American Mineralogist, Volume 78, pages42-48, 1993

A Raman spectral study of -monticellitesolid solutions

K-c.xKALaMorrlNaN, SHrv K. Srr.lnNaA' Hawaii Institute of Geophysics,School of Ocean and Earth Scienceand Technology,University of Hawaii, 2525 Conea Road, Honolulu, Hawaii 96822, U.S.A. FrNr-Bv C. Brsnop Department of Geological Sciences,Northwestern University, Evanston, Illinois 60208, U.S.A.

Ansrru.cr Raman spectraof along the forsterite-monticellite (Fo-Mo) join have been mea- sured in the region of 100-1200 cm-' with a multichannel micro-Raman spectrometer.In the spectrum of Fo, strong Si-O- symmetric stretching bands appear at 824 and at 855 r. cm-r, and a medium-intensity antisymmetric Si-O stretching band appearsat 964 cm Thesebands show a systematicfrequency decrease with increasingMo composition. These systematicvariations of the high-frequency Si-O bands are attributed to decreaseddis- tortion of SiOotetrahedra. The mixing characteristicsof the v, arrd v, modes in orthosili- cateswere found to depend strongly on the ratio of intertetrahedral (O-O) to (Si-O) bond lengths.In the low-frequencyregion of the spectra,Fo-rich compositions show progressive broadening of the bands with the addition of up to 16 wo/o monticellite. But there is no appreciablebroadening of the bands in the spectra of Mo-rich samplescontaining up to l8o/oFo. The broadening of the low-frequency bands with increasing Mo component in the solid solution may be attributed to increasingpositional disorder causedby the sub- stitution of large Ca ions on the Ml octahedral sites.

INrnonucrroN a tool to study cation ordering in . Servoin and Piriou (1973) investigatedboth IR and Raman spectraof (Mg' and Mgr(Si' Olivines are consideredto be one of the important con- the solid solutions of ,Ni,)rSiO4 "- stituents of the Earth's upper mantle and of meteorites. Ge,)Ooand identified the v,, u3,and zomodes. Piriou and Among this large,well-studied classof orthosilicates,for- McMillan (1983) studied the high-frequencyvibrational sterite is one of the most common. In the ternary system spectraof several orthosilicates and investigated the na- containing forsterite (Mg,SiOo), (FerSiOo),and ture of the z, and z. mixing characteristics.Iishi (1978) olivine (CarSiOo),forsterite forms a completesolid modeled interatomic forces in Fo single crystals using solution with fayalite but not with the calcium olivine. vibrational data. In fact, under ambient conditions, it does not form a Raman spectra of a number of end-member olivines complete solid solution serieswith the intermediate iso- have been summarized and reported (Piriou and McMil- structural , monticellite, CaMgSiOo (Biggar and Ian, 1983).Chopelas (1991) measuredpolarized single- O'Hara, 1969;Brown, 1980).Ifuowledge of the structural crystal spectraof forsterite, fayalite, and monticellite and behavior, such as cation disorder causedby varying cat- refined the existing observed vibrational data, but little ion content, is important to our understanding of geo- work has been done on the effect of cation disorder on chemical and petrological systems. Adams and Bishop Raman spectra. Isotopic exchange experiments on or- (1985)have studiedthe Fo-Mo solid solutionseries sam- thosilicates (Paques-kdent and Tatte, 1973) showed in- ples with X-ray diffraction techniquesand detectedonly consistencyin the assignmentsof the high-frequencyvi- a small degreeof Ca-Mg disorder. brational modes. These workers concluded that there is Vibrational frequenciesare very sensitiveto interatom- strong coupling between the /r and I,3modes in the vi- ic forces. Raman and infrared (IR) spectroscopycould brational spectraof orthosilicates.Recent theoretical work thus reveal small changesin bond lengths and bond an- by Lam et al. (1990) showed that the v, srrd u3mixing in gles of crystals responding to changesin composition, orthosilicatescould be explained by consideringboth the temperature, and pressure.These techniques have been corresponding (Si-O) bond length and the ratio of the used for both quantitative and qualitative analyses of averageintratetrahedral O-O distanceto the correspond- (Farmer, 1974), and the vibrational spectra of ing (Si-O) bond distance. In this work the Fo-Mo sam- olivine have been extensively studied. Tarte (1963) and ples used in the previous X-ray studieshave been probed Burns and Huggins (1972) investigatedIR spectraofsev- with Raman spectroscopyto determine structural impli- eral olivines. Huggins (1973) used vibrational spectraas cations of systematic variations of composition, mixing 0003-004x/93/0 l 02-0042$02.00 42 MOHANAN ET AL.: RAMAN SPECTRA OF FORSTERITE-MONTICELLITE 43

