American Mineralogist, Volume 71, pages 694-704, 1986

Raman spectra of some tecto silicates and of glasses along theorthoclase-anorthite and nepheline-anorthite joins

DEAN W. MATSON,! SHIV K. SHARMA, JOHN A. PHILPOTTS2Hawaii Institute of Geophysics, University of Hawaii, 2525 Correa Road, Honolulu, Hawaii 96822

ABSTRACT

.Raman spectra are reported for powdered crystalline anorthite, sanidine, leucite, a-nephe-

Ime, an~ ~-carnegie~te ~nd glasses .ofCaA12Si20s (An), KAlSi30s (Or), and NaAlSiO. (Ne)composItIOns. Quahtatlve compansons between features in the spectra of the isochemicalc~stal~ a~d glasses indicate that the glasses maintain short-range (coordination) relation-ShIPS sImIlar to those of the crystalline polymorphs. Intermediate-range order (TO. ringstructures; T = Si,Al) of the glasses is inferred from the frequency of the dominant symmetricstretch v,(T -0- T) band in their Raman spectra. The ring structure of An glass is similar tothat of the crystalline feldspars, consisting predominantly off our-membered TO. rings. ItsRaman spectrum indicates that the melt-quenched glass of Or composition contains do-mains ~fleucite-like structure, consisting of alternating four- and six-membered TO. rings,and ~egIons ~fpredominantly six-membered ring structures. The ring structure ofNe glassconsIsts of sIx-membered TO. rings, similar to its isochemical crystalline polymorphs,although the rings are highly puckered. High-frequency antisymmetric-stretching featuresin the spectra of the tectosilicate glasses are explained in terms of site symmetries of TO.tetrahedra due to the distribution of SiO. and AIO. units in the glass. Variations in thering sizes ~n glasses along the Or-An and Ne-An joins are reflected by shifts of the v,(T -0-T) bands m the glass spectra. Antisymmetric-stretching features in the spectra of glassesalong the Or-An join vary systematically as Si:Al changes from 3: 1 in Or to 1:1 in An butvary only slightly in the spectra of glasses along the Ne-An join because of the commonSi:Al of all glasses along that join.

INTRODUCTION

Crystalline tecto silicates consist of three-dimensionalframework structures containing fully polymerized TO.tetrahedra (T = SiH ,AP+). Such frameworks generallyconsist of rings of TO. tetrahedra enclosing void spacesin which nontetrahedrally coordinated cations may reside.The structures of glasses (quenched melts) having tecto-silicate-forming compositions are of interest to petrolo-gists and geochemists studying the physical and chemicalproperties of alumino silicate melt structures because themajority of natural magmas are highly polymerized, con-taining less than one nonbridging oxygen per tetrahedralcation (Mysen et ai., 1981). Although it is generally ac-cepted that both Si and Al cations maintain fourfold co-ordination in glasses quenched at ambient and high pres-sures from melts having tecto silicate-forming compositions(Day and Rindone, 1962; Sharma et ai., 1978b; Mysen etai., 1981; Seifert et ai., 1982; Navrotsky et ai., 1982; Fleetet ai., 1984; Hochella and Brown, 1985), the intermediate-range order (ring structure) of these glasses has yet to befully investigated.

I Present address: Battelle Pacific Northwest Laboratory, Rich-land, Washington 99352.

2 Present address: Branch of Analytical Laboratories, U.S.Geological Survey, National Center, Reston, Virginia 22092.

0003-004X/86/0506-0694$02.00

Tetrahedrally coordinated cation populations varywidely among the various tectosilicates. Tetrahedral sitesin the structures of the pure silica crystalline polymorphsare completely occupied by SiH cations. Among the feld-spars, SiH: AP+ ranges from 3: 1 for albite (Ab; NaAl-Si30s) and orthoclase (Or; KAlSi30s) to 1:1 for anorthite(An; CaA12Si20s)' Sodic nepheline (Ne; Na2A12Si20s) alsocontains a 1: I Si:Al population, and other crystalline alu-minosilicates such as leucite (Lc; KAlSi206; Si:Al = 2: 1)have intermediate cation populations.

In this study we investigate the Raman spectra of glasseshaving Or, Ne, and An compositions and the spectra ofcrystalline phases having Or, Ne, An, and Lc composi-tions, which are characterized by variations in both TO.ring structures and Si:Ai. A systematic investigation ofthe spectra of glasses having compositions along the Or-An and Ne-Anjoins has also been undertaken to elucidatestructural variations resulting from the compositionalchanges in these glasses.

EXPERIMENTAL METHOD

The compositions and preparation conditions of the glassesused in this investigation are presented in Table 1. Starting ma-terials for the endmember-composition glasses were high purity(>99.9%) oxides and carbonates. Mixes of the appropriate com-positions were ground under acetone in an agate mortar for not

694

Table I. Compositions and preparation conditions of

aluminosilicate glasses used to obtain Raman spectra

Time atStart; 09 Fusion temp. fusion

Composition components (OC) temp. (hr.)

NaA1Si04 (Ne) Na2CO/Si02+A1203 1620 2

KA1Si30S (Or) K2C03+Si02+A 1203 1600 LOt1625 2.0

CaA12Si20S (An) CaC03+Si02+A 12°3 1625 3.5

Ne20AnSO endmember glasses 1600 1.0

Ne40An60 endmember glasses 1500 1.0

Ne60An40 endmember glasses 1500 1.0 'U;

'ENeSOAn20 endrnember glasses 1500 1.0 :J

~jg

Or20AnSO endmember 91asses 1600 1.0 :eOr40An60 endmember 91asses 1550 1.0 5Or60An40 endmember glasses 1500 1.0~Or

SOAn20 endmember glasses 1500 1.0CJ)zI!!

tIndicates multiple heatings with sample powdered between each.

