Vibrational Spectroscopy of Mantle Minerals

Home , Silicate perovskite

Mineral Spectroscopy: A Tribute to Roger G. Burns © The Geochemical Society, Special Publication No.5, 1996 Editors: M. D; Dyar, C. McCammon and M. W. Schaefer

Vibrational spectroscopy of mantle minerals

2 PAUL F. McMILLAN",2 RUSSELLJ, HEMLEy ,3 and PHILIPPEGILLET2 'Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, U,S.A 2Laboratoire de Sciences de la Terre, Ecole Normale Superieure des Sciences, 46 allee d'Italie, 69365 Lyon Cedex 07, France 3Geophysical Laboratory and Center for High Pressure Research, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington D.C. 20015-1305, U.S.A.

Abstract- Vibrational spectroscopy has been used extensively to characterize the high pressure minerals important in the Earth's mantle. The results of IR and Raman studies complement structural information obtained from diffraction experiments, for example in providing information on [6]- coordinated silicon in high-pressure phases such as stishovite and MgSi03 perovskite, on the tetragonal distortion of MgSi03 garnet, and on the presence of the inter-tetrahedral SiOSi linkage in /3_ Mg2Si04• Such measurements provide detailed information on phase transitions and tests of ab initio and empirical calculations of mineral properties. In situ studies at high pressure and/or temperature have been used to study soft-mode behaviour associated with displacive phase transitions, as demonstrated recently for stishovite. The presence of OH in natural minerals and synthetic candidate high- pressure phases has been demonstrated by IR spectroscopy, permitting the identification of potential storage sites for hydrogen in the mantle. In situ high P-T Raman spectroscopy has demonstrated the stability of magnesite (MgC03) to mantle temperatures and pressures, important in determining the carbon budget within the deep Earth. Micro-Raman and (more recently) micro-IR spectroscopies have proven very useful for non-destructive, in situ analysis of minerals in complex natural and experimental sample assemblages. Experimental vibrational data have been used to construct and constrain models for calculation of thermodynamic parameters such as heat capacity and entropy. These have been useful in establishing P-T phase diagrams for mantle mineral assemblages. We summarize these results and collect together vibrational spectra obtained to date for principal mineral phases of the mantle. We also discuss both limitations and future directions of the techniques.

INTRODUCTION grains and in thin sections of multiphase natural samples as well as in experimental charges OUR KNOWLEDGEof the internal structure of the quenched from high pressures and temperatures. Earth has been largely gleaned from analysis of Finally, vibrational data have been used exten- seismic and other geophysical data, bounded by sively for modeling the thermodynamic properties cosmological constraints on its composition (AN- of mantle minerals. This has been an important part DERSON,1989; POIRIER, 1991; GILLET, 1995; BINA of this research because such vibrational data in and HEMLEY, 1996). Some direct sampling of man- many cases have provided the first, and in some tle minerals has been possible, from examination cases the only, constraints on the thermodynamic of ultramafic xenoliths brought to the surface in properties of such materials at high P-T conditions kimberlites or alkali basalts, or as inclusions in corresponding to the deep Earth. diamonds (SAUTTER and GILLET, 1994), However, We review here the uses of Raman and infrared most of our present understanding of mantle miner- vibrational spectroscopy in the study of mantle alogy is based on experimentally determined phase minerals, We begin with an overview of the kinds relations and thermophysical properties of candi- of information that can be obtained from such mea- date minerals and assemblages. Vibrational spec- surements and their relevance to the Earth's man- troscopy has played an important role in this work tle, We discuss in some detail the vibrational prop- (McMILLAN and HOFMEISTER, 1988; McMILLAN, erties of the major mantle minerals, summarizing 1989). Such measurements provide detailed infor- the available vibrational data for each phase. Our mation on structure and bonding in materials, motivation has been to collect the spectra obtained thereby complementing direct structural studies by to date in a single set of figures, for convenience of diffraction techniques and spectroscopy of elec- comparison and for identifying areas where further tronic transitions. Vibrational spectroscopy pro- work is needed. This then extends and comple- vides a straightforward means for identifying phase ments earlier and more general reviews of the ap- transitions, as well as the mechanism of such trans- plications of Raman and IR spectroscopy to Earth formations (e.g. soft modes and order parameters). and planetary sciences (REMLEY et al., 1987a; Minerals can be analyzed both as microscopic McMILLAN and HOFMEISTER, 1988; McMILLAN, 175 176 P. F. McMillan, R. J. Hemley and P. Gillett

1989; SHARMA, 1989, 1990). We also discuss vi- CO2 and F-, Cl- and S-bearing species degassing brational spectroscopic studies of the role of water from deep within the Earth, or returning via sub- and carbon in the mantle. A brief description of duction processes (THOMPSON, 1992; BELL and future directions is given in the concluding re- ROSSMAN, 1992a; GASPARIK,1993; GILLET, 1993a; marks. The review is limited to crystalline miner- CARROLLand HOLLOWAY,1994). In addition, this als, Vibrational spectroscopy has demonstrated that region is characterized by partial melting, so that large structural changes take place in silicate the properties of molten silicates are important in glasses at high pressures (and by inference melts), the upper mantle, and perhaps even deeper within but this is beyond the scope of the present article the Earth (WILLIAMS, 1990; ITO and KATSURA, and is reviewed elsewhere (McMILLAN and WOLF, 1992; WOLF and McMILLAN, 1995). The transition 1995). We also discuss results of theoretical studies zone is dominated by the mineralogy of refractory of the vibrational dynamics of mantle minerals; ultramafic phases including the ,B-(wadsleyite, or excellent reviews of theoretical techniques and the "modified spinel") and y-(ringwoodite, spinel) calculation of other properties have been written forms of (Mg,Fe)zSi04, along with the garnet and by COHEN (1994), BUKOWINSKI(1994), and CHlZ- perhaps ilmenite phases of MgSi03 (GASPARIK, MESKHYAet al., (1994), 1990). Both MgSi03 ilmenite and garnet contain [6]-coordinate silicon, and at least the majorite gar- GENERAL CONSIDERATIONS net will exist as a solid solution with Mgz+, Fe2+ and Ca2+ substituting on the large ([12]-coordinate) Overview of mantle mineralogy 4 H site, Al3+, Si + and Fe on the octahedral site, and The global structure of the mantle is defined by Si4+ and AlH on tetrahedral sites (IRIFUNE, 1994; its major seismic discontinuities near 410 km and KESSONet al., 1994). All of these nominally anhy- 660 km, which delineate the regions known as the drous mantle minerals can contain significant upper mantle (20-410 km), transition zone (410- amounts of dissolved H as "defect" OH species 660 km), and lower mantle (660-2900 km) (Fig. (BELL and ROSSMAN, 1992a; GILLET, 1993a). In 1). Based on extensive laboratory studies and theo- addition, several high pressure hydrous phases, par- retical modeling over the past two decades, it is ticularly within the system MgO-SiOz-H20, likely that the dominant mineral phase in the lower may be present in the transition zone (KANZAKI, mantle is a silicate perovskite with composition 1991; PREWITTand FINGER, 1992). near (Mg., 9Feo.l)Si03, probably containing up to several per cent Alz03 component, which co-exists Structure and bonding with (Mg,Fe)O (magnesiowustite) with the rock salt (Bl) structure, and a nearly end-member Although it is not a technique for direct structure determination, vibrational spectroscopy gives im- CaSi03 perovskite (WILLIAMSet al., 1989; ANDER- SON, 1989; HEMLEY and COHEN, 1992; IRIFUNE, portant insight into bonding in materials and struc- 1994). Free silica may be present as stishovite in ture-property relations, thereby complementing basaltic regions trapped near the top of the lower x-ray and neutron diffraction methods. Morever, mantle from subducted oceanic crust (KESSON et vibrational spectroscopy has revealed transitions al., 1994), which could give rise to aluminous and structural changes that were not apparent from phases (IRIFUNE et al., 1991; YAGI et al., 1994; diffraction measurements (KINGMA et al., 1995), AHMED-ZAID and MADON, 1995). A free SiOz although in other cases, such claims have not been phase near the base of the lower mantle has also supported by subsequent, and more accurate, direct been proposed (KNITTLEand JEANLOZ,1991; JEAN- structural investigations (e.g., DOWNSet al., 1996). LOZ, 1993). The mineralogies of the transition zone Measurements as a function of pressure and/or tem- and upper mantle are likely to be more complex perature permit detailed identification of response (ANDERSON,1989; GASPARIK,1990; GILLET, 1995). of the structure to these thermodynamic variables The upper mantle is dominated by ultramafic min- ti.e., changes in compression mechanisms in the erals, principally olivine, pyroxene and garnet absence of a thermodynamic phase transition). Vi- phases, but substantial amounts of plagioclase feld- brational spectroscopy has played an important role spar, kaersutitic amphibole, phlogopite mica, and in characterization of mantle phases since the earli- many other accessory minerals may be found. The est days of experimental mantle petrology and min- petrology is highly dependent on the type of prov- eralogy. Infrared spectroscopy confirmed the octa- ince encountered, and the thermal and composi- hedral coordination of Si (or Ge in silicate tional regime involved. Many of these phases are analogues) in several of these high pressure phases volatile-rich, both providing host phases for water, (LYON 1962; TARTE and RINGWOOD 1964; WENG Vibrational spectroscopy of mantle minerals 177

MINERALOGICAL MODEL MINERALOGICAL SAMPLING BY XENOLITHS AND PRESSURE AND TEMPERATURE OF THE MANTLE INCLUSIONS IN DIAMONDS CONDITIONS

PERIDOTITE 3 GPa, 1100 °C 'OPx .. "Px OLIVINE PYROXENES . Gt ::~Gt .,(Mg,Fe)2Si04 (Mg,Fe)Si03 ·01 ... '01 (Ca,Mg,Fe)2Si206 ... Px " GARNETS QGt: (Mg, Fe,Ca)3AI2Si3012 . 01' .. :~~ .01

Mj 410 Ion l13 GP" 1400 'C f3-PHASE (Mg,Fe)2Si04 MAJORITE ~ Mj tlnsition zone _ (Mg, Fe,Ca)3(AI,Si)2Si3012 T PEROVSKITE 520km y-PHASE CaSi03 -'1- - --,-----

1600°C _ ~I {1\Jf~-:~:--I 'r ?

PEROVSKITES MAGNESIOWUSTITE (Mg,Fe,AI)Si03 (Mg,Fe)O Mean chemical composition of the mantle in atom % o 58.4 ~ inclusions Mg 20.4 ~ in diamonds Si 15.9 Fe 2.2 xenoliths o Al 1.8 Ca 1.3

CORE

FIG. 1. Schematic (from GILLET, 1995) of the mineralogy of the Earth's mantle, inferred from results of high pressure-high temperature experiments, and natural sampling via xenoliths and inclusions in diamonds.

et al., 1983). In other cases, vibrational spectros- Another example is the identification and charac- copy was used to reveal unusual structural features, terization of ring structures in minerals such as such as the presence of the SiOSi linkage between coesite [SHARMAet al., 1981]. This has been im- tetrahedral Si04 groups in the ,B-("modified" spi- portant in determining compression mechanisms in nel, or wadsleyite) polymorpb of Mg2Si04 (JEAN- glasses as well as amorphization phenomena (HEM- LOZ, 1980; McMILLAN and AKAOGI, 1987). LEY et al., 1994; McMILLAN and WOLF, 1995). 178 P. F. McMillan, R. J. Hemley and P. Gillett

Vibrational spectroscopy has also been essential detailed IR mapping of complex samples with spa- for characterizing structural components such as tial resolution approaching the diffraction limit. FE! hydrogen (bound stoichiometrically or as defects) et al. (1991) used the micro-Raman technique to which are difficult to study by other techniques, as distinguish ,8- from y-(Mg,Fe)Si04 in a study of discussed below. the phase relations in the MgO-FeO-Si02 system. ZHANG et al. (1993) identified Si02 phases in experimental changes quenched from high P-T to Phase transitions determine the phase relations in the system to Because vibrational spectra typically provide a 2800°C at 9-14 GPa. More recently, BERTKA and unique signature of crystal (and amorphous) struc- FE! (1996) used the technique to identify a large ture, the measurement of such spectra has proven number of different silicates in a high-pressure extremely useful for identifying phase transitions, phase equilibrium study of the mineralogy of the particularly in situ at high pressures and/or temper- martian mantle. atures. Moreover, such measurements can be used Vibrational spectroscopy, in particular micro- successfully to determine phase transitions mecha- Raman spectroscopy, has been useful for the analy- nisms. A classic example is the study of soft modes sis of mineral specimens in thin section, in some and the identification of order parameters associ- cases providing identification that was not possible ated with displacive phase transitions (SCOTT, by standard petrographic techniques (MAO et al., 1974; GHOSE, 1985, 1988; McMILLAN, 1985, 1989; 1987). These techniques have played a role in McMILLAN and HOFMEISTER,1988; SALlE, 1989). studying natural mantle xenoliths as well as deep This includes both pressure-induced soft modes crustal samples. GILLET et al. (l984) used Raman (dvldP < 0) as well as the more commonly studied spectroscopy to identify coesite inclusions within temperature-induced soft modes (dvldT < 0). Both pyrope grains in metasedimentary rocks from sub- types of modes are known to occur for materials ducted continental crust (CHOPIN, 1984), placing with the perovskite and rutile structures as func- constraints on the P-T history of the assemblage. tions of pressure and temperature (SAMARA Similarly, identification of coesite in a clinopyrox- and PEERCY, 1973; SCOTT, 1974), including the ene from Norwegian eclogites demonstrated that

MgSi03 perovskite analog CaTi03 (GILLET et al., the rock formation pressure exceeded 3 GPa 1993a,b). Early ab initio calculations suggested (SMITH, 1984; BOYER et al., 1985). Micro-Raman that such critical transitions might be important for spectroscopy has been used to identify coesite in

