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Conrrasr, the Mean Mo Bond Distances I American Mineralogist, Volume 72, pages 748-755, 1987 High-pressurecrystal chemistry of monticellite, CaMgSiOo Z. D. Snnnp Department of Geological Sciences,University of Michigan, Ann Arbor, Michigan 48109, U.S.A. R. M. fllznN, L. W. FrNcnn GeophysicalLaboratory, CarnegieInstitution of Washington,280l Upton Street,N.W., Washington, D.C. 20008, U.S.A. Ansrru,cr Structuralrefinements of a natural monticellite(Ca"nnMg"n,FeoorMnoo,SiOo) were com- pleted at seven pressuresto 62 kbar using a diamond-anvil cell. Compression of monti- o cellite is anisotropic; the orthorhombic b axis is most compressible(0 b : 3.62(l) x I 0 kbar '), whereasthe a and c axeshave similar compressibilities(8" : 1.96(4) x l0-4 kbar-' and B. : 2.05(8) x l0-o kbar-'). Monticellite axial compressionratios are 1.00:1.85:1.05, compared to ratios of 1.00:2.02:1.60for forsterite.The bulk modulus of monticellite, assuminga Birch-Murnaghanequation of statewith K' : 4, is 1.13(3)Mbar. The bulk moduli of the divalent cation polyhedraare 1.5(l) and l.l(l) Mbar for the Ml (Mg) and M2 (Ca) octahedra,respectively. The Si tetrahedron, on the other hand, displays no sig- nificant compression (bulk modulus >3 Mbar) between I bar and 62kbar. Monticellite compressibility and expansivity display a "noninverse" behavior, consistentwith the large differencein octahedral volumes for the Ml and M2 sites. INrnooucrroN in monticellite thus makes it an ideal candidate for com- Crystal structuresof olivine-group minerals have been pressibility study' (l) studied extensivelybecause oftheir importance as phases The principal objectives of this study are to deter- in crustal and mantle rocks, as well as becauseoittt"i. mine.the.bulk modulus and axial compressibilities of intrinsic crystal-chemicalinterest. The olivine group in- m^onticellite;(2) to measurethe relative compressibilities polyhedra; (3) cludes silicites of composition vIM2+rvSiOo,wit:h Ca, fe, of the ordered Mg, Ca, and Si cation to positions Mg, and Mn as the principal M-site cations. Nonsilicate relate pressure shifts in atomic to the aniso- (4) phases with the olivine structure include chrysoberyl tropic compression of monticellite; to combine the (AlrBeOo),triphylite (LiFepO"), and sinhalite (MgAIBO;. high-pressurebehavior with high-temperature structural Brown (1970, l9g2) reviewed the crystal cneriistry of variationsof monticellite (Lagerand Meagher,1978) in silicate olivines at ambient conditions. There have blen. order to derive an empirical equation of state as a func- in addition, numerous studiesof olivine structuresat high tion of temperature and pressure;and (5) to explain the temperatures(Brown and prewitt, lg73; Smyth and H-a- P-Ieffects in terms of the monticellite crystal structure. zen, 1973; Smyth, 1975; Hazen, 1976; I-agerand Meagh- er, 1978; Hazen and Finger, 1987) and at high pressure Expnnrn'rnxTAl- DETAILS (Hazen and Finger, 1980; Kudoh and Tak6uchi, 1985; Specimendescription Kudoh and Takeda, 1986; Hazen, 1987). The monticellitespecimen is iiom CascadeSlide, New york In spite of this effort, there has been relatively little (valleyand Essene, l9g0), and has the