Taele 1. ObservedRaman frequencies for the Fo-Mo solid-solutionserres

Fo,oo Fo." Fo$ Fo.o Fo,u Fo,. Fou

Ae 964.0(m) 963 1(m) 961.7(m) 961.7(m) 949 7(m) 949.7(m) 948.4(m) 949.0 B.n 919.0(w) 917.7(w) 916.3(w) 915.0(w) 901 6(m) 900.2(m) 898.9(m) 900.0 Aq 855.0(vs) 854.5(vs) 8s3.2(vs) 851.8(vs) 847 8(vs) 847.8(vs) 850.5(vs) 851.0 Aq 824.0(s) 822.0(s) 819.5(s) 818.1 (s) 816.8(s) 816 8(s) 816.8(s) 818.0 As 608.0(w) 606 0(w) 60a.0(w) 603.9(w) 595.0(w) 592.0(w) 589.0(w) 589.0 Brn * Bre + B3e 589.0(w) 586 8(w) 586.1(w) 586.3(w) 575 2(w) 580.0(w) s80.0(w) 560.0 Ae 544.0(w) 544.0(w) 543.7(w) 544.3(w) 575.3(w) 578 0(w) 578.0(w) 534.0 B,n * B2e + B3e 433.5(w) 432.1(w) 429.4(w) 427.9(wl 415.0(s) 413 0(s) 409.5(s) 407.0 Bfu 374.0(w) 369 1(w) 368.0(w) 367.8(w) 266.0 Ae 337.0(w) 333.0(w) 327.4(w) 327.3(w) 336.3(w) 337.0(w) 337.0(w) 275.0 Aq 329.0(w) 307.0 Bfu 314.0(vw) 311.6(w) 313.6(w) 313.6(w) 307.5(w) 251.0 Ae 303.0(w) 298 6(w) 297.2(w) 295.9(w) 261 6(m) 260.9(m) 2s6.8(m) 258.0 Aq 225.0(w) 223.0(w) 216.5(w) 216.5(w) 214.4(w\ 172.0 Note.'vs : very strong; S : strong; m : medium; w : weak; vw : very weak. . Datafrom Chopelas(1991)

characteristicsof the high frequency modes, and order- cies of the observed Raman spectra of polycrystalline disorder phenomena.We have usedRaman spectroscopy powdered samplesof Fo-Mo solid solution seriesare giv- for analysisof the samplesalong the Fo-Mo join because en in Table I . Observation of fewer than anticipated Ra- the Raman spectra of crystalline materials usually give man bands of the powdered samplesis attributed to the rise to well-definednarrow bands and, therefore,both the weak intensities of some bands, as well as to accidental frequenciesand band widths can be used to examine the degeneraciesofthe bands, including the tendencyofweak effect of composition and degee of disorder. bands to merge with adjacent bands.