MATSON ET AL.: RAMAN SPECTRA OF TECTOSILICATES AND GLASSES 695

less than 10 min, dried, and loaded into platinum crucibles. Themixtures were allowed to stand at 700-900°C for several hoursto ensure complete removal of C02 from the carbonates prior tofusion at the conditions listed in Table I. Glasses having com-positions along the Or-An and Ne-An joins were prepared fromthe powdered endmember glasses under similar conditions, omit-ting the decarbonation step. The glasses were quenched either byremoving the crucible and allowing it to cool in air or by im-mersing its base in water. AIl glasses appeared clear and opticallyhomogeneous with the exception of the Or-composition glasswhich contained minute bubbles dispersed throughout, but whichwas otherwise clear.

Polycrystalline samples used to obtain Raman spectra werecrystallized from their isochemical glasses or were naturally oc-curring minerals. Anorthite was crystallized from the glass at1000°C and I atm over a 48-h period (Sharma et aI., 1983).Nepheline and camegieite were crystallized at I atm from Neglass at I 100°C for 4 h and at l400°C for 5 d, respectively. Thesanidine and leucite used for this study were natural mineralsfrom Civitacastellana, Italy, and from Swan City, Colorado, re-spectively.

Spectra of all samples were collected using a computer-con-trolled Spex 1403 Raman spectrometer. The 488.0-nm line ofan Ar+ laser having a source power of approximately 600 m Wwas used to excite the samples, and a 90° scattering geometrywas employed. The unpolarized spectra of powdered crystallinesamples (loaded in glass capillary tubes) were collected using 2cm~l slits. Glass spectra were obtained, with 5 cm~l slits, from

unannealed glass fragments mounted on metal needles. A polar-izer sheet mounted in front of an optical scrambler was used toanalyze the parallel (/11)and perpendicular (I,J components ofthe radiation scattered from the glasses, although only the IIIdataare reported here.

RAMAN SPECTRA OF TECTOSILICATE CRYSTALS AND

THEIR ISOCHEMICAL GLASSES

Comparison of tectosilicate crystal and glass spectraThe polarized Raman spectra (III)of glasses having An,

Or, and Ne compositions are presented in Figures 1-3.Each glass spectrum is accompanied by the powder Ra-

1000

RAMAN SHIFT (cm- 1)

Fig. I. The Raman spectra of crystalline anorthite and glass(/11)of An composition.

man spectrum of one or more related crystalline phaseshaving fully polymerized tetrahedral framework struc-tures.

The broadness of bands in the spectra of glasses com-pared to those in the spectra of their isochemical crystal-line phases (Figs. 1-3) results from the inherent disorderof glass networks relative to the highly ordered structuresof crystalline materials. Structural disorder contributes tothe broadening of glass spectral bands because of the rel-atively wide range of structural parameters (T -0- T bondangles and T-O bond lengths) in the glass that determineits vibrational characteristics. The mean values of thesestructural parameters of the glass networks can be esti-mated using X-ray or neutron-diffraction techniques(Wright and Leadbetter, 1976; Taylor and Brown, 1979a,1979b), but estimates of their distributions require sta-tistical modeling (e.g., Soules, 1979).

An initial comparison reveals that the spectra of thecrystalline tectosilicates and their isochemical glasses ex-hibit strong similarities in terms of the frequencies andrelative intensities of major features. This indicates dis-tinct structural similarities between the crystalline andglass phases in these systems, at least over the distancesto which the Raman spectra are sensitive. Specifically, thenext-nearest-neighbor (cation-oxygen coordination) rela-tionships characteristic of the crystalline phases are likelyretained, and these glasses consist of fully polymerizedtetrahedral framework structures. Nonbridging oxygen(NBO) bonds, which would form if the tetrahedral frame-

696 MATSON ET AL.: RAMAN SPECTRA OF TECTOSILICATES AND GLASSES

KAlSi,O.(Or)

I>-f-enzwf-;;:;

OrthoclaseGlass

60

Fig. 2. The Raman spectra of crystalline sanidine and leucite,and glass (III)of Or composition.

work structures were not maintained by the glasses, havebeen shown to produce strong, polarized bands at char-acteristic frequencies (800-1200 cm-I) in the Ramanspectra of both silicate crystals and glasses (Brawer andWhite, 1975; Matson et ai., 1983). The presence of eithersixfold coordinated Al or fourfold coordinated Al actingas a network modifier is also known to have a dramaticeffect on the Raman spectrum, particularly below 800cm-I where these species cause the dominant low-fre-quency spectral band to shift strongly to higher frequency(Sharma et ai., 1983). Although the general correspon-dence between features in the spectra of the crystallinetectosilicates and their isochemical glasses confirms thesimilar structural roles of cationic species in these phases,the spectra must be more carefully scrutinized to deter-mine the effectof vitrification on intermediate-range order(i.e., ring structures).

Features in the spectra of all fully polymerized tetra-hedral framework silicate and aluminosilicate glasses orig-inate from similar vibrational modes but may vary some-what in intensities or frequencies as a result ofcompositional differences (Sharma et ai., 1985; Matsonand Sharma, 1985). The dominant band in the spectra ofall the tetrahedral framework glasses appears in the 450to 520 cm-1 range and is attributed to a delocalized vi-brational mode involving the symmetric stretching ofbridging oxygens in T-0- T linkages (II,(T-0- T)) (Galeener,1979; Sharma et ai., 1983). Because of the similarity be-tween features in the spectra of crystalline tectosilicatesand their isochemical glasses, the corresponding dominantlow-frequency bands in the crystalline tectosilicate spectraare also attributed to the II,(T-0- T) vibrational mode

NaAISi04 (Ne)

I>-

i~~'"~''''' Crystal

l~ ",..,,",Crystal

500 1000RAMAN SHIFT (em-')

Fig. 3. The Raman spectra of crystalline a-camegieite anda-nepheline, and glass (III)ofNe composition.

(Sharma et ai., 1981, 1983). Other features having similarrelative intensities and frequencies in the Raman spectraof tectosilicate crystals and their glasses also likely origi-nate from corresponding types of vibrational modes intheir structures.

