported here for the firsttime. The glass of Lir 5AlSi2O62, composition was in-vestigated to evaluate the effect of nonbridging oxy-gen on the Raman spectrum.

Experimental methods

Glass of LirsAlsiro62, composition was preparedfrom oxide mixes of high-purity SiO2, Al2O3, and re-agent-grade LirCOr. A method analogous to thepreparation of KrO-AlrO3-SiO, glasses (Schairer andBowen, 1955) was used. Glass of spodumene compo-sition was kindly provided by Dr. D. B. Stewart ofthe U.S. Geological Survey.

A sample of LiAlSirOu-I was crystallized fromglass of LiAlSirO. composition in a sealed PtrrAu,capsule at 30 kbar and 1350'C (Munoz, 1967) n asolid-media, high-pressure apparatus (Boyd andEngland, 1960) for 6 hr. Similarly, phase III wascrystallized from the glass over a period of 3 hr in asealed PtnrAu, capsule at l0 kbar and 1000'C in agas-media, high-pressure apparatus (Yoder, 1950). Asample of LiAlSi,Ou-II was prepared by crystallizingglass of spodumene composition for 48 hr in a sealedPtnrAu, capsule at l350oC and ambient pressure. Theglasses at high pressures (10 and 20 kbar) were syn-thesized from glass of spodumene composition pre-pared at I atm by quenching the melt of LiAlSi,Oucomposition from l750oC under pressure in a solid-media, high-pressure apparatus. The samples weresealed in PtnrAur capsules 3 mm in diameter and 4mm long.

Raman spectra were recorded with a Jobin-YvonRaman spectrometer. Samples were excited with the488.0 nm line of an Ar* ion laser with a laser powerof 300-400 mW at the sample. Scattered radiationwas collected at 90o to the exciting beam. Details ofthe Raman apparatus are given elsewhere (Sharma,1978).

The refractive indices of the glasses were measuredby the immersion method. The refractive index of theoil that matched that of the glass was determinedwith a microrefractometer and monochromatic so-dium light. The density of the glasses was measuredin toluene with a Berman torsion microbalance. Both

Results

Raman spectra of the different polymorphs ofLiAlSirO. are given in Figure L The positions andother spectral characteristics of the bands are sum-marized in Table l.

Raman spectra of glasses of the compositionsLiAlSirOu and Li,5AlSi2O.,2i and the spectrum ofLiAlSi,O6-ttI are shown in Figure 2. The spectra ofglasses of spodumene composition synthesized at l0and20 kbar are shown in Figure 3. The observed vi-brational frequencies, the refractive indices, and thedensities are tabulated in Table 2.

Discussion

Raman spectra of crystalline polymorphs

LiA\Si,O6-III (y-spodumene/. This phase belongsto the hexagonal space group P6t22, Z: l, and has astuffed B-qnartz type structure in which Li atoms oc-cupy interstitial positions and an equipoint of rank 3,and Si'* is replaced randomly by Al'* (Li, 1968).Phase III therefore has, in addition to Si-Al disorder,three-fold cationic disorder. Disorder in the structureis reflected in the Raman spectrum of phase III,which shows a strong Rayleigh tail and broad Ra-man bands (Fig. l). Because of the disordered struc-ture it is not possible to apply rigorously the methodsof group theoretical analysis to the spectrum. A rea-sonable assignment of the prominent bands can,however, be proposed by comparing the spectrum ofLiAlSirO.-[I with the spectrum previously reportedfor B-qtartz (Bates, 1972) that is isotypical to phaseIII. The strongest band at 480 cm-' in the spodumeneis assigned to the l, mode and corresponds to the 462cm-' band observed in B-quartz. The shoulders at102 and 440 cm-' are assigned to the E' mode andcorrespond to the E' modes of B-quartz observed at99 and 428 cm-'. The weak band at 742 cm-' is as-signed to the -8, mode and corresponds to the E'mode of B-qaartz at 788 cm-'. The weak and broadbands at 1044 and 1088 cm-' are assigned to the E'and E, modes, respectively, and the correspondingbands in B-quartz were observed at 1067 (E') andll73 (E) cm-'. In the spectrum of LiAlSi,O6-[II thevibrational bands that may be attributed to the eor+e-sponding E, mode of B-quartz at 688, 409, and 245cm-'were not detected. Similarly, bands that may beattributed to LiOo tetrahedra were not detected,

t20 SHARMA AND SIMONS: RAMAN STUDY

LiAts i2o6

t00 500 toooRomon sh i f t (cm- l )