ExpnnrvrrNTAl METHoDS Rnsur,rs AND DISCUSSToN The sampleswere synthesizedunder high pressurein High-frequency spectra a piston-cylinder solid-media apparatus and character- The Raman spectra of Fo-Mo solid solution seriesin ized with X-ray diffraction analysis (Adams and Bishop, the high-frequencyregion, 700-1000 cm-r, are shownin 1985). Raman spectra of six polycrystalline forsterite- Figure la. The high-frequency bands in the spectra of monticellite solid solution samples,ranging from 100 to olivines are attributed to the z, and z, stretching vibra- 6 molo/oFo, were measuredwith a multichannel micro- tions of SiOo tetrahedra. A very weak shoulder at 880 Raman spectrometer. The spectra were obtained at a cm-' is visible in the spectra of the Fo-rich end, but it geometry scattering of 135" (Sharmaand Urmos, 1987; mergeswith the 855 cm ' band in the Mo-rich end. An- Sharma, 1989). geometry The l35o laser-excitation has other weak feature, seenat 749 cm I in the Mo-rich end, proved give to a better signal to noise ratio for a given is not presentin the Fo-rich olivine. The striking feature sample geometry than the 180"excitation using the same of thesebands is the systematicvariation of the frequency parameters spectroscopic (Cooney and Sharma, 1990). with composition. For example, starting from Fo-rich ol- The sampleswere excited using a 488-nm laser line from ivine to Mo-rich olivine, the bandsat 824,855, and 964 a SpectraPhysics model 165 Ar* ion laser.Laser power cm ' shift toward lower frequenciesby approximately 5.4, ' at the samplewas 20 mW. A spectralresolution of 4 cm 8.1, and l6 wavenumbers,respectively. This systematic was used. In order to achieve a good signal to noise ratio frequency shift is largely due to the increasein the inter- in the spectra,1000 scans,each of 4-s exposure,were tetrahedral O-O bond lengths as a result ofthe substitu- added. The frequenciesof Raman bands are accurateto tion of larger cations in the M2 sites (see below). The +0.6 t. cm X-ray data (Adams and Bishop, 1985) indicate an in- creaseup to l9oloin volume from Fo- Sytrlllrprny ANALysrs the unit-cell the rich to Mo-rich olivine. Replacementof Mg cations with Olivine has an orthorhombic structure and belongsto Ca at the M2 site causesincreases in the unit-cell param- group : space Pbnm (point group D2h;Z 4). Factor group eters;in turn, the intertetrahedral O-O distanceincreases predicts analysis the following 84 normal modes for this and SiOo tetrahedra become more relaxed (Lam et al., group structural (e.9.,Lam et al., 1990): 1990).At the forsterite end, Raman frequency decreases I,o, : I lAs (R) + I lBrs (R) + 7B,s(R) + 7B3s(R) linearly with composition, whereasin the monticellite end + l0A, (ia) + 10B,, (ir) + 14B,"(ir) the Raman frequencyincreases, resulting in a reversetrend (Fig. 2). The intermediate solid solutions along the Fo- + 14B,"(ir) Mo join are not stable; as a result, we were not able to where R indicates the Raman active mode. ir. the infra- examine their Raman spectra.The coefficientsof a linear red active mode, and ia, the inactive mode. The frequen- fit to our data are given below. For convenience, the 44 MOHANAN ET AL.: RAMAN SPECTRA OF FORSTERITE-MONTICELLITE

(b)

Foo

a .= c = For+ L(s For L a = _o (UL

.= Foa+n a c o Foaa P C

Fogg

Foroo

800 1( 100 500 700 RamanShift (cm-t; RamanShift (cm-t; I Fig. 1. (a) In the high frequencyregion (700-1000 cm '), the major bands at 964,855, and 824 cm are assignedto the stretching modes of the Si-O(l), Si-O(3), and Si-O(2) bonds, respectively.(b) The details of the Raman spectra of Fo-Mo solid solutionsin the low frequencyregion (100-700 cm-').

equations are normalized to their respective end-member Those with a monticellite-rich composition are volume. Those with a forsterite-rich composition are z,:948.50+0.205V R :0.939 vr:963.99- 0.343V R : 0.963 ry: 850.29+ 0.426V R : 0.939 v,:855.34- 0.432V R : 0.954 Yz: 816.80 constantwith respect \: 824.17- 0.822V R : 0.995. to volume. MOHANAN ET AL.: RAMAN SPECTRA OF FORSTERITE-MONTICELLITE 45