MgSi03 perovskite under mantle P-T conditions eclogites from central China (WANG et al., 1989). (WOLF and BUKOWINSKI,1987; BUKOWINSKIand SOBOLEVand SHATSKY(1990) and SHUTONGet al. WOLF, 1988). However, no soft mode behaviour (1992) used the technique to identify diamond crys- has been observed experimentally in the Raman tals included in zircons and garnets, in high grade

spectrum of metastable MgSi03 perovskite at am- (> 100 km) metamorphic rocks. The technique was bient pressure (DURBEN and WOLF, 1992), consis- also applied to study exsolution of garnet from tent with more recent theoretical calculations clinopyroxene in high pressure rocks (MALEZIEUX, (STlXRUDEand COHEN, 1993), as discussed below. 1990), and to investigate eclogitic garnets included A displacive phase transition associated with in diamonds (Lm et al., 1990). Infrared spectros- pressure-induced mode softening has been identi- copy has been useful in the study of fluid inclusions fied by Raman spectroscopy in stishovite, which in diamonds (NAVON et al., 1988), and was used undergoes a transition to a CaCl, structure at ~50 to demonstrate the presence of solid CO2 in one GPa (COHEN, 1992; KINGMA et al., 1995). sample, fixing the entrapment depth at 220-270 km (SCHRAUDERand NAVON, 1993). Vibrational spectroscopy of solid inclusions in diamonds are Phase identification and multiphase assemblages giving further valuable information on the volatile An important recent application is the identifica- content of the mantle (D. R. BELL, unpublished). tion of minerals in experimental samples, in particular, in polyphase assemblages quenched from high Characterization of impurities and defects pressures and temperature. Micro-Raman has been used in most of these studies because of its high Vibrational spectroscopy has also been very use- spatial resolution (~ 1 µm or the diffraction limit of ful for the identification of impurities, such as dis- visible light) (HEMLEYet al. 1987a). More recently, solved H in nominally anhydrous mantle minerals. new micro-IR spectroscopic techniques have been Quantitative infrared absorbance measurements on developed (REFFNER et al., 1994), which permit nominally anhydrous minerals such as olivines, py- Vibrational spectroscopy of mantle minerals 179

roxenes and garnets have revealed the presence of generally been taken into account by using a quasi- often substantial (up to 0.1 wt%) OH contained harmonic Cp-Cv correction (Cp = C; + TVa2KT) in defect sites within the structure (see BELL and from the measured bulk modulus and thermal ex- ROSSMAN, 1992a), with implications for the possi- pansivity (KIEFFER, 1979b; 1985). This approach ble water content of the mantle. The OH sites in ignores the effects of intrinsic mode anharmo- hydrous phases in the system MgO-Si02-H 0 2 nicity; i.e., variations in vibrational mode frequen- have been characterized by Raman and infrared cies with temperature at constant volume due to spectroscopy (AKAOGIand AKIMOTO, 1986; FINGER phonon-phonon interactions (GILLET et al. 1989a, et al., 1989; McMILLAN et al., 1991), and the im- 1990, 1991, 1992, 1993a; HEMLEY, 1991; STIX- portance of hydrogen-bonding in stabilizing such RUDE and HEMLEY, unpublished). LIU (1993) has high pressure hydrous phases has been discussed in fact suggested that such anharmonic effects are (WILLIAMS, 1992). These materials may store large not present for minerals. However, there is ample amounts of water within the mantle. SMYTH(1987) experimental and theoretical evidence for the exis- has suggested that the wadsleyite (,8-) phase of tence of intrinsic mode anharmonicity in many Mg2Si04 might provide an important storage site minerals (GERVAIs et al., 1972, 1973; GERVAISand for H in the mantle. This possibility has been con- PIRIOU, 1975; GERVAIS, 1983; WOLF and JEANLOZ, firmed by vibrational spectroscopy (McMILLAN 1984; REYNARDet al., 1992; WINKLER and DOVE, and HOFMEISTER, 1988; McMILLAN et al., 1991; YOUNG et al., 1993). Quantitative IR absorbance 1992), although anharmonic effects do tend to de- measurements have been used recently to investi- crease with increasing pressure (e.g., HEMLEYand gate the OH content of the a-, ,8- and y-phases of GORDON, 1985). These anharmonic contributions permit the high temperature heat capacities to ex- Mg2Si04 as a function of composition and synthesis pressure by KOHLSTEDTet al. (1996). Finally, ceed the Dulong-Petit limit, and cause the C, values the H content and structural position of OH groups calculated within the quasiharmonic approximation contained in defect sites in synthetic stishovite and to be underestimated by several per cent at mantle (Mg,Fe)Si03 perovskite have been examined using temperatures (RICHET et al., 1992). The magnitude such techniques (PAWLEY et al., 1993; Lu et al., of anharmonic corrections can be evaluated experi- 1994; MEADE et al., 1994). mentally by separately measuring vibrational mode frequencies at high temperature and high pressure (SAMARA and PEERCY, 1973; MAMMONE and Thermodynamic properties SHARMA, 1980; GILLET et al; 1989a; 1990). How- Thermodynamic modeling based on vibrational ever, it is also found that intrinsic anharmonic cor- spectroscopic data has played an important role in rections to Cp do not substantially affect the calcu- the development of our present knowledge of the lated phase boundaries over a wide temperature properties of mantle minerals. A systematic and range (AKAOGIet al., 1987; NAVROTSKY,1989; FEI highly successful approach to this problem for min- et al., 1990; YUSA et al., 1993; AKAoG! and ITO, erals was developed by KIEFFER (1979a,b,c, 1980, 1993a), and differences between calculations and 1982, 1985). This method permits the heat capacity experiment are likely to be associated with other and vibrational entropy of a mineral phase of inter- uncertainties (e.g., free energies of formation, ther- est to be calculated from a model density of states mal expansivity). The effects of intrinsic mode an- function, constructed from experimental infrared harmonicities are important for calculating the and Raman data, along with acoustic velocities. high-temperature heat capacity and isotopic frac- This application spurred many measurements of tionation between minerals, properties that depend the IR and Raman spectra of high pressure silicates strongly on vibrational anharmonicity (GILLET et (AKAOGI et al., 1984; NAVROTSKY, 1985, 1989; al., 1996). McMILLAN and Ross, 1987; HOFMEISTERet al., There have been suggestions to use experimen- 1989; FEI et al., 1990; CHOPELAS, 1990a,b, tally determined vibrational frequencies to estimate 1991a,b; HOFMEISTER and CHOPELAS, 1991a,b; mineral elastic properties such as the bulk modulus, HOFMEISTERand ITO, 1992; CHOPELASet al., 1994). since KT is proportional to Lv[IVI/3 in the quasihar- The resulting thermodynamic models have proven monic approximation (HOFMEISTER,1991). The ap- useful in establishing phase diagrams consistent proach involves a large number of approximations with direct phase equilibrium data (AKAOGI et al., for materials beyond highly symmetric structures 1984, 1989; NAVROTSKY, 1985, 1989; FEI et al., but has nevertheless has been applied to several 1990; SAXENAet al., 1993). mantle phases (HOFMEISTER,1991; Lu et al., 1994). In these calculations, anharmonic effects have LIU (1992) has suggested estimating the bulk mod- 180 P. F. McMillan, R. J. Hemley and P. Gillett ulus from the pressure shift of an easily observable McMILLAN and HOFMEISTER,1988; CHOPELASand vibrational mode; however, this approach suffers BOEHLER,1992), so that not only mode frequencies from an unfortunate circularity because it requires a and relative intensities are available for comparison prior determination of the mineral compressibility. between theory and experiment, but also their Such applications of vibrational spectroscopy may changes with pressure and temperature. Recent de- be useful for exploring possible relationships be- velopments in ab initio techniques permit calcula- tween (high-frequency) optical modes and elastic tion of mineral lattice dynamics and thermophysi- properties (low-frequency acoustic modes). How- cal properties with increasing accuracy (COHEN, ever, their predictive value may be questioned in 1994; BUKOWINSKI,1994), and it appears that we view of the fact that high P- T elastic properties can are entering a new era of sophistication in modeling now be measured directly with higher accuracy structure and dynamics of minerals under mantle (ANDERSONet al., 1993; DUFFYet al., 1995). More- P-T conditions. Infrared and Raman spectra ob- over, first-principles theoretical calculations in tained in situ at high pressures and temperatures many cases are more reliable (e.g., COHEN, 1992). are invaluable for evaluating and constraining the quality of these calculations.

Density of states and lattice dynamics LOWER MANTLE MINERALS Perhaps of most concern in the construction of (Mg,Fe)Si0 perovskite vibrational models for thermochemical calculation 3 is the very incomplete nature of the vibrational data Extensive studies of (Mg,Fe)Si03 perovskite set available from infrared and Raman spectros- have been performed in recent years because of its copy. These techniques give information on vibra- importance for lower mantle mineralogy (NAVROT- tional modes at the Brillouin zone centre, and gen- SKY and WEIDNER, 1989; HEMLEY and COHEN, erally give little or no insight into dispersion of 1992). (Mg,Fe)Si03 perovskite was first synthe- vibrational frequencies (McMILLAN, 1985). In ad- sized by Ltu (1974, 1975) and characterized in dition, many zone center modes may not appear in detail in subsequent years (see HEMLEYand COHEN, IR or Raman spectra due to selection rules imposed 1992). KNITTLEand JEANLOZ(1987) showed exper- by crystal symmetry, and others may not be imental evidence for its stability at P-T conditions observed experimentally due to low intensity corresponding to most of the lower mantle, indicat- (McMILLAN and HOFMEISTER,1988). Because the ing that the phase may be the dominant mineral in heat capacity and entropy are obtained as integrals the Earth. The structure and thermodynamic prop- over the entire vibrational density of states g(w), erties of this phase are reviewed by WILLIAMS et the result is a vibrational model for calculating al. (1989) and by HEMLEYand COHEN (1992). The these which may be poorly constrained by experi- infrared spectrum of a sample of pure MgSi03 per- ment. In principle, this problem can be solved by ovskite, synthesized in the diamond anvil cell, was directly determining g(w) from inelastic neutron obtained by WENG et al. (1983), and the Raman scattering; such measurements gives Sew), the vi- spectrum was first recorded by WILLIAMS et al. brational density of states weighted by an atomic (1987). The number of IR-active modes, and the participation factor in each vibration, and its neu- appearance of a first-order Raman spectrum, con- tron scattering cross section (e.g., GHOSE, 1985, firmed the distortion from cubic symmetry of this 1988). This technique, however, requires large phase (Fig. 2). samples (typically a few grams), and is not usually Perovskite with the ideal cubic structure (Fm3m) possible for high pressure silicates, especially in would have three IR-active modes and no Raman situ at high pressure. Alternatively, lattice dynam- spectrum: from symmetry analysis, ics calculations may be carried out using either

ab initio (first-principles) or empirically derived lvib = 3F1u(lR) + F2uCinactive). (1) interatomic potentials (CATLOW and PRICE, 1990.; COHEN, 1994; BUKOWINSKI, 1994). Both ap- Within such a hypothetical cubic structure, the proaches have been applied successfully to mantle highest frequency mode involves asymmetric minerals. The empirical calculations require exper- stretching of the octahedral groups (Si06), the mid- imental data as input, which may include vibra- frequency vibration corresponds to a deformation tional spectroscopic data. It is of particular interest vibration of the octahedra, and the lowest fre- that both IR and Raman spectra of minerals can quency IR mode involves translational motions of be obtained readily at high pressures and (most the alkaline earth cation in the dodecahedral site. recently) high temperatures (REMLEY et al., 1987a; Based on systematic infrared spectroscopy of other Vibrational spectroscopy of mantle minerals 181

(b)

MgSi0 perovskite 3 78K (a)

0.91 "n"':I"\~~. I 0.6

0.7

.?:' 0.6

;::"> 0.5 o ~ 0.4 200 300 400 500 600 700 t!:.- 0.3 Raman shift (em-I) 0.2

0.1 -ocy 0.0 1 Il 0.0 C 60 -0.5 .2 VV\ '0 50 C I III I I ~ -1.0 :J u, 40 " 0 j Ilil II ~ -1.5 L: 30 '0 CIl 20 f £2 II. II ~ -2.0 Qi "" s 10 ~ / l " I \ I II I- -2.5

0 -3.0 0 200 400 600 800 1000 1200 250 SOO Frequency, crn-t Raman shift (em-I)