Fig. l. Raman spectra of (A) LiAISi2O6-III, (B) LiAlSi2O6-Il,and (C) LiAlSi2O5-I (a-spodumene) (laser 488.0 nm, Ar+ ion, 300mW, slit 3 cm-').

probably because they are too weak. Hase and Yo-shida (1979) have recently identified Raman bandsdue to Li-O vibrational modes of the LiOn tetrahe-dron in LirCO3. These bands are weak. In LiAlSirO.-III, the presence of disorder at the Li site will makethe Li-O vibration too weak to detect.

The strongest band at 480 cm-' and the weakbands in the 1000-1200 cm-' region are characteristicof Si(Al)O. tetrahedra with four bridging oxygens.According to theoretical studies on SiO, glass (Belland Dean, 1972; Sen and Thorpe, 1977; Laughlinand Joannopoulos, 1977), the strongest Raman bandin the spectrum is associated with the motion ofbridging oxygen along a line bisecting the Si-O-Siangle. Galeener (1979) suggested that this motionmay be described as Si-O-Si symmetric stretch be-cause, when acting along, the motion of oxygen re-sults in identical distortion of two neighboring Si-Obonds. Similarly, the highest frequency modes, which

give rise to weak Raman bands but strong infraredbands in the 1000-1200 cm-' region, are associatedwith the rrotion of bridging oxygen atoms along aline parallel to Si-Si. This motion can be called anti-symmetric stretch because it results in opposite dis-tortion of the two neighboring Si-O bonds. By anal-ogy, the strongest Raman band at 480 cm-' in thespectrum of spodumene is assigned to the symmetricT-O-T stretching (4) mode, where T : Si or Al inthe network, and the bands at llM and 1088 cm-lare assigned to antisymnetric T-O-T stretch modes.

The T-O-T bond angle in LiAISi,O.-III is smaller(151.6") than the Si-O-Si bond angle in B-quaftz(Li, 1968; Taylor, 1972). k seems that the smallervalue of the T-O-T angle in LiAlSirO6-III causes thea $-O-T) band to appear at higher frequency (480cm-') than the z" (Si-O-Si) band (462 cm-t) in B-qtrafiz. The lowering of the 2." (T-O-T) frequencies

Table L Raman frequen"t:ilj:l.Lt diferent polymorphs of

y-spodumene B-spodumene 0-spodumene

102 ( sh ) t

440 ( sh )480 s

742 w,bd.

1,044 (sh)

1088 w ,bd

r75 ( sh )116 ( sh )'ii ''i1."4L2 w

492 s

: : :

"z io *,ua770 w ,bd

tl1.*'oo

990 ( sh )

: : :

1094 w ,bd

I29 m189 w225 w2 4 1 n

296 m326 w356 s389 n

412 w (sh )436 w

. : : ') L Z V

542 w,bd,

583 w614 w7O7 s

782 w

esi ' *973 w

IO12

IOOO ' *

1095 w (sh )

*Measutement accuracg is 12 cm-L.+Abbreviations: v, verg; w, weak; m, redium; s,

strong; bd, broad; sh, shouTder.

SHARMA AND SIMONS: RAMAN STUDY t2l

too 500 toooRomon shiff (cm-l )

Fig. 2. Raman spcctra of LiAISi2O5-III (A), and of galsses ofthe compositions LiAlSirOu @) and Li1 sAlSi2O6 2s (C). 1n and 1,on the glass spectra, respectively, refer to the spectra recorded withthe electrical vector of the scattered light parallel to andperpendicular to the electrical vector of the laser beam (laser 488.0nm, Ar+ ion, 200 mW, slit 6 cm-t).

in LiAlSirO.-II[ compared with z* (Si-O-Si) in B-qtrartzis due to elongation of the T-O bonds and iso-morphous substitution of Al for Si in the network.The modes of vibration of SiO. and AlO4 tetrahedrainteract strongly and produce coupled modes (Iishi eta1.,1971; Moenke, 1974). Milkey (1960) investigatedthe infrared spectra of 57 tectosilicate crystals andfound that the z* (T-O-T) absorption peaks show anirregular but systematic shift to lower frequencies asAllSi is increased.