Focomposition (mol%) of SiOo tetrahedra. The lattice dynamics of Fo have at- tracted the attention of a number of workers (e.g., Price 806040200 and Parker,1984; Price et al., 1987;Rao et al.,1987; Lam et al., 1990).It is proposedthat the band at 964 cm ' contains large contributions from Si-O(l) bond stretch (Lam et al., 1990). The stretching bands at 855 and 824 cm ' originate largely from Si-O(3) and Si-O(2) bonds,respectively. Their frequenciesand halfwidths vary systematicallywith the cation content. c In general,the observedfrequency of a bond-stretching vibration mode is inversely proportional to the corre- sponding bond length. In solids, however, such a simple o relationship may not be strictly valid becauseof near- neighbor interactions and crystal field effects.Table 2 lists the Si-O bond lengths, the ratio of averageintertetrahe- dral O-O distancesto the Si-O bond distances,and the c observedRaman frequenciesin forsterite,monticellite, and E o calcium olivine. It is evident that within each of these c( minerals, a given vibrational band varies strongly as a function of corresponding inverse bond length. For ex- ample, in forsterite, this dependenceis clear: for bond lengthsSi-O(l) (1.620A), Si-O(3)(1.636 A), and Si-O(2) (1.656 A), the correspondingfrequencies are 964,855, and824 cm-r, respectively.But among other olivine crys- 800L- tal structures,a simple consideration of the bond lengths 290 300 310 320 330 340 of the Si-O bonds alone does not explain the observed Fo Volume(A3) Mo frequencies.To resolve this dilemma, Piriou and Mc- Millan ( I 983) investigatedthe high-frequencyvibrational Fig. 2. High frequencybands vs. unit-cellvolume. Sample spectraof several orthosilicates.They studied the vibra- compositionis givenat the top of the abscissa.Legends: circle tional coupling of the v, and z, modes by considering : 964cm-t; seuare : 855cm-,; and triangle : 824cm-'. simple harmonic oscillators. Their simple theoretical model showedthat the high-frequencyvibrational modes vary with some physical parameter, X. They found that For all the equations, Z is the unit-cell volume in the the strong band A, at 856 cm ' in forsterite has more z, range 290-340 A'. The nonlinear behavior of the high- character,whereas in the monticellite spectrum the band frequency bands is due to the complex nature of the vi- at 814 cm-' has more z, character.Among orthosilicates, brations of the SiOotetrahedra that exist in the family of monticellite lies closest to the crossoverbetween /r and olivines (Piriou and McMillan, 1983). The linear corre- z. modes. It has therefore been suggestedthat in monti- lation coefficients(R) for Mo are not very high, indicating cellite the mixing characteristicsof z, and z, modes are that frequencyvariation of coupled /r-l3 modes with vol- more or lessequal, and forsterite and calcium olivine are ume are not linear. at the extremes(Piriou and McMillan, 1983). Let us consider the behavior and characteristicsofthe The simple theoretical model of Piriou and McMillan high-frequencybands in the Fo-Mo solid solution series. (1983) explainedthe behavior ofthe u, and z, modes of The bands of interest for Fo in this study are the two olivines, but the relationship of the physical parameter X intense,\ modesat 855 and 824 cm-' and the medium to the crystal structure was not clear. Lam et al. (1990) band at 964 cm-\, originating from the stretching modes theoretically simulated the forsterite structure using a pair-

Trale 2, Variation of the high-frequencySi-O stretching modes with their respective bond lengths and the ratio of intertetrahedra (O-O) to Si-O interactions

(o-o) (o-o) Bond lenoth' Frequency Bondrensth -#l- Frequency Bond length Frequency' Bond (A) Si-O (cm.) (cm') (A) Si-O (cm-') si-o(1) 1.620 1.840 964.0 1.615 1.957 949.0 1.637 2.0504 925.0 si-o(3) 1.636 1 822 856.0 1.640 1.927 851.0 1.642 2.0442 839.4 si-o(2) 1.656 1 800 824.0 1.656 1.908 817.0 1.665 2.01s9 813.6 Note: The mixing of v, ?trdv" modes in olivines is a combinedeffect of the two factors tabulated here. * Data from Piriouand McMillan(1983). 46 MOHANAN ET AL.: RAMAN SPECTRA OF FORSTERITE-MONTICELLITE