Tectosilicate crystal spectraBefore beginning a detailed examination of the struc-

tural information encoded in the spectra of the tectosili-cate glasses, we briefly discuss the spectra of the poly-crystalline tectosilicates shown in Figures 1-3. The Ramanspectra of some ofthese materials (anorthite and sanidine)have been presented and discussed elsewhere (e.g., Fabelet ai., 1972; McMillan et ai., 1982; Sharma et ai., 1983;von Stengel, 1977) but are included here for the sake ofcompleteness. To our knowledge the Raman spectra ofleucite and the nepheline polymorphs have not been re-ported previously.

The vibrational spectrum of a crystalline material isdetermined by the characteristic vibrations of the crystal'sunit cell and should therefore account for all degrees offreedom of the entire cell. The number of vibrationalmodes expected to be Raman and infrared active may bedetermined by factor group analysis if the unit-cell pa-rameters are precisely known from X-ray crystallographicstudies (White and DeAngelis, 1967; Adams, 1973). Care-ful examinations of the Raman spectra of single-crystal

MATSON ET AL.: RAMAN SPECTRA OF TECTOSILICATES AND GLASSES 697

Table 2. Factor group analysis of vibrational modes, andobserved frequencies* and relative intensities** of bands

in the Raman spectra of some powderedcrystalline aluminosilicates

Anorthite (pT, z =8a; rb

= 78 A9(Rt)

+ 75 Au (i.r.)); 63 m,

88 m, 139 m, 182 m, 200 (sh), 253 m, 273 m, 281 m, 316 vw, 369 vw,

4001'1, 427 1'1, 484 (sh), 503 vs, 553 m, 590 vw, 620 vw, 681 m, 741(sh), 756 1'1, 908 w(sh), 949 w(sh), 974 m, 998 (sh), 1044 (sh),

1072 (sh), 1124 w.

Sanidine (C2/m, z =4c; rb

= 20 A9(R) + 19 B9

(R) + 17 Au (i.r.) +

19 Bu (i.r.)); 681'1,80 vw. 1101'1,1241'1,142 w(sh), 156 m, 178 w(sh),

198 w(sh), 228 vw, 270 w(sh), 282 m, 330 vw, 336 vw, 372 vw, 406 vw,

454 w(sh), 476 m, 514 vs, 584 vw, 632 vw, 658 vw, 7541'1,8121'1,

1008 w(sh), 1122 m.

Leucite (141/a; z =16d; re

= 30 A9(R) + 30 B9

(R) + 30 E9(R) +

29 Au (i.r.) + 30 Au (i.a.) + 29 Eu (i.r.)); 761'1,1121'1,152 m, 1801'1,

216 W, 266 VW, 272 vw, 304 w. 338 w, 394 VW, 432 VW, 498 VS, 528 m(sh),

618 vw, 678 vw, 786 vw, 984 w(bd)(sh), 1066 w(bd).

a-Nepheline (P63/m, z =2f; re

= 14 A9(R) + 14 B9 (i.a.) + 13 E19

(R)

+ 15 E29(R) + 13 Au (i.r.) + 14 Bu (i.a.) + 13 E1u (i.r.) + 13 E2u

(i.a.)); 1231'1, 138 vw, 151 1'1, 214 m(bd), 2641'1, 331 w(sh), 3991'1,427vs, 469 m, 4971'1,616 VW, 690 vw(bd), 973 w(sh), 984 m(bd). 10B1 w(bd).

a-Carneqieite (low synmetry); 1141'1, 154 w(bd), 217 w(bd), 262 vw,

313 w(sh), 340 w(sh) , 347 vw(sh), 379 vs, 404 w(sh) , 4331'1,444 w(sh),

487 w(sh), 637 vw. 685 w(bd), 721 vw, 803 vw(bd), 949 w(sh), 964 w(sh) ,

982 m(bd), 1072 w(bd).

aSmith (1974); bWhite (1974); cOeer et a1. (1966); dWyart (1940,1941); ethis study; fOo11ase (1970), Foreman and Peacor (1970),

*Heasurementaccuracy ;s:t2 cm-1 for strong bands and :t4 cm-1 for veryweak or broad bands.

**Abbrev;at;ons: 5, strong; m, moderate; w. weak; v. very; (sh),shoulder; (bd), broad.

tR, Raman active; i.r.. infrared active; i.a., inactive mode.

olivines and pyroxenes have shown, however, that acci-dental degeneracies may account for the appearance offewer bands then expected on the basis of factor groupanalysis (White, 1975). A summary of the vibrationalmodes calculated on the basis of factor group analysis andofthe experimentally observed frequencies and intensitiesof bands in the Raman spectra of the crystalline samplesinvestigated here is presented in Table 2.

Crystalline An and Or are feldspars whose structuresconsist of four-membered rings of TO. tetrahedra inter-connected to form characteristic crankshaft-like arrange-ments (Deer et ai., 1966; Smith, 1974). The Ca and Kcations occupy cavities in the framework and balance thecharges of AP+ in tetrahedral coordination. Crystallineanorthite, containing half AP+ and half SiH in its tetra-hedral sites, exhibits a high degree of Si-Al order, largelyobeying the aluminum avoidance rule proposed by Low-enstein (1954)(Kempster et ai., 1962; Megaw et ai., 1962;Smith, 1974). Orthoclase contains Si and Al in a 3:1 ratio,and its tetrahedral cation distribution is not so con-strained. The orthoclase structure may, however, exhibitcation bias because of crystallographic site preferences andbonding-energy considerations. Consequently, the cationdistribution in the high-temperature (sanidine) form of

this composition is relatively random, although the low-temperature form (microcline) exhibits a high degree oftetrahedral cation order (Stewart and Ribbe, 1969).

Crystalline leucite (which the orthoclase compositionmelts incongruently to form; Schairer and Bowen, 1947)contains alternating four- and six-membered rings of TO.tetrahedra, with K ions occupying cavities along the linesjoining the centers ofthe six-membered rings (Zoltai, 1960;Papike and Cameron, 1976). The broadness of the bandsin the spectrum of crystalline leucite (Fig. 2) indicates thatthe natural sample used in the present study likely hassignificant Si-Al cation disorder, causing some loss ofspectral resolution.

