Conrrasr, the Mean Mo Bond Distances I" Botl'ili;; I#:N\Liltil:L."

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 significant 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. Values in oarenthesesreoresent esd's. .. Data in this column collectedwith 4-mm-diameterapertures; all other data collectedwith 8-mm-diameterapertures. t lsotropic extinction coefficientas defined by Zachariasen(1967). 750 SHARP ET AL.: HIGH-PRESSURE CRYSTAL CHEMISTRY OF MONTICELLITE

TneLe3. Monticellite:Selected bond distancesand anglesat variouspressures

Bond or angle 1 bar | 1.1kbar 2O.7kbat 29.1 kbar 4'1.2kbal 53.0 kbar 61.7kbar Ml-O1[2] 2.21O(71 2.173(71 2.187(61 2.198(8) 2.172(7) 2 154(6) 2.145(6) M1-O2[2] 2.071(7) 2.086(7) 2.081(s) 2.060(7) 2.066(6) 2.070(6) 2.065(6) M1-o3[2] 2.118(7) 2.107(6) 2.10s(5) 2.095(8) 2.098(6) 2 081(5) 2 078(6) MeanM1-O [2] 2.133 2.122 2.126 2.118 2.112 2.102 2.096 01-M1-O2[2] 83.10(29) 82.84(26) 83.50(20) 83.45(30) 83.18(26) 83.09(23) 82.88(27\ 01-M1-O2[2] 96.90(29) 97.16(26) 96.50(20) 96.s5(30) 96.82(26) 96.91(23) 97.12(27\ 01-M1-O3[2] 87.40(31) 87.17(31) 87.34(23) 87.78(36) 87.03(29) 87.16(27) 87.27(291 01-M1-O3[3] 92.60(31) 92.83(21) 92.66(23) 92.22(36) 92.97(29) 92.84(27) 92.73(291 o2-M1-O3[2] 75.09(23) 74.96(28) 75.45(23) 7s 50(34) 75.17\29) 75.72(261 75.65(28) o2-M1-O3[2] 10491(32) 105.04(28) 104.55(23) 104.50(34) 104.83(29) 104.28(26) 104.35(28) M2-O1 2.502(9) 2.463(9) 2.463(7) 2.464(11) 2.434(9\ 2.416(8) 2.418(8) M2-O2 2.328(10) 2.302(10) 2.2s0(8) 2.303(10) 2.281(91 2.264(9) 2.264(9) M2-o3 [2] 2.407(7) 2.413(71 2.394(s) 2.388(8) 2.377(61 2.375(6) 2.36s(6) M2-O3[2] 2.295(5) 2.278(61 2.265(51 2.278(7) 2.268(6) 2.257(51 2.2s0(6) MeanM2-O 2372 2 358 2345 2.350 2.334 2.324 2.320 01-M2-O3[2] 75.03(28) 74.50(28], 75.30(21) 75.69(31) 75.35(27) 7s.11(24) 74.99(25) 01-M2-O3[2] 97 29(231 98.30(21) 97.48(17}. 97.s2(25) 97.75(21) 98.16(1 9) 98.13(20) o2-M2-O3I2l 9899(29) 99.06(29) 98.37(23) 97.89(30) 97.91(26) 97.85(25) 97.64(24) o2-M2-O3[2] 8672(251 86.05(23) 86.78(17) 86.77(27) 86.73(22) 86.59(20) 86.82(22) o3-M2-O3[2] 9172(18) 91.75(1 6) 91.28(12) 91.70(1 8) 91.4s(1 s) 91.51(1 3) 91.35(1 5) 03-M2-O3 6s.21(34) 6s.69(31) 6s.93(26) 65.24(36) 65.15(32) 65.99(28) 66 01(29) 03-M2-O3 11 1 .1s(38) 110.69(34) 11 1.36(28) 11 1 .25(41) 111.84(24) 110.90(30) 11119(33) si-o1 1.574(12) 1.628('t2) 1.582(9) 1.s56(13) 1.593(1 1) 1.601(10) 1.593(1 1 ) si-o2 1.673(1 1) 1.647(1 0) 1.643(8) 1 672(121 1.646(10) 1.654(9) 1.64s(s) si-o3 [2] 