potential model by taking into account the ionic and co- valent interactions, as well as the core-core repulsion forces.Their model showsthat the high-frequency/r and 960 z. modes of olivine are not only affectedby the variations of the Si-O bond but also by the variation of intertetra- hedral O-O interactions.Lam et al. (1990) showedthat the intertetrahedral O-O distancevaries by l0o/owith the substitution oflarger cations. They proposed that the in- tensity of z, and z, mixing is attributed to the net effect of the intertetrahedral (O-O) force constant and the Si-O 940 force constant. We have calculated the ratios of intertetrahedral (O- O) to (Si-O) distances for forsterite, monticellite, and calcium olivine and tabulated them, along with the Si-O bond lengthsand their correspondingfrequencies (see Ta- ble 2). It is noted that for a given composition,both across 920 and down the table, the lower the value of this ratio, the Fo higher the Raman frequency.When a larger cation is sub- E 824 R2 stituted into the forsterite structure, Si-O bond length in- () creases,and the relative degreeofcovalency ofthe bond decreases.Moreover, the intra- and intertetrahedral (O- O) distancesincrease. When the Raman frequencies964, C) 855, and 824 crn' of Fo and the correspondingband 3 B2o frequenciesof Mo and calcium olivine are plotted with f the sum ofthe ratio ofthe correspondingSi-O bond length o q) to the Si-O(l) bond length and the ratio of intertetrahe- Mo^ dral (O-O) to (Si-O,) distances(where n : l, 2,3) in the respectivemineral (Fig. 3), a good linear trend is shown, E 816 with forsterite having the highest value of these modes Cd E and calcium olivine having the lowest. Our present Ra- (d Ca man data on Fo-Mo solid solutions are thereforein agree- E. ment with the predictions of Lam et al. (1990). 812 Low-frequencyspectra Raman spectraof the Fo-Mo seriesin the low-frequen- cy region, 100-700 cm ' (Fig. lb) show band broadening, a systematic decreasein frequency, and the merging of adjacent bands. Frequenciesofthese bands shift toward 850 Iower wavenumbers as the composition of the solid so- lution varies acrossthe Fo-Mo join. The bands are fairly sharp in the Mo-rich samples but broad in the Fo-rich samples. In vibrational spectroscopy,the half width of a band 840 is used to study positional disorder in crystal structures. Two strong bands at 433.5 and 544 cm-t are assigned, respectively,as a B-type mode and an A, mode originat- ing from zovibrations. We have selectedthese bands be- causethey have relatively strong intensities and have flat 830 base lines for half-width calculations. Even though the 2.8 2.9 3.0 3.1 band at 433.5 cm-' could be a combinationof the B,, * B2s+ B3gmodes, in the spectraof polycrystalline samples Bondlength ratio the intensities of the Br, and Br, modes are usually much weaker than that of the B,, mode, and therefore the se- Fig. 3. The dependenceofhigh-frequency bands in olivines 433.5-cm-1band for half-width calculation on the sum ofthe ratio ofSi-O bond lengthsto the Si-O(l) bond lection of the justifiable. length and the ratio of intertetrahedrallengths to the Si-O bond is On the addition of Ca, these bands show lengths.Bond length ratio: Rl : [Si-O(1)]/[Si-O(l)]+ (O-O)/ broadening in the Fo-rich samplesand narrowing in the [Si-o(1)];R2 : [Si-o(2)]/[Si-o(1)]+ (o-o)/[Si-o(2)]; R3 : [Si- Mo-rich samples.For example, at the forsterite end, the o(3)l/[si-o(l)]+ (o-o)/[si-o(3)]. 433.5-cm'band has a half width of ll.5 cm-', and it MOHANAN ET AL.: RAMAN SPECTRA OF FORSTERITE-MONTICELLITE 47