LiAlSi,O6-II (B-spodumene,). According to X-raydiffraction data, LiAlSirO"-II belongs to the tetra-gonal space group P4r2,2 with four molecules (Z: 4)in the unit cell (Li and Peacor, 1968). The structure isisotypical with keatite, and it consists of a three-di-mensional alumino silicate network. The distribution

of Si and Al in the tetrahedra is random. Li atomsare four-fold coordinated and occupy interstitial po-sitions. The four Li atoms per unit cell are distrib-uted among four sets of paired sites of eight-foldcoordination. Phase II thus has Si-Al disorder andalso two-fold cationic disorder.

The Raman spectrum of LiAISLO.-II is composedof broad bands (Fig. l). Eleven Raman bands are ob-served (Table l), much fewer than the expected num-ber of Raman bands from the large unit cell ofLiAlSirO6-IL The presence of disorder in phase IIwill make the bands weak and broad. Some of thebands might be accidentally degenerate. It is alsopossible that the principal features of the vibrationalspectrum of LiAlSi'O.-II are determined by a muchsmaller pseudo cell. White (1975) has pointed outthat in complex silicate structures with large unitcells the long-range forces within the unit cell are notsufficiently strong in most silicates to separate all themotion into discrete spectral bands.

100 500 1000Raman shift (cm -11

Fig. 3. Polarized Raman spectra of LiAlSi2O6 glasses formedfrom the liquid quenched at l0 and 20 kbar (laser 488'0 nm, Ar*ion, 200 mW, slit 6 cm-').

I

rr

122 SHARMA AND SIMONS: RAMAN STUDY

Table 2. Raman frequenciesi (cm-r), densities, and refractive indicesofglassespreparedatdifferentpressures

Compos i t ion :

Pressure (kbar ) :

Dens i ty (g /cm3) :Ref rac t lve index :

L iA lS i2O6 L i r , 5A l s i 2o6 . 25

0 . 0 0 1t a 1 q

L . 5 2 7

0 . 0012. 3401 . 5 1 8

102 . 4 3 5L . 5 2 9

202 . 4851 . 5 3 6

Frequenc ies*

ru80 s ru80 s !100 s

"sao i " t ) ,p t4 7 6 s , p

r u 5 8 8 ( s h ) , p1 7 6 0 w , b dtu980 w,bd,wp

! 1 0 5 2 w , b d , w p

4 8 0 s , p5 8 8 ( s h ) , p

1756 w ,bdN964 w,p1 0 4 8 w , b d , p

4 7 6 s , p5 8 8 ( s h ) , p

tu764 w,bdt u 9 6 4 w , b d , p1 0 4 8 w , b d , p

4 9 0 s , pru568 ( sh ) , pN744 w ,bd

11012 w ,bd ,p

xMeasurement accuracq is+Abbreviations. w, weak;

Inlarized.

t7o cn-f tor weak and broad bands and ).4 c*-t fo,s . s t rong; bd , b road; sh , shou jder ; p , poTar ized ;

< f r ^ n d b ) h d a

dp, depoTarized; wp, weakTg

The Raman spectrum of keatite is not known.Comparison of the spectra of LiAlSirO.-II and -III(Fig. l) reveals that in the 400-1200 cm-' region thespectra of these polymorphs are very similar. This re-semblance indicates that both these phases havethree-dimensional network structures in which Al3*is present in four-fold coordination. In LiAlSirOu-IIthe strong band due to the 4 (T-O-T) mode appearsat 492 cm-', and the y* (T-O-T) modes appear at990 and 1094 cm-'. The diferences in the T-O-Tbond angles and T-O bond lengths in LiAlSirO5-IIand - III are responsible for the differences in the po-sitions of the bands in these polymorphs.