1.626(8) 1.631(7) 1.64s(6) 1 615(9) 1.612(7) 1.624(6) 1.628(7) MeanSi-O 1.62s 1.634 1.629 1 615 1616 1.626 1.625 o1-si-o2 11 6.08(54) 11 5.40(51) 11 6.3s(40) 11 6.23(60) 116.32(50) 11 5.e9(46) 115.89(46) 01-si-o3 11 5.1 7(35) 114.42(33) 11 4.56(25) 115.1 3(39) 114.48(33) 114.87(281 115.41(27) [2] 't02.23(37) o2-si-o3 [2] 10137(41) 102.49(29) 101.40(44) 102.43(37)' 101 .99(32) 101.66(35) 03-si-os 105 82(s4) 106.69(49) 104 75(39) 105.67(61 ) 105.1 0(49) 105.54(42) 104.89(45) o1-si 1.574(121 1.628(1 2) 1.582(9) 1.556(1 3) 1 593(11) 1.601(1 1) 1.593(1 1 ) o1-M1[2] 2.210(7) 2.173(7) 2.187(6) 2.1e8(8) 2.172(7) 2.154(6) 2.145(6) o1-M2 2.502(101 2.463(9) 2 463(71 2.464(11) 2.434(9\ 2.416(8) 2 418(6) si-o1-M1 [2] 12s 88(31) 12467(311 12514(24',) 126.05(37) 124.95(30) 124.69(281 124 90(28) si-o1-M2 11 s 54(48) 11 4 84(45) 11 5 94(3s) 11 5.91(53) 115.79(45) 115.22(401 114.57(42]. M1-O1-Ml 92.40(39) 94 14(411 93.00(31) 92.18(44) 93.30(38) e4.06(36) 94 39(36) M1-O1-M2[2] 94.s6(32) 9s.79(33) 9s.03(25) 94.17(38) 95.24(32) 95.55(29) 95 48(30) o2-si 1 673(11) 1.647(1 0) 1.643(8) 1.672(12) 1.646(1 0) 1.654(9) 1.649(9) o2-M1 [2] 2.071(71 2.086(7) 2.081(s) 2.060(7) 2.066(6) 2.070(61 2.065(6) o2-M2 2 328(10) 2.302(10) 2.290(8) 2.303(10) 2.281(s) 2.264(e) 2.264(el si-o2-M1 [2] 91.83(36) 91.3s(33) 91.41(26) 91.25(36) 91.1 6(32) 90.75(29) 91.13(31 ) si-o2-M2 117.21(61) 11 8.68(62) 117.82(48) 116.55(63) 11 7.38(55) 117.57(54) 11 7.0s(53) M1-O2-M1 100.78(46) 99.40(45) 99.36(36) 100 47(45) 9972(41) 99.17(40) 99.29(40) M1-O2-M2l2l 123.48(27) 123.77(24) 124.08(1 9) 124.20(26) 124.25(231 124.65(21) 124.57(23) o3-si 1.626(8) 1.631(7) 1.645(6) 1.615(9) 1.612(7) 1.624(6) 1.628(7) o3-M1 2.118(7) 2.107(6) 2.10s(5) 2.095(8) 2.098(6) 2.081(s) 2.078(6) 03-M2 2.4O7(7) 2.413(7) 2.394(s) 2.388(8) 2.377(6) 2.375(6) 2.369(6) 03-M2 2.29s(7) 2.278(6) 2.265(5) 2.278(71 2.268(6) 2.257(5) 2.250(6) si-o3-M1 19.49(37) 91.05(33) 90.38(24) 91.62(40) 90 97(32) s1.21(27) 91.29(30) si-o3-M2 94.47(29) 93.77(26) 94.60(21) 94.52(33) 94.81(27), 94 20(24) 94.s2(26) si-o3-M2 130.1 0(49) 130.87(44) 130.24(36) 130.35(52) 130.75(441 130 32(39) 129.92(40) M1-O3-M2 99.8s(30) 99.09(27) s9.20(221 99.20(32) 98.97(27) 98 80(24) 98.81(26) M1-O3-M2 115.72(28) 116.45(24) 115.94(21) 115.72(31) 115.33(26) 11621(22], 116.01(24) M2-O3-M2 118.58(30) 11 8.50(27) 119.32(21) 118.62(31) 118.93(26) 119.00(23) 119.25(25) Note.'Bracketed values represent bond or angle multiplicities;values in parenthesesrepresent esd's.