FIG. 2. (a) Infrared and (b) Raman spectra of MgSi03 perovskite. The infrared reflectivity data are redrawn from Lu et al. (1994; Fig. 1, p. 11, 798). The peaks and minima in the dielectric functions E2(V) and Im(lh(v» (middle left) indicate the positions of the TO and LO mode frequencies, respectively (Lu et al., 1994). The Raman spectrum at top is redrawn from DURBEN and WOLF (1992; Fig. 2, p. 891), for a sample cooled to 78 K. The spectrum at bottom (WILLIAMSet al., 1987) shows the detail of the low-frequency region, to 30 cm ",

cubic perovskites, and calculations for hypothetical (WILLIAMS et al., 1987; WOLF and BUKOWINSKI, cubic MgSi03 perovskites, these modes would be 1987; HEMLEYet al., 1989). expected to occur in the regions 750-1000 cm-1, 1 The mid-IR transmission spectra exhibited bands 500-700 cm- , and 250-400 cm ", respectively with maxima at 797,683,614 and 544 cm " (WENG (WILLIAMS et al., 1987; HEMLEY et al., 1987b; Lu et al., 1983; WILLIAMS et al., 1987). MADON and et al., 1994). Reduction of the symmetry to ortho- PRICE (1989) observed low frequency peaks at 390, rhombic Pbnm with Z = 4 units of MgSi0 in the 3 347, 320 and 282 cm -I, and also suggested splitting unit cell causes splitting of the IR-active modes, in some of the higher frequency bands. The powder the occurrence of additional peaks in the IR spec- transmission minima correspond quite well with trum, and the appearance of a first order Raman spectrum: TO (transverse optic) mode frequencies obtained by Lu et al. (1994) from their single crystal and Cib = 7AiR) + 7B1g(R) + 5B2g(R) powder reflectivity studies of pure MgSi03 and (Mgo.9Feo.I)Si03 samples, and several additional + 5B3g(R) + 8Au(inactive) + 7B1u(JR) peaks were recognized in this study (Fig. 2). The + 9B2u(IR) + 9B3u(IR) (2) frequencies of longitudinal (LO) modes were also 182 P. F. McMillan, R. 1. Hemley and P. Gillett deduced from the reflectivity data. WILLIAMSet al. descent in symmetry associated with the Brillouin (1987) observed Raman peaks at 251, 280, 378 and zone folding arising from R- and M-point instabili- 499 cm ", which likely correspond to four of the ties within the cubic aristotype, so that the Raman seven expected Ag vibrations. Additional weak frequencies are particularly sensitive to details of modes at 536 and 990 cm " were observed by HEM- the calculation (HEMLEY and COHEN, 1992; CHIZ- LEY et al. (1989). The best resolved spectrum ob- MESHYAet al., 1994; BUKOWINSKIet al., 1996). tained (at 78 K) to date is that of DURBEN and These modes are also particularly sensitive to po- WOLF (1992), who observed eleven peaks between tential symmetry changes in the perovskite struc- 249 and 666 cm " (Fig. 2). In neither the IR nor ture with pressure and temperature (HEMLEYet al., the Raman spectra have all the expected modes 1987b; McMILLAN and Ross, 1988; WOLF and Bu- been observed, nor their symmetries reliably as- KOWINSKI,1987). In contrast to the approximate signed experimentally (DURBEN and WOLF, 1992; ionic model calculations, vibrational frequencies Lu et al., 1994). This should be pursued in further predicted from full electronic structure methods (e.g., linearized augmented plane wave) give much studies, for the following reasons. First, the vibrational data have proved useful in better agreement with experiment as demonstrated calculations of the specific heat and vibrational en- for stishovite, as discussed below (KINGMA et al., tropy of this phase (NAVROTSKY, 1989; AKAOGI 1995). Although calculations at this level have and ITo, 1993b; GILLET et al., 1993a; Lu et al., been performed for MgSi03 perovskite (STIXRUDE 1994). These calculated properties have helped to and COHEN, 1993; WENTZCOVITCHet al., 1993), determine and rationalize the negative P-T slopes the vibrational frequencies have not been reported. of reactions forming perovskite from lower pres- The lowest lying modes in the Raman spectrum sure phases (NAVROTSKY, 1980; AKAOGI and ITO, are associated with coupled rotations of the linked 1993a). Of critical importance in determining the Si06 octrahedra, and these are expected to be ex- heat capacity and entropy are the positions of the tremely sensitive to the degree of orthorhombic lowest frequency vibrations of this phase. The low- distortion of the perovskite structure; in particular, est frequency Raman mode found by DURBEN and they should soften if MgSi03 perovskite undergoes WOLF (1992) occurred at 249 ern" " and no obvious a displacive phase transition to tetragonal or cubic additional modes were found down to 30 cm " in structures at high temperature (WOLF and JEANLOZ, the study by WILLIAMS et al. (1987) (Fig. 2). Lu 1985; WOLF and BUKOWINSKI,1987; BUKOWINSKI et al. (1994) found the lowest lying IR mode to and WOLF, 1988; HEMLEY et al., 1989; SALJE, occur at 180 cm ", although their data do not abso- 1989). Accurate calculations indicate a wide stabil- lutely rule out the possibility of features at -lower ity field for the orthorhombic form, at least for wavenumber. Further careful IR and Raman spec- the MgSi03 endmember (HEMLEY et al., 1987b; troscopy in the region below 200 cm " will be D' ARCO et al., 1993; STIXRUDEand COHEN, 1993; required to fully resolve this question. WENTZCOVITCHet al., 1993; BUKOWINSj<:Iet al., Information on the low-lying vibrational modes, 1996). In situ X-ray diffraction measurements along with their dispersion across the Brillouin show that the orthorhombic form remains stable zone, is readily obtained from lattice dynamics cal- over the P-T range of the lower mantle explored culations. HEMLEYet al. (1989) used a first princi- so far experimentally (MAO et al., 1989; FUNAMORI ples method based on the modified electron gas and YAGI, 1993; WANG et al., 1994; YAGI et al., approach, allowing for charge relaxation via spher- to be published). The possibility of such critical 2 transitions still exists (WANG et al., 1992; MEADE ical breathing of the 0 - anions (COHEN et al., 1987), to calculate the structure and vibrational et al., 1995; BUKOWINSKIet al., 1996), because the entire P-T range of the lower mantle has not been properties of MgSi03 perovskite. There was good agreement between some of the calculated mode explored experimentally with in situ techniques, frequencies and experimental values, but in general nor have detailed experiments been carried out for such approximate calculations are not expected to Fe-bearing perovskite. This could be tested by car- be able to reproduce the measured frequencies. For rying out Raman spectroscopy in situ under simul- example, because such ionic model calculations taneous high P-T conditions, via laser heating in underestimated the degree of distortion from the the diamond anvil cell (GILLET et al., 1993b). high-symmetry cubic structure, the frequencies of In the IR spectrum, the bands between 770 and the lower octahedrallibrational modes were under- 950 cm ? observed by Lu et al. (1994) could be estimated. Likewise, the frequencies of some of the assigned to the 1/3 asymmetric stretching vibration higher modes are overestimated. The occurrence of the Si06 octahedra (Bill + 2B2u + 2B3ll symmet- of the first-order Raman spectrum results from the ries) from lattice dynamics calculations (HEMLEY Vibrational spectroscopy of mantle minerals 183

et al., 1989), and bands in the 600-725 cm " and zone center vibrational modes «I') = LCvi'Y/LCvh 380-540 cm " regions to the 1/4 and 1/6 deformation Yi ). where C is the contribution of each mode to Cy vibrations (Bj, + 2B2u + B3u> and 2B1u + B2u Experimentally, HEMLEY et at. (1989) found fre- + B3u, respectively) (Fig. 2). Five modes (B1u quency shifts ranging from 1.7 to 4.2 cm-1/GPa, for + 2B2u + 2B3u) are expected in the low frequency seven Raman active modes measured to 26 GPa. region (~250-350 cm ") due to Mg2+ translation, CHOPELAS and BOEHLER (1992) found slightly and additional low frequency modes will arise from smaller shifts (1.37-3.28 cm-1/GPa) for the same acoustic lattice modes, through folding of the Bril- modes observed over a greater pressure range (to louin zone from the ideal cubic structure (2Blu 47 GPa). These values correspond to high mode + 2B2u + 2B3u) (WILLIAMS et al., 1987; WOLF Gruneisen parameters (REMLEYand COHEN, 1992). and BUKOWINSKI,1987; HEMLEYet al., 1989). The Measurements of the anharrnonicity of the material behavior of these low. frequency translational under lower mantle conditions could be obtained .modes is of interest, because they determine the from analysis of the Raman linewidths and infrared ferroic character of many perovskite materials reflectivity spectra under high pressure and high (SCOTT, 1974). In their calculation of CaSi03 pe- temperature conditions (GERVAIs, 1983; GILLET et rovskite during decompression, HEMMATI et al. al., 1993c). In situ high P-T vibrational spectro- (1995) showed that a low frequency mode associ- scopic experiments could also yield information on 4 ated with off-centre Si + displacements within the melting (and possible pre-melting) behaviour of Si06 octahedra became unstable, contributing to silicate perovskite (MATSUI and PRICE, 1991; the amorphization phenomenon observed in this WINKLER and DOVE, 1992; WASSERMAN et al., phase at or near ambient pressure. This type of low 1993; BELONOSHKO,1994; RICHETet al., 1994), of frequency mode would result in an increased far- considerable importance for mantle dynamics (VAN IR reflectivity due to the large static dielectric con- KEKEN et al., 1994). stant, Eo (SPITZER et al., 1962; GRZECHNIKet al., The temperature dependence of the Raman spec- 1996). No such behaviour is indicated for MgSi03 trum has been investigated at ambient pressure by perovskite: the far-IR reflectivity at low frequency DURBEN and WOLF (1992) (Fig. 3). These authors is quite "normal" (~0.35; Fig. 2) (Lu et al., 1994). found no observable shift in the lowest frequency However, this should be investigated more fully in modes (the 249-254 cm-I doublet), which might further experiments. In a recent theoretical study, be expected to be extremely sensitive to changes BUKOWINSKIet al. (1996) have found that the low- in orthorhombic distortion, and only a small tem- est lying ferroic mode frequency for MgSi03 per- perature dependence (~ - 0.001 em -IlK) for the ovskite lies above 300 cm " at 50 GPa, and only other modes. An obvious conclusion is that any softens near 3500 K, above the predicted instability critical behaviour in perovskite must lie at very temperatures for the soft modes associated with high temperature, especially at high pressure (DUR- octahedral rotational motions. BEN et al., 1991; STIXRUDEand COHEN, 1993; Bu. Vibrational spectroscopy at high pressure and KOWINSKIet al., 1996). DURBEN and WOLF (1992) ambient temperature has been used to determine also observed that the metastable perovskite phase vibrational mode Gnmeisen parameters ('Yi = -d In transformed irreversibly to an amorphous material I/Jd In V), for comparison with the thermodynamic above approximately 300°C. During the transfor- Grtineisen parameter 'Yth (=aKTVICv)' WILLIAMSet mation, the Raman peaks of the remaining perov- al. (1987) found frequency shifts (dI/JdP) of 2.6 skite material were found to be shifted irreversibly cm-1/GPa for four IR-active modes between 500 to higher wavenumber, as if the crystal had been and 850 cm" measured to 27 GPa, giving 'Yi in placed under compression by the presence of the the range 1.15-1.45. Using assumptions for non- glass (DURBEN and WOLF, 1992). This illustrates observed vibrational modes, they estimated an av- the problems inherent in studying the material out- erage value of l' ~ 1.9 ± 0.2, close to the experi- side its stability field (HEMLEYand COHEN, 1992). mental value 'Yth = 1.77 obtained from the results The behavior of the vibrational spectra of silicate of KNITTLE et al. (1986). Recent "best" estimates perovskites during amorphization has been exam- for this parameter at ambient density include 'Yth ined theoretically (HEMMATIet al., 1995; see also = 1.7-1.9 (HEMLEY and COHEN, 1992); 1.3 ± 0.2 HEMLEY and COHEN, 1992). (WANG et al., 1994), and 1.5 ± 0.2 (ANDERSONet To date, most vibrational studies have been fo- al., 1995). In the theoretical study by HEMLEY et cused on end-member MgSi03 perovskite. Lu et al., (1989), the value of 'Yth was estimated to be al. (1994) and WANG et al. (1994) obtained the 2.0 at ambient temperature, decreasing to 1.59 in infrared and Raman spectra of (Mg,Fe)Si03 perov- the high temperature limit, by summing over all skites, and found little change in the spectra with 184 P. F. McMillan, R. J. Hemley and P. Gillett

properties of CaSi03 perovskite, in part because MgSi0 perovskite this phase tends to amorphize upon decompression. 3 In situ IR would be of particular interest to observe the behaviour of the far-IR reflectivity as a function of pressure, to give information on the low-lying ferroic mode as the sample is decompressed (HEM- MAT!et al., 1995; BUKOWINSKIet al., 1996). HEM- MATIet al. (1995) have proposed that this vibration, which involves off-centre motions of the Si4+ ions within their octahedral sites, plays a role in the amorphization process. It is possible to recover at ambient pressure perovskites synthesized under

pressure along the CaSi03-CaTi03join (RINGWOOD and MAJOR, 1967, 1971). LEINENWEBER et al. (1994, 1996) have recently obtained Raman and

infrared spectra for CaSi03-CaTi03 perovskites synthesized at high pressure. This system forms an orthorhombic or tetragonal solid solution between

CaSi03 and 50 mol% CaTi03, and a new ordered compound at the Ca2SiTi06 composition. Strong Raman bands appear near 750 and 450 cm " as the silicate content is increased, which can be assigned to Si-O stretching and OSiO bending vibrations associated with the Si06 octahedral groups. 150 250 350 450 550 650 -1 Raman shift (em ) (Mg,Fe)O magnesiowiistite

FIG. 3. Raman spectrum of MgSi03 perovskite as a MgO and FeO have the cubic NaCl (Bl) struc- function of temperature at ambient pressure, taken from DURBEN and WOLF (1992; Fig. 1, p. 891). At high temper- ture, which has no first order Raman spectrum. The ature, the perovskite bands begin to weaken, and are re- zone centre optic mode has symmetry placed by broad bands due to MgSi03 glass (causing the rise in intensity to high and low frequencies), as the meta- (3) stable perovskite reverts to an amorphous phase. This triply degenerate vibration is actually split into transverse (2 TO) and longitudinal (LO) components by the macroscopic electric field associated Fe2+/Mg2+ substitution, except that the Raman with the vibration (FERRARO, 1984; McMILLAN, peaks were broadened, indicating some structural 1985; McMILLAN and HOFMEISTER,1988). The in- disorder. Additionally, the Raman frequencies frared reflectance spectrum under ambient condi- were shifted slightly to lower wavenumber, consis- tions shows a broad reflectance band extending 1 tent with the small lattice expansion (KUDOH et al., from ~350 cm- to ~750 cm ? with low- and high 1990; MAO et al., 1991; WANG et aI., 1994). frequency limits determined by the TO and LO frequencies (PIRIOU, 1964, 1974; PIRIOU and CA- BANNES, 1968). The pure absorption frequency, CaSi03 perovskite which would be visible in thin film measurements, Phase equilibrium studies, together with con- would give rise to a single peak near 380 cm ", However, additional features are often observed in straints from cosmochemical constraints, indicate 1 powder measurements in the 550-650 cm- re- that CaSi03 perovskite is the next most abundant silicate in the lower mantle (HEMLEY and Co- gion, due to vibrational mode coupling, and particle HEN, 1992; IRIFUNE, 1994). It was first synthe- size effects (LUXON et al., 1969; PIRIOU, 1974; sized by LlU and RINGWOOD (1975). Unlike McMILLAN, 1985). The characterization of the thermophysical prop- (Mg,Fe)Si03 perovskite, this phase appears to re- main cubic over the entire P-T range of the lower erties of (Mg,Fe)O solid solutions is crucial for mantle (MAO et al., 1989; HEMLEY and COHEN, understanding the lower mantle (ANDERSONet al., 1992; BUKOWINSKIet al., 1996). To date, there 1992, 1993). There has been a large number of has been no experimental study of the vibrational theoretical studies of the lattice dynamics of MgO Vibrational spectroscopy of mantle minerals 185