LiAlSi,Oo-I (a-spodumene/. This phase has amonoclinic structure of pyroxene chains, and Al3* ispresent in six-fold coordination. Change in the coor-dination of Al3* from four- to six-fold has a drasticeffect on the Raman spectrum of spodumene (Fig. l).In the spectrum of LiAlSirOu-I, the bands are muchsharper and the intensities of the bands in the 900-1200 cm-' region are enhanced owing to creation ofnonbridging oxygen (see below).

Many clinopyroxenes belong to space group A/cwith Z : 4 (Clark et al., 1969). The number of for-mula units in the primitive cell is two, and thereforethe expected 57 1: 31,' - 3) optic modes will be dis-tributed among the following symmetry species(Etchepare, 1972):

f : l4r"(R) + 164(R) + l3l"(rR) + 14,8"(rR)

where R and IR refer to Raman and infrared activemodes, respectively.

Etchepare (1972) studied Raman spectra of an ori-

ented single crystal of diopside (CaMgSirOu) and de-tected 13 A" and 15 .8" modes. In polycrystallinediopside samples, however, only 23 Raman modeswere detected because some of the bands overlap. InLiAlSi,O.-I only 2l bands were observed (Fig. l,Table l). On the basis of X-ray diffraction studies ithas been proposed that spodumene belongs to spacegroap A rather than to space group C2,/c (Clark etol., 1969). For space group C2 all the 57 optic modesshould be both Raman- and infrared-active. If thedeviation of the structure from the A/c space groupis small, lhe A"and B" modes in the Raman spectrumare expected to be very weak. Because only 2l Ra-man bands are observed and the previously reportedinfrared bands (Ignat'eva, 1959) do not coincide withthe positions of the Raman bands, we can concludethat distortion of the structure of spondumene fromthe A/c space group is small, if present. Graham(1975) has pointed out that spodumene belongs tothe C2/c space group, and the extra reflections ob-served by Clark et al. (1969) are probably due to mi-croscopic inclusions in the sample. Infrared reflec-tance and Raman measurements on orients6 singlecrystals of LiAlSirOu-I are needed to resolve thequestion of the space group of spodumene.

The strong band at 707 cm-'in the spectrum ofLiAlSirO6-I is a characteristic band of pyroxenechains and is due to symmetric stretch of the bridgingoxygen a (Si-O-Si) in the chain (White, 1975). Inthe Raman spectra of other pyroxene minerals, e.g.,diopside (CaMgSi,O.), clinoensratite (MgSiOr), andenstatite (MgSiOr), one or two intense bands in the650-680 cm-' range were also observed and were as-

SHARMA AND SIMONS: RAMAN STUDY 123

signed to the motion of bridging oxygen in the chains(White, 1975). In the 1000-1200 cm-' region a bandof medium intensity at 1066 cm-'is due to symmetricstretching of the terminal nonbridging oxygen [2"(Si-O-)l in the chain. In diopside and other pyroxenes,which do not contain Al'* ions, the z" (Si-O-) modeappears near 1000 cm-' and is very intense (White,1975). In the LiAlSi,Ou-I spectrum the medium in-tensity of this band compared with the z" (Si-O-Si)band is probably due to the more covalent characterof some of the Al-O bonds. In fact, the X-ray diffrac-tion study (Clark et al., 1969) has indicated that inLiAlSirO.-I the Al-O bond lengths in AlOu octa-hedra vary (two A1-O distances of 1.818, two of1.943, and two of 1.997 + 0.002A).

Raman bands due to the AlO. group are usuallyweak in intensity, and their positions depend uponcoupling with the neighboring groups (Tarte, 1966;Adams, 1975). The bands attributed to the Al-Ostretching mode of the AlOu group are 520,474, and438 cm-' (Adams, 1975). In the spectra of LiAlSi,Ou-I, the A106 groups, which have strong interactionswith pyroxene chains, may be contributing to theweak bands at 435 and 518 cm-'.