composition Ca"rrMgr,- high-pressureresearch on the Ca-bearing olivines. Ca is FeoorMnoo,SiOo(Sharp et al., 1986)as determinedby electron- by far the largestcation that entersthe octahedralsite in microprobeanalysis.Aclear,inclusion-free40x 100 x l00pm natural olivine (Lager and Meagher, 1978). In monticel- fragmentwas taken from a crushed2-mm-diameter grain and lite, Ca is strongly partitioned into the larger M2 site mountedfor analysis' (Onken, 1965; Lumpkin et al.,-ii 1983), with a resulting High-pressurerr!_L __^^^_-_.crvstallographv mean M2-o bond distance z.l1-'A,;;";;;; nl$ata' bothat high-pressureand at I bar'were collected on mean Ml-o bond distance of 2.13 A (onten', rs6sl. rn conrrasr,themean M-o bond distances i" botl'ili;; i#:n\lilTil:l."tirY.",:it":ffi::-JJ"T fl11#:;x1ll1#:l M2 sitesof Fe--,Mg- and Mn-bearing olivines rangefrom il;;; ;"-e experimentalconditions as the high_pressure 2.l0 to 2.22 L (Brown, 1982).In many oxidesand sili- ;;;;"';;"ilrystematicerrors. Datacollectionwasmadeusing cates,polyhedral compressibility is proportional to poly- an automaredfour-circle diffractometer with monochromatized hedral volume (Hazen and Finger, 1982; Hazen, 1985). MoKa radiation(I : 0.70930A) fottowingthe procedureof The largedifference in Ml and M2 octahedralvolumes Hazenand Finger (1982). A mixtureof 4:l methanol:ethanol 0003404x/87l0708-0748$02.00 748 SHARP ET AL.: HIGH-PRESSURE CRYSTAL CHEMISTRY OF MONTICELLITE 749 TABLE1. Unit-cellparameters of monticelliteat various pres- method of Barnett et al. (1973). Pressuremeasurements have an sures estimatederror of + I kbal. Intensities were measured for all accessiblereflections in a Pressure reciprocal spacewith sin g/tr < 0.7 A-'. Omega (+1 kbar) a (A) b (A) c (A) v(A) vtvo hemisphereof step scans,with 0.025" increments and 4-s counting periods per 0.001 4.821(21 11.105(3) 6.381(1) 341.6(1) 1.0 step were used.The fixed-4 mode of collection was employed in 11 1 4.812(3) 11.051(4) 6.364(2) 338.4(2) 0.991 maximize reflection accessibility and minimize atten- 207 4.800(2) 11.004(2) 6.346(1) 33s.2(1) 0.981 order to 29j 4.793(21 10.971(2) 6.335(1) 333.1(1) 0.975 uation of the diamond cell. A correction was made for X-ray 41.2 4.779(21 10.922(3) 6.317(1) 329.7(11 0.965 absorption by the diamond and Be componentsof the diamond 53.0 4.770(21 10.882(3) 6.305(1) 327.3(2) 0.958 cell (Hazen and Finger, 1982). Digitized data for each step scan 61.7 4.763(1) 10.849(2) 6.294(1) 325.3(1) 0 952 were converted to integratedintensities following the algorithm Nofe; Values in parenthesesrepresent estimated standard deviations oflehmann and Larsen (l 974). Backgroundswere selectedman- (esd's). ually where necessary.Conditions of high-pressurerefinements, refined atomic positional parameters,and isotropic temperature factors are given in Tables 2 and 3. Refinements were made was used as the hydrostatic pressuremedium, and several 5- to using an ordering schemewith Ca fixed in the larger M2 site. lO-pm-diameterruby chips were added for pressurecalibration. Mg and Fe were allowed to partition equally over the Ml and Lattice parameters at each pressure(Table l) were refined from remaining unfilled fraction of the M2 site (Birle et al., 1968). diffractometer