under lower mantle conditions (COHENet al., 1987; peaks than expected from the symmetry analysis. AGNON and BUKOWINSKI,1990; BURNHAM, 1990; However, the powder spectrum closely resembles BUKOWINSKI,1994; CHIZMESHYAet al., 1994; Bu. the polycrystalline IR reflectance spectrum ob- KOWINSKIet al., 1996): however, the frequency of tained by HOFMEISTERet al. (1990) (Fig. 4). Analy- the LO modes is typically overestimated in these sis of the reflectivity data gave the zone centre TO studies because of constraints imposed on the elec- modes at ~650 cm " (Azll), and 470, 580 and 820 1 tronic relaxation associated with the vibration. The cm- (Ell)' These frequencies do not correspond zone centre frequency fixes the upper limit of the simply to the positions of transmission minima in vibrational spectrum. The IR spectra at ambient the powder spectrum, because of the variation in conditions are well understood (PIRIOU, 1964; optical constants due to overlapping modes of dif- BREHAT et al., 1966; FARMER, 1974; FERRARO, ferent symmetry with large TO-LO splitting (HOF- 1984). The infrared reflectance spectrum of MgO MEISTERet al., 1990). The reflectivity data are noisy has been studied at high temperature, giving infor- in the low-frequency region, and do not constrain mation on the anharmonicities of the lattice modes the IR spectrum well in the region below 700 cm ", (PIRIOU, 1964, 1974; PIRIOUand CABANNES,1968). WILLIAMS et al. (1993) measured the pressure To date, there has been no direct study of the IR dependence of peaks in the powder IR transmission spectrum at high pressure. The far IR spectra of spectrum of stishovite. Two bands were followed several halides with the Bland B2 structures have to approximately 36 GPa, with pressure shifts of been studied at high pressure in the diamond anvil 2.4 and 3.3 cm-I/GPa, respectively. The two bands cell (FERRARO, 1984), and this type of study could followed have absorbance maxima at 611 and 861 be extended to magnesiowUstite in future studies. cm " at room pressure, and were assigned to two CHOPELAS and NICOL (1982) noted that the side of the Ell modes observed by HOFMEISTERet al. 2 H bands present on the V + or Cr luminescence for (1990). As noted by WILLIAMSet al. (1993), these these ions doped in MgO contained information on frequencies do lie close to (within 40 cm ") of the the vibrational density of states of the host oxide TO frequencies determined in the reflectivity study: crystal, due to vibronic coupling between the elec- however, this region of the stishovite spectrum is tronic excitation of the transition metal ion and the considerably complicated by overlap between Ell lattice vibrations of MgO. They used these data to and the AZll modes, and resulting interference ef- construct a density of states function g(w) for MgO, fects (HOFMEISTERet al., 1990) (Fig. 4). Based on and to calculate thermodynamic properties of this the resemblance between the powder reflectance phase within the quasiharmonic approximation, up and the transmission data (HOFMEISTERet al., 1990; to 10 GPa. CHOPELAS(1990a, 1992) has revised WILLIAMS et al., 1993), it is most likely that the and extended these studies to over 20 GPa. transmission minima (inverted to give absorbance maxima, via A(v) = -loglO T(v») contain contribu- Stishovite tions from both TO and LO components, of both Ell and AZllmodes in the case of the lower frequency Stishovite with the rutile structure is the high bands. However, the pressure shifts measured by pressure phase of SiOz, stable above 9 GPa (STI- WILLIAMS et al. (1993) give information on the SHOV and POPOVA, 1961). The natural occurrence average Grtmeisen parameter for these modes. The of this mineral was first described as a product of high pressure behaviour of the IR spectrum should impact metamorphism of quartz in sandstone be subjected to further analysis, especially because (CHAO et al., 1962). However, its high pressure of the impending transition to the CaClz-structured stability means that it could be present throughout phase at ~50 GPa, as discussed below. the mantle, being formed in disproportionation re- The Raman spectra of natural and synthetic stish- actions between silicate minerals and oxides (HEM- ovite have been reported by HEMLEYet al. (1986a) LEY et al., 1994; KINGMA et al., 1995). The and HEMLEY(1987) (Fig. 4). The results of earlier expected vibrational modes for the stishovite struc- studies were complicated by the introduction of ture are: impurities due to the extraction process of stishovite from natural samples and the presence of glassy material: this only became apparent once the

+ B2g + Eg(R) + 2B1uCIR) + 2EueIR) (4) spectrum of synthetic samples had been recorded (HEMLEYet al., 1986a; HEMLEY, 1987). The Raman A powder transmission spectrum of stishovite sep- spectrum shows all four expected modes: A1g at arated from a natural sample was first recorded by 753 cm ", B2g at 967 cm ", Eg at 589 cm ", and Big (LYON, 1962). This spectrum showed many more at 231 cm ", HEMLEY(1987) measured the pressure 186 P. F. McMillan, R. J. Hemley and P. Gillett

STISHOVITE RAMAN STISHOVIT!'

0.... SYNTHETIC

~ 0.035 Z etO.OlO b .....111°_ u, 111 0.020 II: 'i!- 0.015

0.00'

0.000 (b) 0.' NATURAL 0.' 111 0 0.7 Z c( 0.' I- 0 O.S .....111 U. 111 II: 0' 0'1- 0.2

0.1 200 400 600 800 0.0 ,,.. '00 ...... WAVENUMBERS. CM""' WAVE NUMBERS

(a) (b)

FIG. 4. (a) Infrared and (b) Raman spectra of Si02 stishovite. The infrared reflectance spectrum is taken from HOFMEISTERet al. (1990; Fig. I, p. 952). The experimental (unpolarized) polycrystalline spectrum is shown at top, along with the TO and LO mode frequencies obtained from analysis of the reflectivity data. The synthetic spectrum at bottom was reconstructed using these parameters, for both mode symmetries (A2u and Eu)' The Raman spectrum at right is from HEMLEY et al. (1986a). The four expected modes are clearly visible in the spectrum of the synthetic sample at top. The

natural sample (bottom) shows broad bands corresponding to amorphous Si02•

shifts of these Raman active vibrations. The be- slope of the transition, which has not yet been determined experimentally). A further transition to a havior of the low-lying Big mode is particularly interesting, because its frequency decreases with modified fluorite or Pa3 phase is predicted to occur increasing pressure (Fig. 5). The atomic displace- at Earth core pressures, above ~ 150 GPa (COHEN, ments associated with this mode correspond to 1994; HAINES et a/., 1996). those required to transform stishovite from the ru- GILLET et al. (1990) measured the temperature tile into the calcium chloride structure (HEMLEY, dependence of the Raman active modes of stisho- 1987; NAGEL and O'KEEFFE, 1971). Recent theo- vite at ambient pressure, and calculated the heat retical (COHEN, 1992; LACKS and GORDON, 1993; capacity and entropy of this phase via vibrational MATSUI and TSUNEYUKI, 1992) and experimental modeling. In this study, the pressure- and tempera- (TSUCHIDAand YAGI, 1989, 1990; KINGMA et al., ture derivatives of the Raman peaks were used to 1995, 1996) work has shown that the rutile struc- estimate the intrinsic anharmonicity of the vibra- ture transforms to a phase with the calcium chloride tional modes, and to obtain a high temperature structure. From theoretical calculations of the vi- heat capacity corrected for anharmonicity, beyond brational mode frequency with increasing pressure, the quasiharmonic approximation. In the high- this transition was predicted to occur near 50 GPa temperature study, broad bands were observed to (COHEN, 1992), in remarkable agreement with the appear due to back-transformation of the high pres- recent experimental result of KINGMAet al. (1995) sure phase into glass dominated by tetrahedrally (Fig. 5). This work indicates that free SiOz would coordinated silicate groups (GILLET et al., 1990; GRIMSDITCHet al., 1994). These broad glass bands exist in the CaCl2 structure below 1200-1500 km within the lower mantle (depending on the P-T are also present in the Raman spectrum of natural Vibrational spectroscopy of mantle minerals 187

• 0 Raman 1200 + PIBH LAPW

1000

.c> 800 ~ ] (f) ~ 600 ~ Rutile c-o, Structure Structure 400~

35.1 BIg

Eg Big I I ~ 2001- ~ ..- ::::::_DV' Ag II .'.' (a) (b) "S:,...... _. .' ~l-Q",j13.5 I o~ 160 200 240 600 640 680 720 0 20 40 60 80 Raman shift (crrr l) Raman shift (crrr+) Pressure (GPa)

FIG. 5. (a) Raman spectra from KINGMA et al. (1995) showing evidence for the rutile-Cafll, transition in Si02 at high pressure. The smooth lines under the observed bands are Lorentzian fits for the expected fundamentals of both phases (KINGMA et aI., 1995). The broad band marked * is not predicted by theory, and is dependent upon the stress state of the sample. The low frequency Big peak for the rutile form softens with increasing pressure. Above the transition pressure (~50 OPal, this becomes the Ag mode of the CaCl2 structure, and its frequency begins to increase with further compression. (b) Frequency shifts of Raman bands of stishovite and CaClz-structured Si0 2 with pressure at ambient temperature, during compression (filled circles) and' decompression (open circles). The experimental data are compared with results of theoretical calculations, using first- principles LAPW (COHEN, 1992: full line) and parametrized PIB (potential induced breathing: dashed line) methods.

stishovite (HEMLEY et al., 1986a). The phenome- marks the 410 km seismic discontinuity (BINA and non of pressure- and temperature-induced amorphi- WOOD, 1987; AKAOGI et al., 1989; KATsuRA and zation of Si02 phases has been studied extensively ITO, 1989; MENG et al., 1993; MORISHIMAet al., using vibrational spectroscopy (HEMLEY, 1987; 1994). The Raman and infrared spectra of the ,B_ GILLET et al., 1990; WOLF et al., 1992; WILLIAMS and v-polymorphs of Mg2Si04 were first reported et al., 1993; GRIMSDITCHet al., 1994). by AKAOGI et al. (1984), who used the vibrational data to construct model density of states functions TRANSITION ZONE MINERALS g( w) for specific heat and entropy calculations. This ,B- (Mg,Fe)2Si04 information was then combined with calorimetric data to establish a P-T phase diagram for the Along with the spinel (1') polymorph discussed Mg2Si04 polymorphs (AKAOGI et al., 1984). This below, a (Mg,Fe)2Si04 polymorph with the wad- study has been superseded by more recent work sleyite, or "modified spinel" , (,8) structure is likely (AKAOGI et al., 1989; GASPARIK,1990; CHOPELAS, to form one of the dominant constituents of the 1991 b), but it represented the first application of mantle transition zone. The transition from the low KIEFFER'S (1979a,b,c) approach to mantle pressure olivine (a-(Mg,Fe)2Si04) to the ,B-phase mineralogy. occurs at approximately 13 GPa at ~ 1700 K, and The powder IR spectrum of ,B-Co2Si04 was ob- 188 P. F. McMillan, R. J. Hemley and P. Gillett

0.7 I tained by JEANLOZ(1980). Unlike the a-(olivine) (a) ~-(Mg,Fe),SiO. and 'Y-(spinel) polymorphs of M2Si04 phases, this 0.6 spectrum showed a peak at 686 cm ", associated O.S with a bending vibration of the SiOSi linkage be- .~ .:! 0.4 tween tetrahedral groups. Similar features at 700 !l and 675 cm" appear in the IR spectrum of ,B- <;: '0.3 ~ Mg2Si04 (AKAOGI et al., 1984; WILLIAMS et al., 0.2 1986; CYNNand HOFMEISTER,1994) (Fig. 6). These 0.1 weak but characteristic bands reveal the presence of the unusual Si207 units in the structure (JEAN- o 200 400 600 800 1000 1200 LOZ, 1980; HORIUCHI and SAWAMOTO, 1981), Wavenumber (an") which require the presence of oxygen bound only to Mg atoms in the ,B-polymorph. This structural (b) feature is important for the ability of this nominally anhydrous phase to accept large quantities of H into its structure (SMYTH, 1987; DOWNS, 1989), discussed further below. The expected vibrational modes for ,B-Mg2Si04 are:

+ 12B3g(R) + 7Au(inactive) + 14Blu(IR) ~

+ 13B2u(IR) + 11B3u(IR) (5)