Effect of Al'* coordination change on the Ramanspectra of LiAl Si rO "

polymorphs

The change in the coordination of aluminum fromfour- to six-fold modifies the structure of the silicateframework from a three-dimensional aluminosilicatenetwork without nonbridging oxygens to a pyroxenestructure with two nonbridging oxygens per siliconatoms. As pointed out above, the vibrational modesof AlOo and SiOo tetrahedra are coupled, and the vi-brational modes due to AlOu octahedral groups areexpected to be weak and may also be coupled toother vibrational modes. It is not possible, therefore,to evaluate directly the effect of the coordinationchange in terms of vibrational modes of the AlOo andAlO. groups. The effect of the coordination changecan, however, be evaluated indirectly by consideringchanges in the frequencies and intensities of thebands associated with the motion of bridging andnonbridging oxygen atoms. A comparison of the Ra-man spectra of three polymorphs of LiAlSirOu (Fig.l) in the spectral range 400-1200 cm-' indicates thatthe change of coordination of Al3* from four- to six-fold coordination causes a large shift in the positionof the strong band associated with the motion ofbridging oxygen. The z. (Si-O-Si) stretching mode inLiAlSirO.-t appears at 707 cm-', whereas the z" (T-O-T) stretching modes in phases II and III appear at

492 and 480 cm-', respectively. The change of coor-dination of Al'* from four- to six-fold also causes anincrease in the intensities of the bands in the 1000-12000 cm-' region relative to the band associatedwith the motion of bridging oxygen. The band at1066 cm-' in the spectrum of LiAlSirOu-I is due to asymmetric stretching motion of the terminal non-bridging oxygen in the chain. The antisymmetricstretching motion of the bridging oxygen in thechain, which gives rise to the characteristic infraredspectrum, is subdued in the Raman spectrum be-cause of its weak intensity.

Raman spectra of glasses

Glasses of LiAlSi'Oo and Li' llSirOo" compositionquenched at I atm. The prominent features in the Ra-man spectrum of glass of LiAlSirOu compositionquenched at I atm are the strong band at 476 cm-'and the broad, weakly polarized bands in the 900-1200 cm-' region (Fig. 2). The positions and the in-tensities of the Raman bands in the spectrum of glassof spodumene composition are closely related to thepositions and intensities of the most prominent bandsin the spectra of crystalline LiAlSirO.-II and -III,

both of which have a three-dimensional networkstructure with Al'* in four-fold coordination. Thebands in the glass spectrum are, however, muchbroader than their counterparts in the spectra ofLiAlSirO.-II and -III polymorphs. The broadening ofthe Raman bands in the spectrum of the glass is dueto additional disorder in the glass structure.

The close resemblance of the relative intensitiesand positions of the Raman bands in the spectrum ofglass of LiAlSirO6 composition to their counterpartsin the spectra of LiAlSiO.-II and -III indicates thatin the glass Al3* is present in four-fold coordinationand acts as a. network former.

A qualitative description of the local structure inthe three-dimensional network of LiAlSirOu glass canbe made by taking into account the positions and po-larization behavior of the Raman bands. It is knownthat the intertetrahedral angle (T-O-T, where T: Sior Al) in real glasses is not everywhere the same butis distributed about the most likely value, estimatedby X-ray diffraction to be l44o in SiO, and l33o inGeO, glass (Wong and Angell, 1976, p.409-507). Infact, the glass probably has an ensemble of local en-vironment with a statistical distribution of the inter-tetrahedral angle. The variation of the T-O-T anglein the glass is the basis of the disorder and is respon-sible for the broadening of the Raman bands. In thissituation, the peak position of the nondegenerate 4

124 SHARMA AND SIMONS: RAMAN STUDY

tS(Al)-O-Si(Al)l mode would be related to the mostprobable T-O-T angle in the glass structure (Galee-ner,1979). The peak position of the strongest Ramanband in the glass spectrum is close to that of the 4lSi(Al)-O-Si(Al)lband in the spectrum of LiAlSi,Ou-lII. It seems, therefore, that the most probable T-O-T angle in the glass of LiAlSirO. composition is closeto that of LiAISi,O.-III. In SiO, glass, which has a B-quartz-like arrangement (3Si at the D, site), the Ra-man bands in the 900-1200 cm-' region are depola-rized (Wong and Angell, 1976, p. 409-507). In thespectrum of the glass of LiAlSirO. composition thebands in the 900-1200 cm-' region are, however,weakly polarized. The weakly polarized nature of thebands in this region can be explained as due to thelower site symmetry of Si(Al) in LiAlSirOu-II[4S(AD atthe C, site and 8 at the C, sitel. The glassmay, therefore, have LiAlSirO6-II-like and -III-likearrangements in which clusters having LiAlSirO5-II-like arrangements dominate. The above structuralmodel of the glass of spodumene composition is alsoconsistent with the observed crystallization behaviorof the glass at I atm. The LiAlSirOu-II crystals growrapidly when the temperature of the melt is lowered25o*50'C below the liquidus, 1429" t loC (Munoz,1967;Li and Peacor, 1968), whereas the crystalliza-tion of LiAlSirOu-III at I atm has to be carried out atlower temperatwe (977"C) for at least l/2 hr (Li,1968).