in collecting data from a small crystal (i.e., with relatively small In order to test this aperture-reductionprocedure, data at I I diffracting volume) mounted in a sample chamber that causesa kbar were collectedin two ways-once with fully opened 8-mm high background of scatteredradiation. It is common in high- aperturesand once with half-closed4-mm apertures.By reduc- pressurestudies, therefore, for more than 500/oof reflections to ing the aperturesthe number of observed reflections increased be unobserved,and unobservedreflections may exceed900/o in from 183to 198,andtheweightedR decreasedfrom4.5to3.9o/o some light-atom silicares.Reducing the diameter of the diffract- (Table 2). This improvement was not considered sufrcient to ed-beamaperture provides one method for lowering background warrant recollection of data at other pressures.In experiments scatter and thus increasingthe ratio of peak-to-background.A with poor peak-to-backgroundratios, however, this procedure significant potential drawback of this method, however, is the may result in a significant improvement in the quantity and possibility of cutting out a portion of the radiation diffracted quality ofintensity data, providing that the crystal orientation from the sample. is stable and well known. SHARP ET AL.: HIGH-PRESSURE CRYSTAL CHEMISTRY OF MONTICELLITE 751

2.14

480

2.to ^ 476 rrr € t z.os rro fl -a 2.36

j lO9 ; 2.34 I 638 E z.tz c 634 3 23O

630 1.62 €.* t,60

! 33s P (kbor) Fig.2. Meanmetal--oxygen bond distances in monticelliteas 3 33o l a functionofpressure. = ges