The Raman spectrum of ,B-Mg2Si04 originally re- 1200 1000 800 600 400 200 ported by AKAOGI et al. (1984) was incorrect, be- Wavenumbers cause the sample partly back-transformed during heating by the laser in the Raman experiment. The (c) Raman spectrum for this phase was later obtained by McMILLAN and AKAOGI (1987) and by CHO- PELAS (1991b). In both of these studies, a strong peak (most likely of Ag symmetry) was observed at 723 cm", due to the SiOSi linkage vibrations, (Fig. 6). REYNARDet al. (1996) have recently found some differences with the previous spectra. They did not reproduce a peak found at 86 cm -1 by CHOPELAS(1991b), which is important for defining the low frequency limit of the density of states function for heat capacity and entropy calculations 250 500 750 1000 (AKAOGI et al., 1984; CHOPELAS, 1991b), and they Raman shift (em 1) did not observe the weak peaks in the 1000-1100 cm " region reported by McMILLAN and AKAOGI FIG. 6. Infrared (a,b) and Raman (c) spectra of /3- (1987). REYNARDet al. (1996) have suggested that (Mg,Fe)2Si04' The infrared reflectance spectrum (a) is these peaks could be due to Si30lO units, created redrawn from CYNN and HOFMEISTER (1994; Fig. 1, p. 17,718). The weak peaks associated with the SiOSi link- locally within the structure associated with the ages in the structure are more easily distinguished in the presence of stacking faults in the structure (BREAR- powder transmission spectrum, from AKAOGIet al. (1984) LEYet al., 1992). If this assignment is correct, Ra- (b). The Raman spectrum (c) was obtained by McMILLAN man spectroscopy could provide a powerful in situ and AKAOGI (1987). The weak peak at 1100 cm-' was method for characterizing the defect structure in not observed in the recent study by REYNARDet al. (1996), and may correspond to a structural defect in the sample ,B-Mg Si0 at high pressure and temperature (e.g., 2 4 (see text). with a diamond-anvil cell). The kinetics and mechanism of transformations

between (l'-, ,B- and -y-polymorphs of (Mg,Fe)2Si04 are important for understanding the seismicity Vibrational spectroscopy of mantle minerals 189

within subducting slabs (SUNG and BURNS, 1976; TER, 1994), in contrast to the case for stishovite Wu et al., 1993; BREARLEYet al., 1992; RUBIE and mentioned above. It has been noted that, as the BREARLEY, 1994; REYNARD et al., 1996). Raman complexity of the mineral phase increases and spectroscopy provides a useful technique for char- more peaks are present in the IR spectrum, the acterizing these structural changes (CHOPELAS, powder transmission spectrum gives a better repre- 1991b; DURBEN et al., 1993; REYNARD et al., sentation of the true TO mode frequencies, at least 1996). The accidentally back-transformed spec- for modes which are not strongly polar (such as trum of AKAOGI et al. (1984) resembled that of the Si-O stretching vibrations) (McMILLAN and Mg2Si04 olivine, but with considerably broadened HOFMEISTER,1988). The average mode Gruneisen bands. In a high temperature single crystal X-ray parameter was determined to be (I') = 1.28 ± 0.02, diffraction study of j3-Mg2Si04 at ambient pres- in good agreement with the thermodynamic value sure, TSUKIMURAet al. (1988) had observed anom- 'Yth = 1.05 ± 0.2 (WILLIAMSet al., 1986). REYNARD alous changes in the lattice parameters above et al. (1996) measured the temperature shifts of the 580°C which might be associated with a structural Raman peaks of j3-Mg2Si04 up to 800-900 K (the transformation. McMILLAN et al. (1991) obtained back-transformation temperature), and combined a Raman spectrum for a sample that had been the data with pressure shifts to determine intrinsic heated to 58SOC at ambient pressure. This spectrum mode anharmonicity parameters (GILLET et al., showed the broadened bands corresponding to a 1991). In her study of the Raman spectrum at high disordered olivine-like material found by AKAOGI pressure, CHOPELAS(1991b) observed changes in et al. (1984), along with weak broad bands in the the pressure shifts at 9.2 GPa and perhaps also 650-700 cm " region. These suggested the pres- at 17 GPa, which might indicate some structural ence of SiOSi linkages persisting within the transformation in j3-Mg2Si04. Similar changes in structure, presumably as the j3-Mg2Si04 back- slope are apparent in the IR data taken at high transformed to olivine via a series of spinelloid pressure (CYNN and HOFMEISTER,1994). Although intermediate structures (HYDE et al., 1982; MADON the possibility of weak structural distortions de- and POIRIER, 1983). REYNARD et al. (1996) have serves further study, no anomalies have been noted recently carried out a detailed kinetic study of this in X-ray diffraction measurements of j3-(Mg,Fe)z- transformation at ambient pressure via Raman Si04 to 26 GPa and 900 K (FEI et al., 1992). LIU spectroscopy. DURBEN et al. (1993) have used Ra- et al. (1994) has also investigated the Raman man spectroscopy to investigate the compression spectrum of this phase to 18 GPa at ambient mechanism in olivine (a-Mg2Si04) at room temper- temperature. ature, and observed changes which might be asso- Because the composition of j3-Mg2Si04 is that ciated with formation of SiOSi linkages within the of an orthosilicate, formation of the SiOSi linkage structure, perhaps accompanied by Si coordination in the high pressure phase results in one oxygen changes (GUYOT and REYNARD, 1992). Further per asymmetric unit being bound to only Mg atoms, compression of the olivine end-members at room in an irregular five-fold coordinated site (HORIUCHI temperature leads to amorphization, as shown by and SAMAMOTO,1981; SAWAMOTOand HORIUCHI, WILLIAMS et al. (1990) and RICHARD and RICHET 1990). SMYTH(1987) and DOWNS(1989) suggested (1990) for FezSi04 and by ANDRAULTet al. (1995) that this might provide a potential site for proton- for (Mg,Fe)2Si04. Further experiments using both ation of the nominally anhydrous mineral, so that Raman and infrared spectroscopy under mantle j3-Mg2Si04 could act as an important source or sink P-T conditions are likely to yield new information for water in the mantle (DOWNS, 1989; SMYTH, on the nature of the forward- and back-trans- 1987). McMILLAN et al. (1991) examined a syn- formations between the a-, j3- and v-phases of thetic sample of j3-Mg2Si04 using micro-Raman (Mg,Fe)zSi0 . 4 spectroscopy, and found that an asymmetric band WILLIAMSet al. (1986) measured the powder IR was present at 3322 cm ", due to OH groups within spectrum of j3-Mg2Si04 to 27 GPa at room temper- the structure (Fig. 7). Measurement of the ab- ature, and CYNN and HOFMEISTER(1994) have ob- sorbance via micro-infrared spectroscopy allowed tained the mid- and far-IR spectrum of a sample an estimate of the OH content as approximately with composition (Mg1.8oFeols)Si04, to ~23 GPa. 0.06 % by weight (as g OH per 100 g Mg2Si04: The spectrum of the Fe-containing sample is less this corresponds to ~ 10,300 Hl106 Si) (McMILLAN well-resolved, presumably due to Fe/Mg disorder. et al., 1991). These authors considered that this The peaks in the powder absorption spectrum cor- value represented a maximum degree of hydration respond quite closely with the TO modes deter- for the j3-Mg2Si04 phase, because other stable hy- mined from the IR reflectivity (CYNN and HOFMEIS- drous magnesium silicates were known to be pres- 190 P. F. McMillan, R. J. Hemley and P. Gillett

KOHLSTEDTet al. (1996) have recently prepared a (a) suite of (Fe,Mg)2Si04 samples from natural olivine at pressures and temperatures up to 15 GPa and 1100°C, and have found water contents ranging up to -2.4 wt% (400,000 Hl106 Si) via quantitative IR absorbance spectroscopy (Fig. 7). This is consistent with the recent determination of OH in j3-Mg2Si04 (to give an actual composition close to MgL7sHo.sSi04) via SIMS (INOU et al., 1995). KOH- LSTEDT et al. (1996) also documented a similar quantity of hydrous component dissolved in 1'-

3200 3400 3600 3800 (Fe,Mg)zSi04, prepared at 19.5 GPa and 1100°C. Raman shift (em-I) These results are extremely important for the mineralogy of the transition zone, and water storage (b) within the Earth. CYNN and HOFMEISTER(1994) have investigated the behaviour of the OH stretching vibrations in the (Mg,Fe)2Si04 sample prepared 100 by YOUNG et al. (1993), and have documented an increase in hydrogen bonding in this phase with 90 increasing pressure. Such behaviour could help in stabilizing hydrous phases at high pressure (WIL- 80 LIAMS, 1992; KNITTLEet al., 1992). w 70 'Y-(Mg, Fe)zSi04 0 Z 60 « a At pressures above 18-20 GPa for the tempera- CO cr: 50 ture range encountered within the transition zone 0 b of the normal mantle, j3-Mg2Si04 transforms fur- (/) ~ ther into a phase with the spinel structure, y- CO« 40 .., c MgzSi04 (AKAOGI et al., 1989; HIGDEN et al., 30 d 1991). Unlike the magnesian end-member, Fe2Si04 transforms directly from olivine into spinel at much 20 e lower pressure, -6-7 GPa, over the same tempera- 10 ture range. For this reason, the Fe content of the f (Mg,Fe),Si04 component is an important parameter 0 in determining the depth and width of the a-(j3)-'Y 3000 3200 3400 3600 3800 transitions occurring within the mantle (AKAOGIet FREQUENCY, cm-1 al., 1989; BINA and WOOD, 1987; KATSURA and ITo, 1989; Wu et al., 1993; GILLET, 1995). FIG.7. (a) Raman and (b) infrared spectra of /3-Mg2Si04 There have been several vibrational studies of in the 3000-3800 cm" region, showing evidence for the Mg Si0 and Fe2Si04 spinel end-members, and OH groups within the structure (McMILLANet al., 1991; 2 4 YOUNGet al., 1993). Samples (a-f) on the right were their solid solutions (Fig. 8, 9). The expected vibra- determined to contain up to 65,000 H/I06Si (YOUNGet tional modes for a cubic (Fd3m) spinel structure al., 1993). Spectra from YOUNGet al. (1993; Fig. 6, p. are: 415) used by permission of the editors of Physics and Chemistry of Minerals (Springer-Verlag GmbH). lvib = Alg(R) + Eg(R) + TIg(inactive) + 3T2g(R)

+ 2Azu(inactive) + 2Eu(inactive)

+ 4Tlu(lR) + 2T2uCinactive) (6) ent. From quantitative IR absorption spectra on single crystal samples of (Mgo.92,Feo.os)2Si04,prepared JEANLOZ(1980) obtained a powder IR absorption at 14 GPa and 1550-1650 K, YOUNG et al. (1993) spectrum of Fe2Si04 spinel, and observed three in- determined much higher OH concentrations, be- frared peaks at 848, 503 and 344 cm " (Fig. 8). tween 10,000 and 65,000 Hll06 Si. It was sug- The lower frequency peaks correspond quite well gested that these higher values could be due to the with the TO mode frequencies estimated from the presence of Fe in the sample (YOUNG et al., 1993). reflectance spectrum obtained by HOFMEISTER(see Vibrational spectroscopy of mantle minerals 191

(a) cal interatomic potentials, but the vibrational frequencies were not reported (MATSUI and BUSING, ~ 1984; PRICE and PARKER, 1984; PRICE et al., 1987). Il 8 AKAOGIet al. (1984) obtained a Raman spectrum for Mg2Si04 spinel, which showed the strong peaks expected for the symmetric (VI) and asymmetric j 4 (V3) stretching vibrations of the Si04 tetrahedra, in the 780-900 cm " region. A similar spectrum was obtained by GUYOT et al. (1986) for a natural sam- 1200 1000 800 600 400 200 ple of 'Y-(Mgo.74Fe026)2Si04, although an additional Wavenumbers weak peak was observed in that study at 844 em -1, (b) which has not been assigned (and could be due to coating from the microscope objective). The remaining weak peaks expected in the spinel spectrum were recorded by McMILLAN and AKAOGI

(a)

800 600 400 Wavenumbers

FIG. 8. (a) IR reflectance (obtained by A. HOFMEISTER, reported in McMILLAN and HOFMEISTER, 1988) and (b) powder transmission (JEANLoz, 1980) spectra for 'Y- Fe2Si04' Three of the expected four resonances are clearly visible in these spectra. Spectra from JEANLOZ(1980; Fig. 2, p. 331) used by permission of the editors of Physics and Chemistry of Minerals (Springer-Verlag GmbH).

1200 1000 800 600 400 200 Wavenumbers

McMILLAN and HOFMEISTER, 1988), and corre- (b) spond to the deformation vibration (V4) of the Si04 tetrahedra, and an octahedral Fe06 stretching mode, respectively. The highest frequency mode is attributed to the V3 asymmetric stretching vibration of the tetrahedral Si04 groups. This highly polar vibration is associated with a large TO-LO splitting, and gives rise to a broad reflectance band (Fig. 8). The upper limit of this band occurs near 960 cm ", which fixes the approximate upper limit in the density of states function, g(w). The powder IR absorption frequency lies close to the inflexion 1000 on the low-frequency side of the reflectance band; Raman shift (cm-l)

which corresponds to the TO mode frequency FIG. 9. (a) Infrared and (b) Raman spectra of 'Y- (McMILLAN and HOFMEISTER, 1988). The fourth Mg2Si04• The Raman spectrum shows all five peaks ex- mode expected in the IR spectrum has not yet been pected for the spinel structure. McMILLAN and AKAOGI observed. (1987) assigned the strong peak at 794 cm " to the Ag mode, and the 836 crn " peak to the T mode, expected AKAOGI et al. (1984) observed broad bands at 2g from the Si04 symmetric and asymmetric stretching vi- 830 and 445 cm " in their powder transmission IR brations. Based on results of a single crystal polarized spectrum for 'Y-Mg2Si04 (Fig. 9). The remaining study, CHOPELAS et al. (1994) have now reversed this infrared modes for Mg2Si04 have still not been assignment. The mid-IR transmission spectrum (a) was characterized. There have been several lattice dy- obtained by AKAOGI et at. (1984). This shows only two bands, instead of the expected four. Further work is re- namical calculations for 'Y-Mg2Si04 using empiri- quired to locate the additional resonances. 192 P. F. McMillan, R. J. Hemley and P. Gillett

(1987). In their unpolarized powder spectrum, et al. (1984) for the value calculated at room tem- l these authors assigned the strong peak at 794 cm- perature, the difference at 1000 K is only ~2%. This serves to illustrate the relative insensitivity of peak to the VI symmetric stretching vibration of the calculated high temperature heat capacity and the Si04 groups (Alg symmetry), and the 836 cm+ I entropy to details of the vibrational modeling. The band to the V3 asymmetric stretch with T2g symmetry, based on their relative intensities (Fig. 9). pressure shifts in the Raman spectrum of 1'- Based on oriented spectra of single crystals, CHO- Mg2Si04 were measured in the study by CHOPELAS PELAS et al. (1994) re-assigned these peaks, sug- et al. (1994) to 20 GPa. gesting that the symmetric stretching vibration cor-

responded to the higher frequency component (Fig. MgSiOJ ilmenite 10) (this has consequences for the Si-O stretch- The ilmenite phase of MgSi03 would be stable stretch interaction force constant of the Si04 tetrahedron: McMILLAN and HESS, 1990). These au- in a narrow P-T range within cold subducted re- thors used the Raman data to determine a model gions at approximately 600-700 km depth (ITO vibrational density of states for quasiharmonic cal- and YAMADA, 1982). It enters into solid solution culations of heat capacity and entropy. It is interest- with Al203 component (Lnr, 1977). There have ing to note that, although the calculated entropy been several studies of this phase by Raman and differs by ~ 10% from the earlier model of AKAOGI infrared spectroscopy, and by computer simulation

0.1

0.6· >< t: 0.5- ~ t; 0.4 J.l.l ,...l u... OJ J.l.l 0:: 0.2

0.1-'

(1.(1'" " ;.,,"\ 100 200 aoo 400 500 600 700 600 900 (000 1100 (b) FREQUENCY, cm'

(a)

0.8 I CALCULATED 0.7

U.G >-- b 0.5 t>

t; 004 w ...:J ~ 0.31 Eu 0:: . 0.2 .