In order to evaluate the efect ofnonbridging oxy-gen in the glass on the Raman spectrum, 0.25 M LirOwas added to the melt of LiAlSirO. composition. Thepresence of excess Li* in the glasses causes an in-crease in the intensity of the bands in the 900-1200cm-' region relative to that of the z" [Si(Al)-O-S(ADI band, which shifts toward higher frequency(Fig. 2). Similar changes are.observed when alkalimetal oxides are added to SiO, melt (Simon, 1960).These changes are caused by the creation of non-bridging oxygbn in the network structure. In the glassof Li,,AlSirO.r, composition the ratio r, correspond-ing to the integrated intensity of the contour from900 to 1250 cm-' to the integrated intensity of theband from 200 to 650 cm-' is 0.7, whereas in the glassof LiAlSirOu composition this ratio is 0.52. The pres-ence of 0.08 nonbridging oxygen per network-form-ing cation ts(Al)l on the average thus results in a 35percent increase in the relative intensity of the bandsin the 900-1200 cm-' region. Raman spectroscopy is,therefore, a sensitive tool for detecting the presenceof nonbridging oxygen in the silicate network.

In the spectrum of glass of Li, ,AlSirO.r, composi-

tion the strong and broad band at 100 cm-', which iseasily resolved from the descending Rayleigh tail(Fig. 2), is characteristic of the glassy state. Similarlow-frequency bands have been observed in thespectra of pure B2Or, SiO2, and GeO, glasses (Stolen,1970) and also in sp€ctra of glasses of diopside(CaMgSi,Ou) (Etchepare, 1972), akermanite (Ca,MgSi.O,), and sodium melilite (Sharma and Yoder,1979) compositions. It has been shown that theselow-frequency bands in the glasses and melts are dueto disorder-induced phonon density of states arisingfrom low-lying optic modes as well as the higher-lying acoustic modes (Shuker and Gammon, 1970,l97l; Winterling, 1975). A close examination of thespectrum of glass of spodumene composition (Fig. l)shows that a very low-frequency band, which is notwell resolved from the Rayleigh tail, is present. Theshift in the band position to higher frequency in thespectrum of Li,rAlSirOu, composition is partly dueto the higher density of this glass (D : 2.375) com-pared with that of I atm glass of LiAlSirOu composi-tion (D : 2.340). These very low-frequency bands inthe spectra of glasses correspond to intramolecularmotions. These motions are not well understood butare very important because they dominate thethermodynamical properties of melts (Shuker andGammon, l97l; Angell et al.,1969).It should be em-phasized that these low-frequency bands do not in-fluence the high-frequency intermolecular modesthat provide information about the structural units inthe glasses and melts.

Glasses of LiAlSirOo composition quenched at highpressures. Both the density and the refractive index ofthe glasses of spodumene composition formed byquenching the melt at high pressure increase with in-creasing pressure (Table 2). The spectra of thesehigh-pressure glasses in the 300-1200 cm-' regionare, however, similar to that of the quenched I atmglass except that with increasing pressure the weakand broad T-O-T asymmetric stretch bands at -980and -1052 cm-' in the I atm glass shift to lower fre-quency and show strong polarization (Table 2).

In general, the change in the polarization characterof the Raman band in the 900-1200 cm-' region inthe spectra ofhigh-pressure glasses can be attributedto a further lowering of local symmetry of the tet-rahedrally-coordinated units in the network with in-creasing pressure.