increasesby sevenwavenumbers with the addition of l6 molo/oCa (Fig. a). On the other hand, in the Mo-rich end, its half width decreasesby one wavenumber with the ad- dition of 8 molo/oCa. The samebehavior is observedwith E ' the 544-cm band. This unusual behavior can be ex- c plained on the basis of order-disorder phenomenain the =12 crystal. Positional disorder in binary olivine is causedby r the occupancy of larger cations, such as Ca (ionic radii I t.0 A;, in the smallerMl site.In forsterite,the two struc- tural sites M I and M2 are of the same size. In monticel- 5 10 15 2080 85 90 95 100 lite, the M2 site is larger than the Ml site. By preference, Focomposition (mol%) the larger cation (Ca) fills the M2 site and the smaller Fig. 4. Variationof half width for two major low-frequency cation (Mg) the Ml site. For this reason, monticellite is bands,433.5 arrd 544 cm r. In the Fo-richcompositions the a highly ordered mineral. The addition of Ca to forsterite steepslope is dueto the substitutionof largercations in the Ml affectsits crystal structure in two ways: the occupancyof site(see text). either site by Ca distorts the crystal structure and hence the SiOotetrahedra and the presenceof Ca in both M I and M2 sitesintroduces positional disorder into the crys- The mixing betweenthe r, and /3 modes is correlatedwith tal structure. On the other hand, in monticellite, the M2 the following structural parametersof olivine: the respec- site is already big enough to accommodate the Mg ion tive bond length and the ratio ofthe averagebond length without affecting the crystal structure, but it does intro- of the O-O bond to the correspondingSi-O bond length. duce disorder. For these reasons,the addition ofCa into In the low-frequency region, the A,, and Bre modes the forsterite-rich composition causes considerable in- show significantbroadening on substitutingCa2* for Mg2*, creasein the half width of the Raman bands, whereasin indicating that some of the Ca2t ions occupy the M I site. the monticellite end, addition of Mg causesonly a slight The substitution of Mg in the Mo crystal, however,causes increasein the half width. Adams and Bishop (1985)de- only small variation in the half width of the low-frequen- termined the presenceof a maximum of I loloCa in the cy modes, indicating a smaller degreeof Ca-Mg disorder Ml site in the Fo-rich samples and 2o/oCa in the Mo- near the Mo end. rich samples.A comparison of the observedchange (Fig. In summary, the present study has shown that Raman 4) in the slope of the bands of SiOo near the Fo end to spectroscopynot only is a sensitive tool for examining those of minerals near the Mo end further indicates that distortion of SiOo tetrahedra but also provides informa- Fo-rich olivines exhibit higher (Ca,Mg) disorder. Disor- tion about cation disorder in crvstals. der in general tends to decreasethe frequencies of the Raman bands. AcxNowr,sncMENTS In silicate solid solutions, we expected to observe a The authors thank Li-Chung Ming negativeexcess-volume (Newton and David Muenow for helpful dis- effect and Wood, 1980). cussionand commentsand Diane Hendersonfor editorial assistanceThis The data of Adams and Bishop (1985),however, show a work was in part supported by NSF grant EAR-8915830 S K S. would nearly linear relationship between the unit-cell volume like to rhank the Earth ScienceEquipment Program ofNSF for providing and composition acrossthe given end-members.The ab- partial support for the micro-Raman spectroscopicfacility at the Univer- senceof such excess-volume sity of Hawaii. Critical reviews of an early version of this manuscnpt by effectsin these samplesim- Ann Hofmeister plies and Paul McMillan are very much appreciated.This is that the substitution of the larger Ca cation at the SchoolofOcean and Earth Scienceand Technologycontribution no. 2985. Ml site in the Fo end causesthe volume to increaselin- early. It is, however, evident from Figure 2 that the fre- RnrrnnNcns ct:tro quencies of the high-frequencybands of SiOoare not lin- Adams, G.E., and Bishop, F.C. (1985) An experimental investigation of early dependent on unit-cell volume along the Fo-Mo thermodynamic mixing propertiesand unit-cell parametersof forster- JOrn. ite-monticellite solid solutions American Mrneralogist,7O, 7 l4-7 22 Biegar, G M., and O'Hara, M.J (1969) Monticellite and forsterite crys- CoNcr,usrous talline solutions. Joumal of the American Ceramic Society, 52, 249- 252. The Raman spectraof crystalline samplesalong the Fo- Brown, G E. (1980) Olivines and silicatespinels. In Mineralogical Society Mo join show well-defined bands. Analysis of the bands of America Reviewsin ,5,275-381 in the high-frequency (700-1000 cm ') region provide Bums, R.G., and Huggins, F.E. (1972) Cation determinative curves for information about the mixing of z, Mg-Fe-Mn olivines from vibrational spectra.American Mineralogist, and z, internal modes 57,967-985. of SiOotetrahedra. The 964 cm-r band of Fo, which has Chopelas,A. (1991) Single crystal Raman spectra offorsterite, fayalite, a pure v, eharacter,decreases linearly from Fo to Mo, and monticellite.American Mineralogist, 76, I l0l-l109. indicating decreaseddistortion ofthe SiOotetrahedra. The Cooney,F T , and Sharma,S.K ( I 990) Structureofglasses in the systems 824 and 855 cm-' bands of Fo do not show linear rela- MgrSiOn-Fe,SiOo,MnrSiOo-Fe,SiOn, MgrSiOrCaMgSiOo, MnrSiOo- CaMnSiO". of Non-Crystalline 122, tionships with composition join, Journal Solids, 10-32. along the most likely Farmer, V.C., Ed. (1974) The infrared spectraof minerals, 539 p. Min- becauseof the mixed u, and v, character of these bands. eralogicalSociety, London. 48 MOHANAN ET AL.: RAMAN SPECTRA OF FORSTERITE-MONTICELLITE