Four polymorphs of the nepheline composition areknown (Smith and Tuttle, 1957). ~-carnegieite is stablefrom the liquidus to -1250°C where it transforms to~-nepheline. At -900°C ~-nepheline inverts to a-nephe-line which is hexagonal and stable to room temperature.The transformation from ~-carnegieite is sluggish, and byrapid cooling it may be obtained in the stability field ofnepheline. At -690°C ~-carnegieite undergoes a displa-cive inversion to a-carnegieite which has low symmetry.The structures of nepheline and carnegieite may be de-scribed in terms of stuffed derivatives of the high-tem-perature silica polymorphs tridymite and cristobalite, re-spectively. These, unlike the a- and ~-quartz structures,are sufficiently open to accommodate interstitial Na ions(Buerger, 1954). Both structures are dominated by six-membered rings of TO. tetrahedra (Papike and Cameron,1976). The alternation of Si and Al cations in the crys-talline nepheline structure is mandated by the aluminumavoidance rule, and high degrees of Si-Al ordering havebeen observed in natural nephelines to temperatures inexcess of 900°C (Foreman and Peacor, 1970).

The structure of pure synthetic sodic nepheline has notbeen thoroughly investigated. Naturally occurring nephe-line (NalNa,K)Al.Si.016) is hexagonal and belongs to theP63 space group, although some specimens may possesshigher pseudosymmetry close to P6/m (Dollase, 1970;Foreman and Peacor, 1970). The structure of the Na end-member is closely related to that of the natural mineral(Smith and Tuttle, 1957). A general lack of coincidencebetween the positions of bands in the Raman spectrumof nepheline presented here (Table 2) and the bands ob-served in the infrared spectrum of a naturally occurringnepheline (Moenke, 1974) indicates that the syntheticsample likely belongs to the P6/m space group in whichthe Raman and infrared active modes are mutually ex-cluded (Table 2).

In nearly all cases presented here, the number of de-tected Raman bands is less than predicted on the basis ofgroup theoretical considerations (Table 2). This may re-sult, in part, from accidental degeneracies between bandsin the spectra or from the inability to distinguish weakbands from background noise (Sharma et ai., 1983). Al-ternatively, the spectra of some of the crystalline mate-rials, particularly those having larger unit cells, may bedetermined by smaller (pseudo) unit cells (White, 1974).

"s(T-0- T)

Minera 1 (cm'l )

a-Quartz 4648,Quartz (700°C) 464 (a)

a-Cri s toba 1i te 416

a- Tridymite 407 (b)

Coes i te 521 (c)

y-Spodumene 4aO

(LiA1Si206-IlI)(d)

B-Spodumene 492

(LiA1Si206,II)

Low a1bite 506 (e,f)

(NaA 1Si 30a)

Sanidine 513 (e,f,g)

(KA1Si3Oa)

Anorthi te 503 (e,g,h)

(CaA12Si2Oa)

Leucite 4ga (g)

(KA1Si206)

a-Carnegieite 3a1 (g)

(NaA1Si04)

a-Nepheline 427 (g)

(NaA1Si04)

476 (d) 2.34 (d) 6 (d)

480 (u) 2.3a (w) 6 (w)

491 (g) 2.37 (w) 6 (g ,w)

50a (g,h) 2.69 (w) 4 (g ,h ,w)

698 MATSON ET AL.: RAMAN SPECTRA OF TECTOSILICA TES AND GLASSES

Table 3. Relationship between v,(T -0- T) and ring structures in some tecto silicates and their isochemical glasses

Crys ta 1

S~alle~t~~tnng SlZe

Dens; ty

(g/cm3)Vs

(T-0-

T)

(cm,l )

GlassDens; ty Prom; nent

(g/cm3) ring structure

2.65 (m)

2.33 (m)

2.26 (m)

3.01 (n)

437 (c) 2.21 (v) 6 (c)

2.399 (p)

2.374 (r)

2.63 (m)

2.563 (m)

2.76 (m)

4 + 6 2.47-2.50 (m)

2.513 (s)

2.598 (t) 485 (g) 6 (g, t)

(a) Bates and Quist (1972)(b) Etchepare et a1. (1978)(c) Sharma et a1. (1981)(d) Sharma and Simons (1981)(e) White (1975)(f) van Stengel (1977)(g) This study(h) Sharma et al. (1983)(k) Zoltai (1960)

2.50 (t)

(m) Oeer et a1. (1966)(n) Zoltai and 8uerger (1959)(p) Li and Pea cor (1968)(r) Li (1968)(s) Bowen (1912)(t) Taylor and Brown (1979b)(u) Sharma et a1. (1985)(v) Seifert et a1. (1983)(w) Taylor and 8rown (1979a)

Interpretation of ring structures in tetrahedralframework crystals and glasses

It has been demonstrated (Sharma et aI., 1981, 1983,1985) that a relationship exists between the intermediate-range TO. ring structures of crystalline tecto silicates andthe frequencies of v,(T -0- T) bands in their Raman spectra.Those tectosilicates consisting of structures containing four-membered rings (e.g., coesite and the feldspars) have v,(T-0- T) modes appearing above 500 cm-I whereas thosecontaining six-membered ring structures (e.g., a- andi3-quartz, cristobalite, tridymite, -y-spodumene, a-carne-gieite, and i3-nepheline) have v,(T-O-T) modes rangingfrom 381 to 480 cm-I. v,(T-O-T) bands in the spectra ofother crystalline tecto silicate phases whose structures con-sist of five-membered rings (e.g., i3-spodumene) or alter-nating four- and six-membered rings (e.g., leucite) appearat intermediate frequencies (Table 3). Comparisons be-tween the spectra of crystalline tectosilicates and theirisochemical glasses have led to the conclusion that thefrequencies of v,(T-0- T) bands in the spectra of tetrahe-dral framework glasses are also indicators of ring struc-tures in the glass networks (Sharma et aI., 1981, 1983,1985).