The ratios 4, corresponding to the integrated in-tensity of the contour from 900 to 1250 cm-' to theintegrated intensity of the band from 200 to 650 cm-'in the spectra of glasses of LiAlSirOu composition

SHARMA AND SIMONS: RAMAN STUDY r25

prepared at l0 and 20 kbar (Fig. 3), are estimated tobe 0.34 and 0.32, respectively. For the 2O-kbar glassthe value of n is 34 percent less than the value of 4(0.52) for the l-atm glass and is in contrast to the ob-served 35 percent increase in the value of r, in thespectrum of glass of Li,rAlSirOu' composition dueto the contribution of the nonbridging oxygen to thecontour in the 900-1200 cm-' region. The decrease inthe value of n in the spectra of high-pressure glassesfurther reinforces the conclusion that Al3* remainspredominantly in tetrahedral coordination, and de-fect structures involving nonbridging oxygen areminimized in the high-pressure LiAlSirOu glasses.

In the spectra of high-pressure glasses of LiAlSirO.composition a strong band at 80 cm-' is observed(Fig. 3). This band, as pointed out before, is due todisorder-induced phonon density of states arisingfrom low-lying optic modes as well as the high-lyingacoustic modes. In the l-atm glass of LiAlSirOu com-position a similar low-frequency band is not re-solved. The presence of a well defined low-frequencyband in the high-pressure glasses is probably relatedto more ordered and less open structure of theseglasses.

In the case of high-pressure jadeite glasses,Sharma et al. (1979) have proposed that local short-range ordering in high-pressure glasses may in partresemble that in coesite. The changes observed in thespectra of high-pressure glasses are sirnilar to thoseobserved in the spectra ofhigh-pressure glasses ofja-deite composition. The observed decrease in the fre-quency of the 2." (T-O-T) in the spectra of high-pres-sure LiAISLO. glasses can be attributed to a decreasein the average T-O-T angle, which may be associ-ated with a slight increase in the average T-O bondlength. It can be concluded, therefore, that the high-pressure LiAlSirOu glasses may also have, in part, acoesite-like arrangement. A coesite model of thestructure of high-pressure LiAlSirOu glasses is consis-tent with the observed increase in density and refrac-tive index of the LiAlSirO5 glasses with increasingpressure.

It should be pointed out that within the accessiblepressure range in the apparatus used for the presentwork a-spodumene (LiAlSirO6-I) does not melt con-gruently (Munoz, 1967). It will be interesting to seewhether or not Al'* remains six-coordinated in themelt at higher pressures (65 kbar), where LiAlSirO6-Imelts congruently.

Conclusions

The spectra of LiAlSi,Ou-[I and -III polymorphsshow a close resemblance but differ greatly from the

spectrum of LiAISLO.-I (c-spodumene), owing toAl'* coordination and Si-Al ordering. The positionsand number of Raman bands in the spectrum of c-spodumene are compatible with the pyroxene struc-ture having the A/c space group.

This study of different polymorphs of LiAlSirOushows that change in the coordination of Al'* fromfour- to six-fold produces large changes in the fre-quencies and relative intensities of the Raman bandsassociated with the motion of bridging and non-bridging oxygen atoms. The bands associated withthese motions give well defined Raman bands evenin the spectra of silicate glasses, and therefore can beused to determine the role of Al'* in aluminosilicatecrystals and glasses.

On the basis of similarity in the Raman spectra ofLiAlSirO6 glasses, prepared by quenching up to 20kbar pressure, and that of LiAlSirO6-II and -III, weconclude that Al3* is predominantly tetrahedrallycoordinated in these glasses. On the basis of the po-laization characteristics of the Raman bands andposition of the z" (T-O-T) mode, we propose that thel-atm glass has LiAlSi,Ou-IIlike and -[[I-like ar-rangements in which the clusters having LiAlSirO6-Il-like arrangements dominate. With increasing pres-sure the local symmetry of Si,Al in the network islowered compared with that of LiAISLO.-III, and theglass structure at high pressures may in part resemblethat of coesite.

AcknowledgmentsThe manuscript has benefited from critical reviews by Drs. R.

M. Hazen, T. C Hoering, L. W. Finger, and J. D. Frantz. Wethank Dr. H. S. Yoder, Jr., for valuable comments.

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Manuscript received, March 25, 1980;acceptedfor publication, May 9, 1980.