Huggins,F.E. (1973) Cation order in olivines: Evidence from vibrational Rao, K.R., Chaplot, S.L., Choudhury, N., Ghose, S., and Price, D.L. spectra Chemical Geology, I 1, 99-109 (198?) Phonon density ofstates and specificheat offorsterite, MgrSiOo. Iishi, K. (1978) Lattice dynamics of forsterite. American Mineralogist, Science,236, 64-65. 63,I 198-1208. Servoin, J.L., and Piriou, B. (1973) Infrared reflectivity and Raman scat- Lam, P.K., Yu, R., Lee, MW., and Sharma,SK. (1990)Relationship tering of Mg'SiO. singJecrystal. PhysicaStatus Solidi, 55 B,677-686. betweencrystal structure and vibrational mode in Mg'SiOr. American Sharma,S.K. ( I 989) Applications ofadvanced Raman spectroscopictech- Mineralogist,75, 109-I l9 niques in earth sciences.In H.D Bist, J.R. Durig, and J.F. Sullivan, Newton, R.C., and Wood, B.J. (1980) Volume behavior of silicate solid Eds., Raman spectroscopy:Sixty years on-Yibrational spectra and solutions. American Mineralogist, 65, 733-7 45 structure,p. 513-568 Elsevier,New York. Paques-I-edent,M.Th., and Tarte, P. (1973) Vibrational studies of oliv- Sharma,S.K., and Urmos, J.P. (1987)Micro-Raman spectroscopicstudies ine-type compounds. I. The i.r and Raman spectra of the isotopic of materialsat ambient and high pressureswith cw and pulse lasers.In speciesof Mg,SiOo.Spectrochimica Acta, 29A, 1007- l0 16. Roy H. Geiss,Ed., Microbeam analysis,p. 133-136. San Francisco Piriou, B., and McMillan, P. (1983) The high frequencyvibrational spec- Press,San Francisco. tra of vitreous and crystalline orthosilicates.American Mineralogist, Tarte, P. (1963) Etude infra-rougedes orthosilicateset des orthogerman- 68,426-443. ates.II Structuresdu type olivine et monticellite. SpectrochimicaActa, Price, D.G., and Parker, S.C. (1984) Computer simulations of the struc- t9,25-47. tural and physical properties of the olivine and spinel polymorphs of MgrSiOo.Physics and Chemistry of Minerals, 10, 209-216. Price, G.D., Parker, S.C.,and Maurice, L. (1987) The lattice dynamics of Menuscnrsr RECEIvEDApnlr 20, 1992 forsterite. Mineralogical Magazine, 5I, | 57- 170. MeNuscr.rsr ACCEPTEDAvcvst 29, 1992