Frequencies of the v,(T-O-T) bands in the spectra ofanorthite crystal and glass agree closely (503 vs. 508 cm-1,respectively; Table 3), indicating a close correspondencebetween the ring structures in the two phases of this ma-terial. On the basis of v,(T -0- T) band positions alone, itis inferred that four-membered rings of TO. tetrahedracharacteristic of the crystalline feldspar structure likelypredominate in the isochemical An glass. This interpre-tation is supported by X-ray RDF (Taylor and Brown,1979a) and thermochemical studies (Navrotsky et aI.,1980) of anorthite glass and is also consistent with a num-ber of physical, chemical, and optical properties of Anglass summarized by Sharma et al. (1983).

The v,(T-O-T) band frequency of Or glass (491 cm-I)is significantly lower than that of its crystalline feldsparanalogue,sanidine(513cm-I; Table 3).The frequencyofthe Or glass band is, however, considerably closer to thatobserved in the spectrum of its liquidus phase, leucite(498 cm-I). Careful inspection of the low-frequency fea-tures in the spectrum of Or glass (Fig. 2) indicates thatthe relatively sharp v,(T -0- T) band appearing at 491 cm-Iis probably superimposed over a much broader featurecentered at approximately 15 to 20 cm -I lower frequency.

MATSON ET AL.: RAMAN SPECTRA OF TECTOSILICA TES AND GLASSES 699

Because the v,(T-O-T) band is believed to result from adelocalized vibrational mode occurring over a large num-ber of tetrahedral linkages (Bell et aI., 1970), the appear-ance of vibrationally distinct v,(T -0- T) bands indicatesthat relatively large domains of more or less homogeneousring structure must exist in the Or glass. The close cor-respondence in frequency between the v,(T-O-T) band inthe spectrum ofleucite and the sharp v,(T -0- T) feature inthe Or glass spectrum suggests that domains of leucite-like structures consisting of alternating four- and six-membered rings may persist in the glass framework. Theunderlying broader feature indicates the presence of a vi-brationally distinct structure containing a higher propor-tion of six-membered rings. The Raman spectrum of aglass prepared from a gel of Or composition (McMillanet aI., 1982) exhibits only a single asymmetric v,(T -0- T)feature, perhaps attesting to a greater structural homo-geneity of gel-derived glasses as opposed to those quenchedfrom melts (Mackenzie, 1982).

The presence ofleucite-like structures in Or-rich meltshaving compositions along the Ab-Or join has also beenproposed by Rammensee and Fraser (1982) to accountfor positive deviations in t:J{mix of the endmembers onthe basis of high-temperature Knudsen cell mass-spectro-metric measurements. The glasses of Rammensee andFraser (1982) were carefully examined in transmitted lightbut reportedly showed no signs of leucite crystallizationproducts and are considered to represent supercooled liq-uids above Tg, the glass transition temperature. Taylorand Brown (1979a) noted that the X-ray RDF of Or glasscompared well with that of leucite to >4.5 A, althoughbeyond 5 A the RDFs of the leucite structure and Or glassappear uncorrelated. These observations appear to sup-port the proposal of Kushiro (1975) that in some casesthe highest-temperature crystalline phase of a particularcomposition influences the melt structure.

The v,(T-O-T) bands at 427 and 381 cm-I in the spectraof a-nepheline and a-carnegieite crystals, respectively, oc-cur at low frequencies compared with the frequencies ofcorresponding bands in the spectra of a- and i3-quartz (464cm-I; Bates and Quist, 1972). The nepheline and carne-gieite bands do correspond more closely with the fre-quencies of v,(T-O- T) bands in the spectra of a-tridymite(407 cm-I; Etchepare et aI., 1978) and a-cristobalite (416cm -I; Bates and Quist, 1972), of which nepheline andcarnegieite are stuffed derivatives (Buerger, 1954).

The v,(T -0- T) band in the spectrum ofNe glass appearsat considerably higher frequency (485 cm -I; Table 3) thanthe corresponding band in the spectra of either of thecrystalline polymorphs. This may result, in part, from adistribution ofT04 ring sizes (including three-, five-, sev-en-, and larger-membered rings) in the disordered structureof Ne glass. Such a distribution could contribute to therelatively high frequency of the v,(T -0- T) band in thespectrum of Ne glass because of the highly delocalizednature of this mode. However, the rapid crystallizationbehavior of the Ne glass to its isochemical crystallinephases containing six-membered rings (Bowen, 1912) sug-

gests a predominance of six-membered rings in the glassstructure because a redistribution of ring structures oncrystallization would require the breaking of strong T-Obonds. Further support for the assignment of six-mem-bered rings to the structure of Ne glass may be found inthe Taylor and Brown (1979b) interpretation of the X-rayRDF data of this material.

The tendency of An glass (and melt) to retain the four-membered T04 ring configurations characteristic of itscrystalline feldspar phase is probably related to the natureof its charge-balancing cation (CaH). The divalent char-acter of the Ca cations requires that two AIO. units bepresent in close proximity to satisfy charge-balance con-ditions. This-in addition to the small size of the CaHrelative to the alkali cations and the tendency of the Si04and AI04 tetrahedra to alternate in the structure owing tothe Al avoidance principle-can account for the stabilityof four-membered rings in An glass. The larger size ofNa+ and K+, the single charge on these cations, and/or thelower Si:Al ratio in Or and Ne glasses undoubtedly con-tribute to the greater stability of six-membered T04 ringsin these materials.

