Single crystal Raman spectra of forsterite, fayalite, and monticellite

Polarized single crystal Raman spectra of the fundamental modes offorsterite (Mg2Si04), olivine (M&.88,Feo.12),fayalite (Fe2Si04), and monticellite (CaMgSi04) are presented. Seven low energy modes «450 cm-I) in forsterite differ from those of all previous studies. All modes predicted by symmetry for forsterite and monticellite were observed; 34 out of 36 modes were observed for fayalite. Assignment of the modes was determined by systematics in frequency changes resulting from cation substitution. Although the lattice modes are generally mixed, likely mode assignments are made using the following observations. The lowest energy modes are assigned to Si04 translations and appear to be mixed with the cation translations. The lattice modes between 330 and 435 cm-I in forsterite that changed the least with composition were assigned as Si04 rotations. The modes from 300 to 390 cm-I that showed the greatest variation with composition or showed signs of two-mode behavior in the olivine Mg88were assigned as M2 translations. The Si04 internal stretching and bending modes were assigned to the highest frequencies; they vary little from forsterite to monticellite to fayalite and appear to depend more on cation mass than volume. These assignments are consistent with previous single-crystal infrared studies of forsterite and fayalite and with mode Griineisen parameters 'Yimeasured vs. pressure in that the highest 'Yivalues should be associated with the M2 cations and the lowest with the Si04 internal modes. INTRODUCTION important modes are those below 450 cm-I, as the estiThe vibrational modes of olivines, important geophysmat~s of heat capacity and entropy vs. tem~~rature at ical and cosmochemical phases, have been studied extenambIent temperatures and below are very sensItIve to the sively by Raman spectroscopy (Iishi, 1978; Servoin and number of these mo~es and their distribution. These are, Piriou, 1973; Piriou and McMillan, 1983; Stidham et aI., h,owever, the most dIfficult to measure because of exten1976; Hohler and Funck, 1973; Chopelas, 1990), infrared ~IV~over~apping: breadth ofp~aks a~~ t~o-mode behavspectroscopy (Hofmeister, 1987. Tarte 1963. Paqueslor In sohd solutIons, and low IntensItIes In Raman specLedent and Tarte, 1973; Kovach 'et aI., 1'975; Hofmeister tra because ~f the low polarizabilities of the octahedral et aI., 1989; plus some references given above for Raman structural unIts. spectroscopy), inelastic neutron scattering (Rao et aI., T~e polarized ,Raman spectrum of forsterite has been 1988), and lattice dynamical calculations (Price et aI., prevl0.u~ly descnbe~ (~ohler and Funck, 1973; Servoin 1987a, 1987b). Study of these properties yields insight and Plnou, 1973; IIS~I, 1978), bu~ none, of the results into the interatomic forces within the crystal structure agree for the ~r~qUencies o,f the low IntensIty modes. The and a basis for understanding the effect of cation substionly other ohvlne for whIch a (nearly) complete singletutions on the thermodynamic properties. For example, crystal, Rama~ spectrum has been des~ribed is tephroi,te the heat capacity and entropy can be estimated to better (Mn2SI04) (StIdham et aI., 1976); partIal data are avallthan 50/0over a moderate temperature range (, , 500 K, a~l~ for Ca-beari~g phases (e.g., ~ohler and Funck, 1973; Kieffer, 1979) from the mode frequency information ob~lnou and McMIllan, 1983). ThIS study was prompted, tained by infrared and Raman spectroscopy. The accuIn part, by the general lack of Raman data in the low racy in estimating these quantities can increase to better frequency range for other olivines and, in part, by disthan 0.5% over a larger temperature range (> 1000 K) if crepancies with the previously reported data for forsterite all of the infrared and Raman modes are enumerated and (Chopelas, 1990). assigned to atomic motions. The detailed mode assignIn this study, single-crystal polarized Raman spectra of ments provide an accurate method for estimating the freforsterite, olivine (Mgo.88,Feo.12)2Si04,monticellite, and quencies of inactive or unobserved modes, thereby yieldfayalite are presented. Mode assignments to molecular ing more precise information on the frequency distribution motions are made by comparison of analogue modes in or density of states (see Hofmeister, 1987; Hofmeister these olivines and previous results of tephroite (Stidham and Chopelas, 1991a; Chopelas, 1990, 1991). The most et aI., 1976) and 'Y-Ca2Si04 (Piriou and McMillan, 1983). 0003-004X/91/0708-1101$02.0