anglesof l5 reflections,each of which was mea- Calculated and observed structure factors for monticellite at sev- sured in eight equivalent positions (Hamilton, 1974; IGng and en different pressuresappear in Tables 4a-4h.' Finger, 1979). The range of 20 for the 15 reflectionswas 34-39' at all pressuresin order to avoid systematic errors that result Data collection with reducedapertures from comparing angular data from different ranges(Swanson et Low peak-to-backgroundratios are a significant problem in al., 1985).Unit-cell parameterswere initially refined as triclinic; high-pressurecrystallographic work. This difrculty is inherent at all pressuresthe unit-cell anglesconform to the expectedor- thorhombic dimensionality within two estimated standard de- ' To obtain copiesof Tables 4a-4h, order Document AM-87- viations. Final values of unit-cell parameters(Table l) were re- 345 from the BusinessOffice, Mineralogical Societyof America, fined with orthorhombic constraints. Pressurewas calibrated 1625I Street,N.W., Suite414, Washington, D.C. 20006,U.S.A. before and after each data collection using the ruby fluorescence Pleaseremit $5.00 in advancefor the microfiche. TneLe2. Monticellite:Refinement conditions and refined parameters at variouspressures 1 bar 11 kbar.' 11 kbar 21 kbar 29 kbar 41 kbar 53 kbar 62 kbar No observed (/ > 2o) 178 198 183 r80 182 181 179 189 B=>[F._F"]t>F.(/4 7.4 5.8 7.4 5.5 o.o 5.6 o.o WeightedR: - tz v,tlF. F"f l> wFll1? ('hl 4.O 38 4.3 3.4 4.4 4.2 4.0 4.4 Extinctiontrt (x 105) 2.s(5) 2.4(4) 0.8(4) 1.s(4) 2.5(5) 1.e(4) 1.e(3) 1.7(3) M1 x 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 z 0 0 0 0 0 0 0 0 B 1.04(8) 1.43(6) 1 13(8) 0.93(6) 1.03(8) 0.e5(8) 1.06(7) 0.86(7) x o.9774(7) o.9770(4) 0.9766(6) 0.9748(4) 0.9751(7) 0.9746(6) 0.9744(s) 0 9733(6) o.2771(3) o.2764(2) 0.2763(3) 0.2766(21 0.2765(3) 0.2761(3) 0.2754(2) 0.2757(2) z o.25 0.25 0.25 0.2s o.25 0.25 0.25 o.25 B 101(7) 1.32(5) 0 ss(6) 1.00(5) 0.98(7) 1.02(6) 1.02(s) 0 s4(5) x 04111(10)o.4122(7) 0.4099(9) 0.4116(7) 0.4117(1 1) 0.4102(1 0) 0.4117(8) 0.4134(8) v 0.0823(4) 0.0811(3) 0.0812(3) 0.0801(3) 0.0811(4) 0.0807(4) 0.0807(3) 0.0807(3) z 0.25 0.25 0.25 0.25 o.25 0.25 0.25 0.25 B 0.96(8) 1.25(6) 0.82(8) 0.86(6) 0.81(8) 0.93(8) 0.88(7) o.77(71 x 0.7474(2310.7462(18) o.7481(23) 0.74't2(18],0.7389(24)0.7434(22) 0.7473(21) o.7477(20) v 0.0773(8) 0.0775(5) 0.0769(7) 0.0773(6) 0.0766(9) 0.0776(8) 0.o770(7) 0.0760(7) z o.25 0.25 o.25 0.25 0.25 0.25 0.25 0.25 B 1.17(18) 1.61(1 3) 1.22(171 't.22('t3) 1.s7(21) 1.32(17) 1.32(15) 1.42(16) x 0.2s23(23)0.2472(17)o.2458(23) 0.2464(18],0.2s14(24)0.2490(21l- 0.2474(21) o.2476(20) 0.4494(9) 0.4478(6) 0.4484(8) 0.4477(6) 0.4483(9) 0.4471(8) 0.4456(7) o.4461(71 z 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 B 1.04(1 9) 1.40(13) 0.88(18) 1.15(1 4) 0.87(19) 0.77(16) 1.17(16) 1.11(16) x 0.2728(16) 0.2714(11)0.2741(',t4)0.2722(12],o.2731(17) 0.2737(14)0.2723(13) o.2715(14) 0.1470(6) 0.1479(4) 0.1465(6) 0.1482(4) 0.1471(6) 0.1476(s) 0.1472(5) 0.1476(5) z 0.0467(10) 0.0455(7) 0.0444(8) 0.0447(7) 0.04s3(10) 0.0474(9) 0.0449(8) 0.04s0(8) B 1.1 4(1 3) 1.34(9) 0.84(12) 1.14(1 0) 1.20(1 4) 1.02(12) 0.92(10) 1.057(1 1) Note.
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