With the exception of the v,(T-O-T) band and the high-frequency antisymmetric-stretching bands discussed be-low, other features in the Raman spectra of the frameworkalumino silicate glasses are not well understood. Shouldersappear on either side of the v,(T-O-T) band in the spectraof each of the tecto silicate composition glasses studied(Figs. 1-3). If the v,(T-O-T) vibrational modes of the ringstructures in glasseswere not strongly delocalized, it wouldbe tempting to assign these features to contributions fromlarger (>6-membered) and smaller (3-membered) ringstructures. We have previously shown, however, that thelow-frequency features in the spectra of Ge-substituted-for-Si framework alumino silicate glasses can be attributedto distinctive vibrational modes to which cation motionmakes a significant contribution (Sharma and Matson,1984; Matson and Sharma, 1985). Unresolved low-fre-quency shoulders in the spectra of the framework alu-minosilicate glasses discussed here are considered to arisefrom corresponding modes. Similarly, we have argued(Matson and Sharma, 1985) that the shoulder appearingbetween 550 and 580 cm-I in the spectra of the alumi-nosilicate glasses corresponds to the 606 cm-1 Si02 glass"defect" band, although its relative intensity appears tobe related to the AIH content of the glass. The specificnature of the "defect" responsible for this feature is notcurrently understood. We do not, however, consider itlikely that this band can be accounted for by AI-O-Allinkages as proposed by McMillan et aI. (1982) as thiswould require a highly localized v,(T-0- T) vibrationalmode. Intermediate-frequency features appearing be-tween 700 and 800 cm-I (Figs. 1-3) are common to thespectra of tetrahedral network glasses and are attributedto an intertetrahedral deformation mode involving sig-nificant cation motion (Sharma et aI., 1984; Sharma andMatson, 1984).

700 MATSON ET AL.: RAMAN SPECTRA OF TECTOSILICATES AND GLASSES

High-frequency features

Systematic Raman spectral investigations of glasseshaving compositions along the Si02-MAI02 and Si02-MAI204 joins (Seifert et aI., 1982; McMillan et aI., 1982)have shown that both intensities and band contours ofhigh-frequency features (850-1200 cm-I) in the spectraof the framework alumino silicate glasses are highly sen-sitive to the glass composition. Although strong bandsattributed to the stretching modes of non bridging oxygens(NBOs) appear in this spectral region (Brawer and White,1975; Sharma et aI., 1978a; Matson et aI., 1983), it isunlikely that glasses having alkali or alkaline-earth cationspresent in just sufficient quantities to charge balance AP+in fourfold coordination contain significant numbers ofNBOs. Difference spectra between pure Ab glass and Abglass containing excess Na20 or AI203 (Matson, 1984)clearly show that high-frequency features in the spectrumof pure Ab glass do not result from NBO stretching modes.On this basis it is inferred that high-frequency features inthe spectra of other tectosilicate-composition glasses re-sult primarily from the anti symmetric-stretching modesof 0 atoms and network-forming cations in the three-dimensional aluminosilicate framework. These high-fre-quency glass modes correspond to similar vibrationalmodes of crystalline tecto silicates (White, 1975; Sharmaet aI., 1983).

The shape of the high-frequency envelope in the spec-trum of Ab glass (McMillan et aI., 1982) is similar to thatin the spectrum of Or glass (Fig. 2). Both compositionscontain Si:AI of 3: 1. The high-frequency antisymmetric-stretching features in the spectra of both Ab and Or glassesconsist of two dominant bands, the higher-frequency bandhaving higher intensity and being more strongly polarized.Similarly, the high-frequency features in the spectra of Anand Ne glasses (Si:AI = 1: I; Figs. 1 and 3) are comparable,consisting of a single predominant band with some smallresidual intensity on the high-frequency side. These ob-servations suggest that the general shape of the antisym-metric-stretching features in the spectra of the frameworkalumino silicate glasses can be directly related to the Si:AIof the glass.

High-frequency features in the Raman spectra of glassesalong the Si02-NaAISi04 join have been interpreted interms of two variable-frequency bands (Virgo et aI., 1979;Mysen et aI., 1980), although that interpretation was sub-sequently abandoned in favor of a more complex modelbased on residual-minimized Gaussian curve fitting of theexperimental data (Seifert et aI., 1982; Mysen et aI., 1982).More recently it has been shown that the high-frequencyfeatures in the Raman spectra of Ga- and/or Ge-substi-tuted glasses are analogous to those in the spectra of thecorresponding sodium alumino silicate glasses and can alsobe interpreted in terms of two variable-frequency (andnon-Gaussian) bands (Sharma and Matson, 1984; Matsonand Sharma, 1985). This brings to question the validityof using residual-minimized fits of Gaussian-shaped bandsto interpret features in the spectra of framework alumi-nosilicate glasses, or of glasses in general.

~-

The predominance of two bands in the high-frequencyregion in the spectra of alumino silicate glasses having Si:Al greater than 1:1 (e.g., Or; Fig. 2) can be accounted for,at least as a first approximation, by the lowering of theSi04 tetrahedra site symmetries resulting from the pres-ence of adjacent AI04 tetrahedra (Sharma and Matson,1984; Matson and Sharma, 1985). The presence of ad-

jacent tetrahedra having distinct T -0 bonding characterproduces a splitting of the high-frequency antisymmetric-stretching (F) mode of the tetrahedral species into modesof Al and E character. The higher-frequency componentof the aluminosilicate-glass antisymmetric-stretching en-velope in the spectra of glasses having Si:AI > 1:1 is morestrongly polarized than the lower-frequency componentand has therefore been assigned to the mode having Alcharacter. Glasses having Si:AI of 1:1 (An and Ne) areexpected to have a relatively high degree ofSi-AI orderingif the aluminum avoidance rule is invoked, and individualT04 units should therefore experience similar environ-ments (e.g., Si(OAI)4 units). This results in the appearanceof a single antisymmetric-stretching feature in the spectraof An and Ne glasses (Figs. I and 3) although the polarizednature of this feature indicates that the symmetry of theT04 units is lower than Td' A slight asymmetry of thehigh-frequency feature in the spectra of An and Ne glassessuggests some deviation from strict adherence to the alu-minum avoidance rule. The spectra of glasses having com-positions along the Ge02-NaAlGe04 and Ge02-Na-GaGe04joins (Sharma and Matson, 1984) clearly indicatethat the frequencies of both bands in the doublet are af-fected by the aluminate content in the glasses, as had beenoriginally proposed to account for features in the spectraof glasses along the Si02-NaAISi04join (Virgo et aI., 1979;Mysen et aI., 1980). This would strongly suggest that somedegree of coupling exists between the high-frequency an-tisymmetric-stretching vibrational modes ofthe AI04 andSi04 tetrahedra. Coupling between the antisymmetric-stretching modes of Si04 and AI04 tetrahedra is also in-dicated by the lack of features between 800 and 900 cm-Iin the spectra of the alumino silicate glasses, which wouldcorrespond to the stretching of AI04 units, as observed inthe spectrum of CaAl204 glass (McMillan et aI., 1982).