0.1

(1.11. 300 400 500 600 700 800 900 100 300 500 700 900 lIOO FREQUENCY, crn-l Raman shift (em-I)

FIG. 10. (a) Infrared and (b) Raman spectra of MgSi03 ilmenite. The infrared reflectance spectrum is redrawn from HOFMEISTERandITO(1992; Figs. 1 and 2, p. 426. Used by permission of the editors of Physics and Chemistry of Minerals, Springer-Verlag GmbH). The unpolarized experimental spectrum is shown at top. The synthetic spectrum at bottom was reconstructed for both mode symmetries (Au and Eu). All expected modes are observed. The Raman spectrum at right is from McMILLANand Ross (1987). Recent careful measurements (REYNARD and RUBIE,1995) indicate that all indicated modes are real, including the weak shoulder at 499 em-'. Vibrational spectroscopy of mantle minerals 193

of its lattice dynamics. The expected vibrational considerably higher wavenumber than the true TO mode symmetries are: mode frequencies, as expected (McMILLAN and

rv;b = 5Ag(R) + 5Eg(R) + 4Au(IR) + 4Eu(IR) (7) HOFMEISTER,1988; HOFMEISTERand ITO, 1992). Lm et al. (1994) recorded the principal Raman The Raman spectrum of MgSi03 ilmenite was first peaks of MgSiOrilmenite to 30 GPa. A more com- recorded by Ross and McMILLAN (1984), who ob- plete study has recently been carried out by REy- served seven of the expected ten modes. A better NARDand RUBIE (1996), in which accurate pressure quality spectrum was obtained by McMILLAN and shifts were obtained for nine Raman modes, with Ross (1987), in which eight peaks were clearly (ov/oPh ranging from 1.7-3.7 cm-I/GPa. These resolved, and a further mode was apparent as a investigators also measured the temperature shifts shoulder on the band at 480 ern"! (Fig. 10). The of the Raman peaks (ov/oT)p.298 = -0.0148 to spectrum is dominated by an intense peak at 799 -0.0285 cm-llK), which permitted evaluation of cm ", assigned to an Ag mode associated with sym- the intrinsic mode anharmonicity parameters. metric Si-O stretching of the Si0 groups 6 These are all very small, indicating that MgSiOr (McMILLAN and Ross, 1987; HOFMEISTERand ITO, ilmenite behaves nearly quasiharmonically over the 1992). WALL and PRICE (1988) carried out a theo- temperature range studied. Because the anharmo- retical calculation of the lattice dynamics of nicity parameters for garnets are much larger (GIL- MgSi03 ilmenite using a polarizable ion (shell) LETet al., 1992), it has been proposed that intrinsic model with empirical potentials. These workers mode anharmonicity corrections might playa role used their results to predict the positions of the in determining the slope of garnet-ilmenite phase missing Raman peaks not observed by McMILLAN boundaries (REYNARDand GUYOT, 1994; REYNARD and Ross (1987), and to reassign the suggested and RUBIE, 1996). From their data, REYNARDand mode symmetries. However, HOFMEISTERand ITO RUBIE (1996) calculated an average value of the (1992) noted that all of the calculated vibrational Gruneisen parameter (defined as (1') = LCi'Y;TI frequencies were overestimated, and they made a Lev;, where 'Y;T = KT(oln v/o Ph) of 1.38 at room further re-assignment of the observed infrared and temperature, and 1.24 in the high temperature limit, Raman active modes. They suggested that the compared with the value of 'Yth = 1.74 (ambient "missing" Raman mode corresponded to an unre- P, T) which can be estimated from the available solved pair of peaks at 620 cm ", with Ag and E, thermodynamic parameters (ASHIDA et al., 1988; symmetry. However, REYNARDand RUBIE (1996) HOFMEISTERand ITO, 1992). have recently obtained a high quality Raman spec- McMILLAN and Ross (1987) used the structural trum for this phase, and find no evidence for split- analogy between MgSi03 ilmenite and Al203 co- ting in the 620 em -1 peak. rundum, for which the full vibrational spectrum MADONand PRICE (1989) first obtained a powder across the Brillouin zone was known from inelastic IR transmission spectrum for MgSi03 ilmenite. neutron scattering measurements, to construct a Unfortunately, these authors observed nine peaks model vibrational density of states g(w) for the between 282 and 825 em:", compared with the high pressure silicate phase, and calculated its vi- eight IR-active modes expected for the ilmenite brational heat capacity and entropy within the qua- structure. HOFMEISTERand ITo (1992) carried out siharmonic approximation. The calculated entropy infrared reflectance measurements for both poly- was combined with calorimetric data by ASHIDA crystalline and single samples of MgSi03 ilmenite, et al. (1988) to obtain phase boundaries between and observed all of the expected modes, although MgSi03 ilmenite, pyroxene, j3-Mg2Si04 + stisho- there may be some remaining uncertainty in the vite, and 'Y-Mg2Si04 + stishovite in a P-T range detailed analysis of the reflectance spectrum, be- appropriate for the transition zone. HOFMEISTER cause of noise in the data below 300 em -1, the and ITO (1992) have constructed an alternative occurrence of overlapping peaks, and the presence model for g( w) based on their infrared spectra and of weak additional oscillators, presumably due to the Raman data of McMILLAN and Ross (1987), combination bands (HOFMEISTER and ITO, 1992) combined with the lattice dynamics calculation of (Fig. 10). There is generally good agreement be- WALL and PRICE (1988). The calculated specific tween the spectra of MADON and PRICE (1989) and heat was in better agreement with the experimental the reflectance data of HOFMEISTERand ITO (1992), data of ASHIDA et al. (1988). except that the lowest frequency peak observed by REYNARD and RUBIE (1996) have recently ob- Madon and Price (282 cm ") is not reproduced, tained the Raman spectrum for a rhombohedral and the transmission minima ("absorption" max- phase with composition Mg3Al2Si3012 (correspond- ima) of the broad high frequency bands occur at ing to pyrope). This showed eight distinct peaks 194 P. F. McMillan, R. J. Hemley and P. Gillett which matched up well with those observed for 1971; OMORI, 1971; TARTE and DELIENS, 1973; DELANY, 1981; SUWA and NAKA, 1975; KIEFFER, MgSi03 ilmenite, indicating that the 3MgSiOr 1979b; DIETRICHand ARNDT, 1982; GEIGER et al., Al203 phase has an ilmenite structure, rather than the corundum structure expected for a fully- 1989; McMILLAN et al., 1989; HOFMEISTERand disordered material. A disordering transition be- CHOPELAS,1991a; PINET and SMITH, 1993). In par- tween the ilmenite and averaged corundum struc- ticular, HOFMEISTERand CHOPELAS(1991a) have tures might be expected at high temperature, as assigned the symmetries for the expected Raman discussed below for (Mg,Fe)Si03 garnet (majorite), and infrared active modes for the principal garnet and this could be studied most easily by vibrational end members, through detailed polarized single spectroscopy. However, the importance of such an crystal measurements (Fig. 11). ordering transition on phase relations or mineral HOFMEISTERand CHOPELAS(1991b) used the vi- properties in the transition zone has yet to be brational spectra of pyrope and grossular to construct model density of states functions, for demonstrated. g(w), calculation of the specific heat and entropy of these phases. The relative heat capacities of these garnets MgSi0 garnet 3 has been of some interest. From crystal chemical

Garnets in the (Ca,Fe,MgMAl,Fe)zSi30I2 (py- and mineral thermodynamics systematics, it was rope-grossular-almandine-spessartine) system are expected that the standard entropy for pyrope important phases in the lower crust and upper man- should be smaller than that for grossular, but in- tle. In addition, a majorite garnet phase, containing stead, the reverse is observed (KIEFFER, 1980; HOF- octahedrally coordinated silicon, with composition MEISTERand CHOPELAS, 1991b). KIEFFER (1979a, in the system (Mg,Fe,Ca)3(Si,Al,Fe)zSi30I2, is 1980) suggested that the "excess entropy" of py- likely to be present within the transition zone rope could be attributed to low frequency vibra- (AKAOGI and AKIMOTO, 1977; KATO and KUMA- tions of the small Mg2+ ion "rattling" in the large ZAWA,1985; AKAOGIet al., 1987; GASPARIK,1990; dodecahedral site in the garnet (DELANY, 1981; YUSA et al., 1993). A majorite garnet with compo- HOFMEISTERand CHOPELAS,1991a,b). HOFMEISTER sition close to (Mg,Fe)Si03 was first identified in and CHOPELAS (l991a) do find the lowest fre- natural samples from the Coorara and Tenham me- quency (IR-active) vibrations of pyrope (140 ern -1) teorites (SMITHand MASON, 1970), following a lab- to lie at lower wavenumber than the lowest fre- oratory synthesis from pyroxene (RINGWOOD, quency mode in grossular (159 cm "). However, 1967). X-ray diffraction and vibrational spectro- they assign the low frequency mode of pyrope to scopic studies of pure MgSi03 garnet indicate that a translation of the Si04 group, and that of grossu- this phase has a slight tetragonal distortion from lar to a Ca2+ vibration, based on the relative cation cubic symmetry (KATO and KUMAZAWA, 1985; masses. It seems that the alternative assignment AKAOGi et al., 1987; McMILLAN et aI., 1989). At proposed by KIEFFER (1979a, 1980) could still high temperatures and pressures, there is extensive hold, if the effect of the lengthened bond in pyrope solid solution in a cubic garnet phase between py- on the Mg-O force constant were taken into account. This question seems to be an obvious candi- rope (Mg3Al2Si30I2) and the majorite end member, extending to approximately 80 mol% MgSi03 com- date for study by 24MgP6Mg isotopic substitution ponent at 1000°C and 16 GPa (AKAOGIet al., 1987; (PAQUES-LEDENT and TARTE, 1973), particularly KANZAKI, 1987). since the result might have implications for the low The expected vibrational spectrum for an ideal temperature heat capacity and hence entropy of cubic garnet structure, such as pyrope, almandine, majorite «Mg,Fe)Si03 garnet), which have not yet grossular and the Mg3A}zSi30I2-Mg4Si40l2 solid been determined (YuSA et al., 1993). solutions, is already quite complex due to the large Raman spectra for cubic garnets have been mea- number of atoms in the unit cell: sured at high pressure and high temperature by several authors (DIETRICHand ARNDT, 1982; MER- Cib = 3Alg(R) + 5A2g(inactive)' + 8Eg(R) NAGH and Lnr, 1990; GILLET et al., 1992; Lnr et al., 1995). GILLET et al. (1992) combined the high + 14Tlg(inactive) + 14T2g(R) + 5AluCinactive) P and high T data for pyrope, grossular and alman- + 5A2u(inactive) + lOEu(inactive) + 17TluC1R) dine to calculate intrinsic mode anharmonicity parameters, showing that a relatively large (~3%) 16T (inactive) (8) + 2u anharmonic correction to the heat capacity would There have been several systematic infrared and be necessary. As already noted, this could affect Raman spectra of garnet minerals (MOORE et al., the slope of the garnet-ilmenite phase boundary, in Vibrational spectroscopy of mantle minerals 195

(b)

... ~ :e (a) ::: 01 CD " co I J ~~ CJ1 0 C ..c0 a'- (I) ..c -e; f

l 400 650 '900 1150

N 0 0 3.6001 ~ ~ ... ,.., ~ M co

M 3.0001 l'l MgSi0 garnet .. -<, 3 a (Mj) ::J (I) z, "",1 I !. QI QI a 1.8001 II ::J " ~IL '" !2'" ! l 200 •• ...... on. ,000.. ,200.. aso 700 10SlJ 1400 WAVENUMBERS WAVENUMBERS Cc)

16·~------~ Pyrope

FIG. 11. Infrared and Raman spectra of pyrope and MgSi03 garnets. The Raman (a) and powder transmission IR data (b) are from McMILLAN et al. (1989), and show the additional peaks which appear due to the tetragonal

distortion of MgSi03 garnet compared with cubic pyrope. The single crystal IR reflectance spectrum of pyrope (c) is taken from HOFMEISTERand CHOPELAS(1991a). Peaks

are assigned to internal vibrations of the Si04 tetrahedra (V3 asymmetric stretching, V4 and V2 deformation) and external modes associated with the Mg and Al sites. Spec- tra from HOFMEISTERand CHOPELAS (1991a; Fig. 1, p. 200 400 600 800 1000 507) used by permission of the editors of Physics and Wavenumbers Chemistry of Minerals (Springer-Verlag GmbH). 196 P. F. McMillan, R. J. Hemley and P. Gillett hot regions of the transition zone (REYNARD and small deviations from local cubic symmetry, during RUBIE, 1996). Lm et al. (l995) have recently re- development of the ordered domains (see, for ex- corded the high frequency (>800 cm ") Raman ample, analogous work on cordierite: McMILLAN spectrum of a garnet with composition 0.9 et al., 1984).