RAMAN SPECTRA OF GLASSES AWNG Or-An ANDNe-An JOINS

Systematic investigations of Raman spectra of glasseshaving compositions along the Or-An and Ne-An joinsare of interest in light of the above interpretation of fea-tures in the endmember glass spectra. Intermediate-rangering species in the endmember glasses of both joins aredistinct. The relative stabilities of those species in thestructures of the glasses having compositions along thejoins may be probed using the frequency ofp,(T-O-T) asan indicator of the T04 ring distributions in the glassstructures. The Or-An join is also characterized by a changein Si:AI from 3:1 to 1:1 along the join, which should bereflected in the high-frequency features of the interme-diate-composition glass spectra because of the contribu-

I1000r

>~aOOr.20Anii5

z

~ji;

600r.40An

400r.60An

Table 4. Raman frequencies (em-I) and refractive indices of glasses along the Or-An join

Composition: Or100 Or80An20 Or60An40 Or40An60 Or20An80 An100

Refract; ve ; ndex: 1. 485 1. 499 1. 515 1. 535 1. 555 1. 573

oo3iij vw,bdt72 s,p 68 s,p 90 s,p

'1,320 vW,bd '\,324 vw,bd '\,326 vw,bd '1,320 vW,bd '1,320 vw,bd452 (sh) ,p491 vS,p 491 vs

')492 vs,p 498 vs ,p 500 vs ,p 508 vs,p

578 (sh) ,p '1,576 (sh ,p '1,576 (sh),p '1,568 (sh) ,p '1,568 (sh),p '1,572 (sh),p'1,802 w,bd '\.790 vw,bd '1,772 vW,bd '1,768 VW, bd '\.764 w,bd '1,770 w,bd1015 w,(sh),bd 1012 w,(sh),bd 1033 w,bd

'1014 m,bd '1,993 m,bd '1,984 m,bd1118 m,bd 1104 w,bd 1079 w,(sh)

MATSON ET AL.: RAMAN SPECTRA OF TECTOSILICA TES AND GLASSES 701

KAISi3 08 (Or) - CaAI2 SI2 08(An)

200r.aOAn

100An

500 1000RAMAN SHIFT (em")

Fig. 4. Raman spectra (/n) of glasses having compositionsalong the Or-An join.

60

tion of both 0 and T cation motion to these modes.Changes in the spectra of glasses having compositionsalong the Ne-An join must be accounted for by factorsother than changes in Si:Al because this parameter re-mains constant at 1:1 for all compositions along this join.

Or- An system

The In Raman spectra of glasses along the Or-An joinare presented in Figure 4. Refractive indices and spectralband positions of the glasses along the join are listed inTable 4. The refractive indices vary smoothly with nom-inal composition along the join and are consistent withthose of both the endmember- and intermediate-com-

'E505

<.>

Us (T-O-T) SYMMETRIC STRE/CH.-;/"..---.......

-- ,-/,/

-><-- ,-/0/ 0

/x-- .9-........../" ,-

/" ,-/"

/;;rj' o-/

./ x x NEPHELINE-ANORTHITE/"

./ 0 ORTHOCLASE-ANORTHITE/"

~

o~ 495

l'

485

o 20 10040 60 80

Mol. % Anorthite

Fig. 5. v,(T-O-T) as a function of composition in the spectraof glasses along the Or-An and Ne-An joins.

position glasses as reported by other workers (Schairerand Bowen, 1947; Taylor and Brown, 1979a).

It is important to note that only a single broad v,(T -0-T) band exists in the spectra of all of the intermediate-composition glasses (Fig. 4) despite the fact that the end-member glasses are interpreted as consisting of distinctring structures. This indicates (1) that the intermediate-composition glasses must consist of mixtures of ringstructures and (2) that the v,(T -0- T) vibrational mode issufficiently delocalized that individual rings cannot be dis-tinguished. The nearly constant frequency of the v,(T -0-T) band between the spectra of pure Or and Or60An40glasses (Fig. 5) could be taken to indicate the retention ofthe leucite-like domains in these glasses without the in-corporation of larger numbers of four-membered ringstructures. This is unlikely since leucite does not appearas the liquidus phase beyond about 20% An content(Schairer and Bowen, 1947) and because the v,(T -0- T)shoulder characteristic of the leucite structure does notappear in the spectra of either the OrsoAn20 or the Or60An40glasses. The very small shift in frequency of the v,(T-O-T) band in the spectra of glasses containing less than 60%An (Fig. 5) may result from a more random incorporationof the four-membered rings throughout the glass upon theloss of distinct structural domains of the Or 100 glass. Glass-es having compositions beyond 40% An almost certainlyrevert back to random ring structures in which the con-centration of four-membered rings increases as the pureAn composition is approached. This is reflected by the

""'Accuracy: :!:5 cm-1 for strong and medium intensity bands and :tlO cm-1 for weak bands and shoulders.tAbbrev;ations: w, weak; m. medium; s, strong; v, very; bd. broad; (sh), shoulder; p, polarized.

100Ne

I 80Ne.20An>-....e;;zw....