MgSi03:0.1 A1203, to 13.6 GPa at room temperature, and 875 K at room pressure. UPPER MANTLE The pure MgSi03 end member garnet exhibits a small tetragonal distortion from cubic symmetry (Mg,Fe)2Si04 olivine (KATO and KUMAZAWA,1985; AKAOGIet al., 1987; This phase, along with (Mg,Fe)Si03 pyroxenes Y AGI et al., 1992), probably resulting from order- and (Mg,Fe,Ca)3Al2Si30lz garnets, dominates the ing Mg and Si atoms on the [6]-coordinated sites mineralogy of the upper mantle (Lnr and BASSETT, (HATCH and GHOSE, 1989). This ordering and te- 1986). The garnet phases have been discussed in tragonal distortion causes the appearance of weak the previous section, and the chain silicates are additional peaks and splittings in its infrared and described below. The lattice dynamics of synthetic Raman spectra, when compared with pyrope (KATO and natural olivines have been thoroughly investi- and KUMAZAWA, 1985; McMILLAN et al., 1989) gated, both experimentally and by theoretical cal- (Fig. 11). It has already been noted that Mg3Al,. culation. The zone centre vibrational modes for the Si30I2-Mg4Si40I2 garnet solid solutions are cubic, orthorhombic (Pbnm) structure are: as are high pressure Fe3Al,Si30I2-Fe4Si4012 garnets (AKAOGI and AKIMOTO, 1977). The X-ray, IR and r'ib = l1Ag(R) + lIB liR) + 7B2g(R) + 7B3g(R) Raman spectra of a natural majorite garnet 10Au(inactive) 9B u(IR) «Mgo.79Fe021)4Si4012) from the Catherwood mete- + + l orite indicated that it was cubic also, consistent + 13B2uCIR) + 13B3u(lR) (9) with some samples studied by KATO (1986). How- ever, (Mg,Fe)Si03 majorites with Fe/(Fe+Mg) up All of the infrared and Raman-active modes have to ~0.24 have been reported to exhibit a tetragonal been assigned via polarized measurements on sin- distortion (KATO, 1986; MATSUBARAet al., 1990; gle crystals (SERVOIN and PIRIOU, 1973; IISHI, OHTANI et al., 1991). It appears that the presence 1978; PIRIOU and McMILLAN, 1983; HOFMEISTER, or absence of complete solid solution between tet- 1987; CHOPELAS,1991a; REYNARD, 1992). The lat- ragonal (Mg,Fe)Si03 and cubic majorite garnets tice dynamics of forsterite have been investigated may be associated with the energetics of cation by single crystal and powder inelastic neutron scat- ordering on the octahedral sites (AKAOGI et al., tering spectroscopy, coupled with lattice dynamical 1987; McMILLAN et al., 1989; ANGEL et al., 1989; calculations within the rigid ion approximation HATCHand Gnoss, 1989; PHILLIPSetal., 1992). On (RAO et al., 1988). The experimental data and the the basis of modulated microstructures observed theoretical modeling have been used to construct by transmission electron microscopy, WANG et al. vibrational density of states functions, for calcula- (1993) have suggested that a tetragonal-cubic tran- tion of the specific heat, entropy, and other thermo- sition associated with octahedral cation disordering dynamic properties within the quasiharmonic ap- occurs near 2400°C for pure MgSi03 garnet proximation (AKAOGI et al., 1984; CHOPELAS, (HATCH and GHOSE, 1989), and that the transition 1990b; CHOUDHURY et al., 1989; HOFMEISTER, temperature drops rapidly with increasing Fe con- 1987; HOFMEISITERet al., 1989; PRICE et al., 1987; tent. This transition would have implications for RAO et al., 1988). The pressure and temperature garnet elastic properties within the transition zone dependence of infrared and Raman modes in for- (WANG et al., 1993). The existence and nature of sterite and fayalite has been measured in several this. possible phase transformation in (Mg,Fe,AI)- studies (BESSONet al., 1982; DIETRICHand ARNDT,

Si03 majorites merits further investigation, by in 1982; Xu et al., 1983; GILLET et al., 1988, 1991; situ studies at combined high P-T conditions, and HOFMEISTERet al., 1989; CHOPELAS, 1990b; GIL- studies on phases equilibrated at high P-T and LETet al., 1991; Lru and MERNAGH,1992; SHARMA quenched to ambient. Although X-ray diffraction et al., 1992; WANG et al., 1993; DURBEN et al., is probably a first choice for these studies, IR (or 1993). ANDERSON and SUZUKI (1983) have dis- perhaps even Raman scattering, but cubic and tet- cussed the effects of anharmonicity on the thermo- ragonal structures might be less easy to distinguish: dynamic properties of olivines at mantle tempera- see Fig. 11) spectroscopy combined with laser tures, and GILLET et al. (1991) and REYNARD et heating techniques in the diamond anvil cell (GIL- al. (1992) have shown the importance of taking LET et al., 1993b) might be useful in determining anharmonic effects into account for calculations of Vibrational spectroscopy of mantle minerals 197 the vibrational heat capacity. More recently, troscopy for (Ca,Mg)Ge04 olivines, and has been

GUYOT et al. (1996) have used available high- related to M 1- M2 disordering at high temperature, pressure and high-temperature IR and Raman data as well as to extreme anharmonicity (FIQUETet al., to calculate the equations of state of olivines at 1992; RICHETet al., 1994). This behavior has been various P- T conditions. The agreement with experi- observed to occur for silicate olivine, pyroxene and ment is excellent. garnet, and has obvious implications for rheology In a high-pressure Raman study of forsterite at of high temperature mineral assemblages in the room temperature, CHOPELAS(1990b) noted breaks mantle, as well as the kinetics of phase transforma- in slope of the pressure shifts in several modes at tions. This phenomenon can now be studied with 9.2 GPa, which was thought to be associated with existing techniques by in situ high P-T IR and Ra- a high order phase transformation occurring in oliv- man spectroscopy to evaluate its importance in the ine at high pressure and ambient temperature. mantle P-T regime. WANG et al. (1993) noted similar breaks in slope, but concluded that no phase transition occurs, sim- Pyroxene ply a change in the compression mechanism with In comparison with the olivine group of miner- pressure. Moreover, DOWNSet al. (1996) found no als, there has been much less work done on the evidence for structural changes at these pressures vibrational spectroscopy of pyroxenes, either at in a single-crystal x-ray diffraction study to 17.2 ambient or under mantle conditions (WHITE, 1975).

GPa with a variety of pressure media. Such changes The (Mg,Fe)Si03 rich enstatite phase shows com- in the strength of pressure media used cause appar- plex polymorphism as a function of pressure and ent changes in the slope of such frequency shifts temperature (ANGEL et al., 1992; LEE and HEUER, with pressures. Compression of forsterite to higher 1987; PREWITT, 1980; SCHRADER et al., 1990; pressure (50 GPa) at room temperature results in ANGEL and HUGH-JONES, 1994a,b). The ortho- the appearance of new bands near 750 cm-I and rhombic phase (Pbca) is stable to approximately 8 960 cm " (DURBEN et al., 1993). These are likely GPa and 1300 K. Above this temperature, there is related to formation of SiOSi (dimer) linkages a displacive transformation to protoenstatite within the structure (perhaps related to the forma- (Pbcm) at ambient pressure for pure MgSi03, but tion of spinelloid structures) or to the formation of Fe-containing phases transform to C2k structures high coordinate Si species (perhaps as a precursor (YANG and GHOSE, 1995). At pressures above 8 to amorphization) (WILLIAMS et al., 1990; GUYOT GPa, orthopyroxene transforms into a high density and REYNARD, 1992). In their infrared study of clinoenstatite structure (PACALO and GASPARIK, pressure-induced amorphization in fayalite, WIL- 1990; ANGELet al., 1992). CHOPELASand BOEHLER LIAMS et al. (1990) noted an increase in intensity (1992) prepared a sample of this phase by CO2 laser of a band in the 600-800 cm " region above 35 heating single crystalline MgSi03 orthoenstatite in GPa, which they attributed to the formation of six- a diamond anvil cell, and investigated its Raman coordinated silicon species. As already noted spectrum during decompression. The spectrum of above, Raman and IR measurements in situ at high the clinonenstatite polymorph is similar to that of pressures and temperatures may be useful in phase diopside at the same pressure, consistent with the identification and possibly for characterizing local C2k symmetry assigned by ANGEL et al. (1992), and intermediate-range structures in kinetic studies and it reverts to the orthorhombic polymorph on of high-pressure transformations of olivine and decompression below 5 GPa (CHOPELAS and other (Mg,Fe)2Si04 polymorphs; such experiments BOEHLER,1992). The pressure shifts of the Raman are needed to further understand seismicity and peaks in both phases are given by CHOPELASand rheology within the upper mantle and transition BOEHLER(1992). DIETRICHand ARNDT(1982) have zone. investigated the infrared spectrum of natural ortho- An area that deserves further attention is the ex- pyroxene at lower pressures, to 5 GPa, at up to ploration of pre-melting behavior of mantle miner- 250°C. SHARMA et al. (1987), SHARMA (1989, als, through in situ high temperature vibrational 1990) and GHOSE et al. (1994) have investigated spectroscopy, combined with diffraction and calo- the effect of temperature on the Raman spectra of rimetric experiments (RICHET et al., 1994). The MgSi03 pyroxene phases. In particular, GHOSE et pre-melting regime is characterized by an anoma- al. (1994) have reported polarized single crystal lous rapid increase in heat capacity and entropy, Raman spectra of protoenstatite. The transitions be- below the thermodynamic melting point (RICHET tween the ortho-, proto- and clino-enstatite phases and FIQUET, 1991). The origin of the phenomenon are clearly observed by the appearance or disap- has been studied in detail using in situ Raman spec- pearance of peaks in the spectra. 198 P. F. McMillan, R. J. Hemley and P. Gillett

At lower pressures, a (Ca,Al)-rich pyroxene sites (BERAN and PUTNIS, 1983; AINES and Ross- phase is found, forming a monoclinic (e21e) solid MAN, 1984a; ROSSMAN, 1988; BELL and ROSSMAN, solution in the system CaMgSi206 (diopside)- 1992a; BELL et al., 1995). Measured OH contents NaAlSi206 (jadeite) (GASPARIK,1992). The Raman for natural olivines, pyroxenes and garnets, which and infrared spectra of diopside have been charac- constitute >90% of peridotites sampled at the sur- terized at ambient conditions via polarized single face, range from 100-1500 Hll06 Si (BERAN and crystal measurements (ETCHEPARE, 1970, 1972; PUTNIS, 1983; AINES and ROSSMAN, 1984b,c; ZULUMYANet al., 1976), and an empirical lattice MILLER et al., 1987; BELL and ROSSMAN, 1992a,b, dynamics calculation has been carried out (TOMI- 1995; KUROSAWA et al., 1992, 1993; BAI and SAKA and lISHI, 1980). A Raman spectrum at 16 KOHLSTEDT,1993). Experiments are under way to GPa is presented by CHOPELAS and BOEHLER determine the maximum OH concentration which (1992). SEKITA et al. (1988) have used Raman can be incorporated in these phases (KOHLSTEDT spectroscopy to study bands in the 600-700 cm " et al., 1996). region of clinopyroxenes, and assigned peaks to The case of garnet is particularly interesting, be- SiOSi, SiOAl and AlOAl linkages. This type of cause grossular (Ca3Al2Si3012) forms a solid solu- calibration could prove useful in future high P-T tion with the hydrogarnet (Ca3Alz(04H4)3), in studies of Ca-rich pyroxene phases. Raman and which each Si4+ site is occupied by a distorted infrared studies of pyroxene phases, both at ambi- tetrahedron of OH groups (COHEN-ADDAD et al., ent conditions and in situ at high pressure and tem- 1967; KOBAYASHIand SHOJI, 1983; LAGER et a!., perature, will be useful to complement diffraction 1987, 1989). This substitution permits the OH con- studies in unraveling the detailed nature of the centration of Ca-rich garnets to become very high. complex transformation mechanisms between the From vibrational spectroscopy, the structural ar- different polymorphs of (Mg,Fe)Si0 (YANG and 3 rangement of OH groups is determined by bifur- GHOSE, 1994a,b; 1995; GHOSEet al., 1994), as well cated hydrogen bonds to adjacent oxygens forming as the onset of pre-melting behaviour as described the tetrahedral site (HARMON et al., 1982). In above for olivine (RICHET et al., 1994). a high pressure study of hibschite (Ca3Alz(Si- 04)Ls(04H4)Ls), KNITTLE et al. (1992) found both WATER AND CARBON IN THE MANTLE Raman-active 0- H stretching vibrations and the Water in the mantle higher frequency IR mode to decrease in frequency with increasing pressure, as expected for increased The questions associated with "water" or the hydrogen bonding, but the lower frequency IR hydrogen content in the mantle (how much? band increased in frequency. It was suggested that where? in what form?) have been and still are a this could result from H ... H or H ... Ca2+ repulsive matter of intense interest and considerable debate. interactions (SHEU and McMILLAN, 1988; WIL- These are critical questions for mantle petrology, LIAMS, 1992). both to understand the volatile budget and recy- The P-T stability and breakdown behaviour of cling processes, and because of the large effects of even trace hydrous species on mineralogical phase hydrous minerals such as amphibole or mica play relations, including melting, and rheology of man- a critical role in volatile recycling. A few studies tle minerals and mineral assemblages (MACKWELL of the vibrational properties of these minerals et al., 1985; Lnr, 1987; WYLLIE, 1988; MACKWELL at ambient conditions have been reported (see and KOHLSTEDT,1990; BELL and ROSSMAN,1992a; McMILLAN and HOFMEISTER,1988), and some high THOMPSON, 1992; DUFFY and AHRENS, 1992; pressure work is now beginning to appear (HOLTZ GASPARIK,1993; GILLET, 1993a). Vibrational spec- et al., 1993). Raman and IR studies under high troscopy, particularly infrared absorbance measure- P- T conditions will be extremely important in help- ments of natural mantle minerals and laboratory- ing determine the thermodynamic properties of synthesized samples, has played a role in helping these phases (GILLET et al., 1989b; ROBIE et al., establish our current estimates of the hydration 1991), and will shed light on their formation or state of the mantle. dehydration mechanisms within the upper mantle. It is well known that hydrous minerals such as A related field which is only beginning to receive phlogopitic mica and kaersutitic amphibole can be attention is the study of hydrous silicate melt stable under the P- T conditions of the upper mantle. phases, under mantle P-T conditions (WILLIAMS, However, IR absorption studies of nominally anhy- 1990; FARBER and WILLIAMS, 1992; WILLIAMS, drous mineral phases have demonstrated that these 1992; CLOSMANN and WILLIAMS, 1995). Vibra- can contain substantial amounts of OH in structural tional spectroscopy is expected to play a critical Vibrational spectroscopy of mantle minerals 199

role in these studies, which can not be easily ad- carbon, and on the other, they provide carriers for dressed by diffraction methods. carbon when sediments are recycled in subduction Considerable interest has been stimulated by the zones. Experimental studies as well as observations possibility that large amounts of water could be of natural samples have shown that carbonates stored in the transition zone, contained in structural could provide major hosts for carbon in the Earth's sites in j3-(Mg,Fe)2Si04, as discussed above upper and lower mantle. High-pressure experi- (SMYTH, 1987; DOWNS, 1989; McMILLAN et al., ments have shown that in the presence of pyroxene 1991; YOUNGet al., 1993; KOHLSTEDTet al., 1996), and olivine, or their high-pressure equivalents (j3_ or within high pressure hydrous minerals in the Mg,Fe)zSi04 and the assemblage (Mg,Fe)Si03 pe- system MgO-Si02-H20 (Lnr, 1987; KANZAKI, rovskite + (Mg,Fe)O magnesiowiistite), MgC03 is 1991; PREWITT and FINGER, 1992). These phases the stable carbonate phase (BREY et al., 1983; (chondrodite, clinohumite, phases A, B, C, D, E, CAA'lLand SCARFE, 1990; KATSURAand ITO, 1990; etc.) have been partly characterized by infrared and BIELLMANN et al., 1993b). Vibrational spectros- Raman spectroscopy (AKAOGIand AKIMOTO, 1986; copy has been used to demonstrate the stability of FINGER et al. 1989; McMILLAN et al., 1991), but carbonates under pressure and temperature condi- much work remains to be done. This is an essential tions relevant to the mantle (GILLET, 1993b; GILLET complement to X-ray diffraction, because it is et al., 1996b) (Fig. 12). much easier to study the H atom environment via The expected vibrational modes of calcite and vibrational spectroscopy. WILLIAMS (1992) has magnesite with space group R3c are: studied a natural chondrodite to ~9 GPa, showing the importance of H-bonding in one of the 0- H IVii> = Alg(R) + 3A2g(inactive) + 4Eg(R) + stretching vibrations, and used IR spectroscopy to 2AluCinactive) + 4A2uCIR) + 5EuCIR) (10) investigate phase changes following laser heating

at pressures between 22 and 44 GPa. The vibrational modes of dolomite (CaMg(C03)z) Two other interesting cases of trace OH in nomi- are related by the disappearance of the c-glide nally anhydrous mantle minerals have also recently (space group R3). been demonstrated by IR absorption spectroscopy. IVib = 4Ag(R) + 4Eg(R) + 5AuCIR) + 5EuCIR) (11) PAWLEY et al. (1993) have shown that stishovite, the high pressure phase of Si02, can accept hydro- The infrared and Raman spectra of these phases gen in defect sites within its structure, and the H are well known, through extensive single crystal content was shown to scale with Al substitution spectroscopy (WHITE, 1974; GILLET et al., 1993a). into the Si02 phase (PAWLEY et al., 1993; SMYTH Several recent infrared and Raman studies of et al., 1995). This is consistent with the observed carbonate minerals have been carried out at ambi- dissolution of H into Ti02 rutile (VLASSOPOULOS ent temperature, to pressures in excess of 30 GPa et al., 1993; SWOPE et al., 1995). Lu et al. (1994) (GILLET et al., 1988; 1993a; Lm and MERNAGH, and MEADE et al. (1994) have also shown by IR 1990; KRAFT et al., 1991; BIELLMANNand GILLET, spectroscopy that (Mg,Fe)Si03 perovskite, the 1992). Calcium carbonate has two stable poly- dominant phase in the lower mantle, can accept morphs, calcite and aragonite, the high pressure trace OH within its structures, with considerable form. Calcite also shows transitions to metastable potential implications for the water content of the forms of CaC03 (calcite-II and calcite-III) in the deep Earth. The study of MEADE et al. (1994) was 1.4-2.0 GPa range (FONG and NICOL, 1971; HESS carried out with the synchrotron infrared technique, and GHOSE, 1988; GILLET et al., 1988; Lnr and a promising new method for micro-spectroscopy of MERNAGH, 1990; COLLERSONet al., 1992; BIELL- materials at ambient and high-pressure conditions MANNand GILLET, 1992). BIELLMANNet al. (1993a) (REFFNER et al., 1994). have shown that, after heating of calcite to 2000 K at 40 GPa, calcite II can be quenched to ambient Carbonates conditions. In contrast to this behaviour, dolomite and magnesite show no evidence for any phase A second problem of intense interest is the car- transitions to the highest pressures studied (34 bon budget and oxidation state of the deep Earth GPa) (Fig. 12), nor does the stable high pressure

(KUSHIRO et al.. 1975; IRVING and WYLLIE, 1975; aragonite phase of CaC03 (KRAFT et al., 1991; KATSURAand ITO, 1990; BLUNDYet al., 1991; GIL- COLLERSONet al., 1992; BIELLMANNand GILLET, LET, 1993b; GILLET et al., 1996a). Carbonate min- 1992; GILLET, 1993b; GILLET et al., 1993a; GILLET erals are fundamental actors in the global carbon et al., 1996a). cycle because, on the one hand, they may store The effect of temperature on the room pressure 200 P. F. McMillan, R. J. Hemley and P. Gillett

(a) (b) C03 symmetric stretch !Magnesit~

P=26 GPa T=1100 ± 200 K

C03 bend

P=26 GPa T=300 K

50 250 450 650 850 1000 1200 200 400 600 800 1000 1200 Wavenumber (cm-!) Wavenumber (em-I)

FIG. 12. (a) Effect of pressure on the Raman spectrum of magnesite, MgC03, to mantle pressures. (b) In situ high P-T spectrum of magnesite, at 26 GPa and 1100 K, showing the stability of this phase under mantle conditions (GILLET,1993b).

infrared spectrum of calcite (SAKURAI and SATO, show that under these conditions magnesite retains 1971), and the Raman spectra of calcite, dolomite its ambient R3c structure (Fig. 12). Optical obser- and magnesite (GILLET et al., 1993a), has been vations during heating as well as Raman spectra investigated up to 800-1200 K. The measured fre- recorded at 30 GPa after laser heating up to 2000- quency shifts with temperature have been com- 2500 K showed that no decarbonation had occurred bined with the observed pressure-induced shifts to and that magnesite was stable. These results are in calculate mode anharmonic parameters (GILLET et agreement with other experimental data that show al., 1993a). These must be taken into account for that magnesite can act as a host for carbon storage a realistic calculation of 160/180 isotopic fraction- down to at least 1000 km, and that this phase can ation factors at high pressure and temperature (GIL- also be the carrier for carbon in subducting plates LETet al., 1996b). It is found that these anharmonic (GILLET, 1993b; GILLET et al., 1995). parameters are larger for the lattice modes than for The Raman data for magnesite, combined with the C03 internal modes. Moreover, they are larger compressibility measurements at high pressures in calcite than in dolomite and magnesite, consis- and room temperature, have been used to compute tent with increasing relaxational component to the the specific heat and vibrational entropy, and calcu- libration of the C03 groups, premonitory to a rota- late the equation of state of magnesite up to 135 tional order-disorder phase transition in calcite at GPa and 3500 K (GILLET et al., 1995), in good high temperatures (GILLET et al., 1993a). agreement with existing experimental data. The KRAFT et al. (1991) have investigated the vibra- model was then used to calculate the decarbonation tional spectrum of dolomite in-situ under high curve MgC03 ~ MgO + CO2, under mantle condi- P-T conditions, at pressures up to 11.5 GPa and , tions, which provides an upper limit for the stability 550 K. The stability of magnesite (MgC03) has of magnesite in the Earth's mantle. been studied by Raman spectroscopy at high pressure and high temperature, using a COz-laser- Diamond heated diamond anvil cell (GILLET, 1993b). Raman spectra recorded at simultaneous high pressure (26 This high pressure form of carbon also provides GPa) and high-temperature (1200 K ± 200 K) an important source or sink for carbon in the man- Vibrational spectroscopy of mantle minerals 201

tle. Diamond is the window material of choice for a diamond-anvil cell at high temperature. An excit- in situ diffraction and spectroscopic experiments ing possibility for future work is the use of micro- under high P-T conditions, in the diamond anvil IR and Raman spectroscopy of mineral and fluid cell, for mineral physics studies of the deep Earth. inclusions in deep mantle diamonds, as well as As discussed above, this same material also pro- defect species in the diamond matrix itself (NAVON vides a "window" of a different type, acting as a et al., 1988; FIELD, 1992), to better understand the host for mineral and fluid inclusions brought to the volatile budget of the deep Earth. surface from great depths within the Earth (KEss ON and FITZGERALD,1992; HARTE and HARRIS, 1993; CONCLUSIONS SAUTTER and GILLET, 1994). BELL (unpublished) has carried out measurements of the OH content Vibrational studies via infrared and Raman spec- in a suite of silicate inclusions in diamond with troscopy provide a set of powerful techniques for new synchrotron micro-IR techniques (REFFNERet the characterization and structural study of mantle al., 1994). In contrast to studies on typical mantle minerals. The vibrational spectra can reveal subtle minerals, OH was not detected in either olivines structural distortions which are not immediately or orthopyroxene inclusions in diamonds, while its obvious in diffraction experiments, and are sensi- presence was clearly demonstrated in kyanite inclu- tive to the presence of hydrogen within the mineral, sions, with IR spectra similar to kyanites from man- even as a defect species. It is usually quite simple tle eclogite xenoliths. These results have important to obtain spectra in situ at high pressure or at high implications for the hydration history of the mantle temperature, and combined in situ high P- T studies and place constraints on processes of diamond for- are beginning to appear, allowing minerals to be mation. Notably, the measurements were not possi- examined directly under mantle conditions. The ble with conventional IR methods. spectra can be used to obtain important thermody- Vibrational studies of this phase have been car- namic information on the high-pressure phases, in- ried out to help demonstrate the stability of dia- cluding heat capacity, vibrational entropy, and Gru- mond to high pressures and temperatures, and as a neisen parameters, and detailed information on testing ground for first-principles theory. The vi- phase transformation mechanisms. These Raman brational spectrum of cubic diamond itself is very and infrared studies are. particularly fruitful when simple: the expected vibrational modes are they are combined with theoretical calculations of the lattice dynamics, which give information on all (12) vibrational modes throughout the Brillouin zone, which gives rise to a single first order peak in the and permit a structural interpretation of the ob- Raman spectrum at 1332 cm ", Weak features due served spectra. Because the vibrational spectrum to second order Raman scattering are observed in provides a fingerprint for a mineral, micro-Raman the 2200-2600 cm-I range. The Raman spectrum and IR measurements are proving very useful for of diamond has been studied to ~40 GPa (BOPPART phase identification of natural and experimental et al., 1985; HANFLAND et al., 1985; HANFLAND polyphase aggregates with spatial resolution close and SYASSEN, 1985; SHARMA et al., 1985; ALEK- to the diffraction limit of the radiation used (e.g., SANDROVet al., 1986). These studies confirm both ~ 1 µm for visible-light Raman; ~ 10 µm at 1000 the high-pressure stability and the singular high cm-I for IR methods). strength of this mineral over a pressure range ex- Most of the principal mantle mineral phases have tending well into the lower mantle. MAO and HEM- been identified, and their infrared and Raman spec- LEY (1991) have carried out micro-Raman mea- tra assigned, but data on relevant solid solution surements on diamond anvils at sample pressures series are often lacking. There are obvious gaps in up to 300 GPa, and found some evidence for struc- the data base: for example, the far infrared spectra tural transformations driven by the large non- of 'Y-Mg2Si04, and infrared and Raman data on the

hydrostatic stresses of the anvils under these condi- polymorphs of (Mg,Fe)Si03. In addition, further tions. VOHRA (1992) has found evidence for a tran- experimental and theoretical work on MgSi03 sition in highly stressed diamond anvils in studies perovskite is required to fully characterize the low- to 420 GPa. ZOUBOULIS and GRIMSDITCH(1991) frequency vibrational spectrum, and determine the have investigated the Raman scattering in diamond presence or absence of soft modes at simultaneous at high temperatures, to 1900 K. Recently, FRANTZ high P-T conditions. In general, in situ studies un- and MYSEN (1996) have exploited the frequency der combined high P-T conditions will be useful shift of the T2g mode in l3C relative to 12C and its to gain a better understanding of anharmonic be- weak temperature shift for pressure calibration in haviour, and to investigate phenomena such as pre- 202 P. F. McMillan, R. J. Hemley and P. Gillett melting, which may play an important role in de- ing in the IR, which can alleviate problems associ- termining transport properties at high temperature. ated with laser-induced sample fluorescence and Vibrational spectroscopy has played a central role degradation at visible wavelengths (CHASE et al., in the characterization of OH-containing phases 1986). Also, there may be important applications likely to be stable within the mantle, and thus help- of time-resolved and non-linear optical techniques ing to constrain the sources and sinks for a hydrous (stimulated Raman scattering) (e.g., SHARMA, component at depth. But more work is required 1989). Notably, although the P-T regime of the under high pressure conditions, to help determine entire mantle of the Earth can now be created in the role that hydrogen bonding may play in stabiliz- the laboratory (MAO and HEMLEY, 1996), there ing these mineral phases, and also to investigate the have been few measurements of material properties dehydration mechanisms of hydrous phases during under simultaneous high P-T conditions. It should processes such as subduction. be possible to combine each of these new spectro- This review has focused on vibrational spectroscopic methods with high P-T techniques for the scopic studies of crystalline minerals within the next generation of in situ measurements on mantle mantle. Much is now known about the structure minerals. and properties of aluminosilicate melts which yield magmas observed at or near the surface, generated Acknowledgements- The work of PFM and RJH on spec- by partial melting events at relatively shallow troscopy of mantle minerals has been supported by the National Science Foundation, Division of Earth Sciences. depths within the lower crust and upper mantle PG has benefited from financial support of the CNRS (STEBBINSet al., 1995). However, melting may also (INSU, programme Terre Profonde). This manuscript was occur deeper in the mantle. Of paramount interest prepared while PM was Professeur Associe at the Ecole is the role of structural changes (e.g., changes in Normale Superieure, Lyon. We thank Q. Williams, P. coordination) in determining the density contrast Conrad, and an anonymous reviewer for their comments, and G. 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