~60Ne.40An

Table 5. Raman frequencies (em-I) and refractive indices of glasses along the Ne-An join

Compos it i on: NelDO Ne80An20 Ne60An40 Ne40An60 Ne20An80 An100

Refractive index: 1. 513 1. 523 1. 533 1. 546 1. 559 1.573

"'285-~w.bdt74 s ,p 90 s ,p

"'283 VW, bd '\.304 vw,bd '\,300,_ vw,bd '\,313 vw,bd '\,320 vw.bd485 vs,p 490 liS,p 498 vs ,p 502 vS,p 508 vs,p 508 vs ,p

558 (sh) ,p 565(sh) ,p 560 (sh),p 564 (sh) ,p 570 (sh),p"'572

(sh) ,p

"'735 w,bd "-'741 w,bd "-745 w,bd ",760 w,bd '\.759 w,bd ",770 w,bd1005 m,bd 990 m,bd 994 m,bd 988 m,bd 980 m,bd 985 m,bd

702 MATSON ET AL.: RAMAN SPECTRA OF TECTOSILICA TES AND GLASSES

20Ne.80An

100An

Fig. 6. Raman spectra (In) of glasses having compositionsalong the Ne-An join.

sharp increase in frequency of the v,(T -0- T) band in thespectra of glasses containing greater than 40% An (Fig. 5).

The spectrum in the high-frequency region (850-1250cm -I) is altered dramatically as the glass compositionchanges from pure Or to pure An (Fig. 4). The Or-glassspectrum contains a pair of unresolved high-frequencyfeatures that shift continuously in the spectra of glassesalong the Or-An join to become a single asymmetric fea-ture in the spectrum of An glass. Changes appearing inthe high-frequency spectra of the glasses along the Or-Anjoin reflect the changing concentration and distributionof SiO. and AIO. units in these glasses as Si:Al changesfrom 3:1 in Or to 1:1 in the An composition.

Ne-An systemThe polarized (In) spectra of all the glasses along the

Ne-An join are presented in Figure 6, and glass refractive

indices and spectral band positions are recorded in Table5. Refractive indices of the glasses along the join varysmoothly with composition and are in close agreementwith those reported for glasses of these compositions byGummer (1943) and Goldsmith (1947).

A shift in the frequency of the v,(T -0- T) band from 485cm-I at the pure Ne composition to 508 cm-I at the Ancomposition (Table 5, Fig. 5) suggests a general increasein four-membered ring concentration in the glasses withincreasing An content. An apparent break in the v,(T-O-T) vs. An content observed near the Ne6oAn.o composition(Fig. 5) may reflect subtle changes in intermediate-rangeorder near this composition. Clearly, differences in Si:Alcannot be invoked to account for these changes as allglasses along the join have the same Si:Al.

Other than a slight shift of the envelope toward lowerfrequency (Fig. 6, Table 5), there is little change in thehigh-frequency antisymmetric-stretching region of thespectra of the glasses along the Ne-An join. This is ex-pected because the constant Si:Al (= 1:1) of all the glassesproduces a single antisymmetric-stretching mode fromTO. tetrahedra surrounded by four other similar tetra-hedra as mandated by the aluminum avoidance rule.

CONCLUSIONS

Similarities between the spectra of the crystalline tec-tosilicates and those of their isochemical glasses are takento indicate that structural similarities exist between thetwo phases, at least in terms of the maintenance of SiO.and AIO. units that are interconnected to form continuousthree-dimensional framework structures. The broadnessof the bands and the lack of fine structure in the spectraof glasses relative to the isochemical crystal spectra resultdirectly from the disordered state of the glassy material.

The frequencies of the v,(T -0- T) bands in the spectraof the crystalline tectosilicates are sensitive to the size ofthe TO. rings present in their structures. Crystalline tec-to silicates whose structures are dominated by four-mem-bered rings of TO. tetrahedra have v,(T-O-T) frequenciesabove 500 cm-I whereas those containing six-memberedand larger rings have v,(T -0- T) bands which appear below480 cm-I. Using a similar criterion to determine the TO.ring distribution in the structures of aluminosilicate glass-es, it is clear that An glass contains four-membered TO.rings similar to those in the crystalline feldspar phase,whereas Ne glass probably consists predominantly of six-

*Accuracy: is cm-1 for strong and medium intensity bands and :tlO cm-1 for weak bands and shoulders.Abbreviations: w, weak;, m, medium; 5, strong; v, very; bd. broad; (sh), shoulder; P. polarized.

MATSON ET AL.: RAMAN SPECTRA OF TECTOSILICA TES AND GLASSES

membered rings. Or glass quenched from melt of Or com-position may contain domains ofleucite-like structure aswell as domains consisting largely of six-membered rings.

High-frequency features in the spectra of the tetrahedralframework aluminosilicate glasses result from the anti-symmetric-stretching modes of the bridging oxygens andnetwork-forming cations. Variations in frequencies andintensities of high-frequency antisymmetric-stretchingbands with changes in Si:AI in these glasses indicate thatthe higher-frequency feature of the antisymmetric-stretch-ing envelope corresponds to the Al component, whereasthe lower-frequency feature can be attributed to the Emode component of this mode which is split by a loweredsymmetry of TO. species in aluminosilicate glass net-works. Shifts of these bands to lower frequency with in-creasing Al content of the glasses indicate that both com-ponents ofthe antisymmetric-stretching mode are sensitiveto the overall T-O bond strength of the glass and thereforemust be at least partially delocalized.

The above interpretations are consistent with observedchanges in the spectra of glasses having compositions alongthe Or-An and Ne-Anjoins. Variations in v.(T-O-T) bandfrequencies with compositional changes in the spectra ofglasses along these joins reflect changes in the TO. ringdistributions in these materials and indicate the delocal-ized nature of the vs(T -0- T) mode. Variations in the shapeof the high-frequency antisymmetric-stretching envelopewith changes in composition in the spectra of glasses alongthe Or-An join are most likely related to the changes inSi:AI of these glasses.

ACKNOWLEDGMENTS

Financial support for this work was provided by NSF GrantEAR80-26091. The authors are thankful to an anonymous refereefor constructive suggestions. Hawaii Institute of Geophysics Con-tribution No. 1690.

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