: + Fig. l. Unit-cell parametersof monticellite as a function of yieldsa bulk modulusKo Ll3 0.0I Mbar if the bulk- temperature. modulus pressurederivative, K', is assumedto be 4.0 and the volume at I bar (V):341.6 t 0.1 43. An earlier compressibility estimate for monticellite, based on the Failed experirnentson CarSiOo expression Zmonrcerrite: r/zVroun,rnI t/zV.^-orrurn"(Sharp et al., We had hoped to combine results on monticellite with high- 1986),is in excellentagreement with the measuredvalue. pressuredata on the olivine form of CarSiO.. A colorless,syn- thetic single crystal with several l0-pm fluid inclusions was ob- Monticellite crystal structure tained from Gordon Brown, Stanford University. Unfortunately The olivine-type crystal structure of monticellite (or- pressure this crystal imploded at a of less than l0 kbar. Such a thorhombic: Pbnm, Z: 4; with the mineralogicalcon- result might have been anticipated for an inclusion-rich sample, vention for olivines: b > c > a) was first described by but this is the first time that we have observedsuch a phenom- Braggand Brown (1926) and was subsequentlyrefined by enon. The high-pressure,single-crystal experiments on 7-CarSiOo (1970). will be commenced as soon as we can synthesizea crystal of Brown The structureconsists of a distorted hex- sufrcient size. agonal close-packedarray of oxygens in which one-half of the octahedral sites are occupied by divalent cations RBsur,rs and one-eighth ofthe tetrahedral sites contain Si. Monticellite compressionand equation of state Crystal-structure parameters of monticellite determined at I bar in the diamond cell are similar to previ- Lattice parametersand unit-cell volumes vary smooth- ously reported values of Brown (1970) and Lager and ly as a function of pressurewith no anomalous values at Meagher (1978). The Mg Ml octahedron(point sym- any pressure(Table 1, Fig. l). Unit-cell edgesas a func- metry 1) has a mean Mg-O distanceof 2.132 A. It is tion of pressureand averagecompressibilities (l bar to distorted, with O-MI-O anglesdeviating by as much as 62 kbar) are as follows: 15"from the ideal 90'value. The Ca M2 octahedron(point a: 4.8207+ 0.0007- (0.00095+ 0.00002)P symmetry z) is significantly larger than Ml, with a mean (0": 1.96t 0.04 x l0-okbar ') (l) Ca-O distance of 2.372 A. ttre O-M2-O angles range b: lr.l04 + 0.002 - (0.0050+ 0.0002)p from 65" to I I l" in this highly distorted polyhedron. The + (0.000015+ 0.000002)P' Si tetrahedron (point symmetry m) has a 1.625-A mean (h:3.62 + 0.01x lO-okbar') (2) Si-O distance. Three O-Si-O angles are as much as 8o c: 6.382+ 0.002- (0.0018+ 0.0001)P lessthan the ideal 109.5'valuefor a regulartetrahedron, + (0.000007a 0.000002)P, whereasthe other three anglesare more than 6o greater (B": 2.05 + 0.08 x l0 4 kbar-'). (3) than this value. The pairs ofoxygens that define the nar- rower anglesform shared edgesbetween the tetrahedron The a cell edge varies linearly with pressure,so that the and adjacent octahedra. second-orderterm is not necessary,whereas the b and c cell edgesdisplay slight curvature as a function of pres- High-pressurecrystal structures sure(Fig. l). Changesin cation-anion bond distancesconstitute the A least-squaresfit of unit-cell volume data as a func- principal structural variations with pressure(Table 3, Fig. tion of pressureto a Birch-Murnaghan equation of state, 2). Between I bar and 62 kbar the mean Ml-O distance P: l.5Ko|(Vo/V1ttt - (Vo/V)u'j decreasesfrom 2.132 to 2.096 A 0.lo/o), whereasthe mean - '{l 0.75(4- K',)l(VJn'','- ll), (4) M24 distan@compresse s from 2.372 to 2.320 A Q.ZVA. 752 SHARP ET AL.: HIGH-PRESSURE CRYSTAL CHEMISTRY OF MONTICELLITE

TneLe5. Monticellite:Polvhedral volume and distortionat variousoressures

Atom/ param- eler 1 bar 11 kbar 21 kbat 29 kbar 41 kbar 53 kbar 62 kbar Ml (Mg) v(A') 12 39(5) 12.20(5) 12 12(4) 12.1 1(6) 12.04(5) 11 .91(5) 11.79(5) 1.0297(141 1.0296(1 3) 1.0291(1 1) 1.0287(1 5) 1.0288(1 3) 1.0270(121 1.0273(12) 100.6 103.8 99.48 v/.o 100.2 94.6 96.0 M2 (Ca) Y (A') 16 63(7) 16 29(6) 16 09(5) 16.17(7) 15.84(6) 15.66(5) 15.59(6) OE 1 048(4) 1.049(4) 1.046(3) 1.047(5\ 1.047(4) 1.046(4) 1.046(4) 164.1 167.1 158.5 159.4 163.1 158.3 158.5 Si Y(A') 2.166(1 7) 2 210(17\ 2.187(1 3) 2.14s(18) 2.133(16) 2 173(141 2.164(1 5) 1.01 1(5) 1 009(s) 1.010(4) 1.01 1(5) 1.010(s) 1 010(4) 1.01 1(4) s0.6 394 43.7 49.7 43.0 4s.3 s0.9 - : Nofe; Valuesin parenthesesrepresent esd's. QE : quadraticelongation : 2i-, [(l,llo)2|(n 1)] (Robinsonet al., 1971).AV : angle variance 2i-,1(0,- 0o)2l(n- 1)l (Robinsonet al., 1971).

Over this same pressurerange, the mean tetrahedral Si- (Si)are 6.8(7),9.5(ll), and l(2) (all x 10 o kbar-'), re- O distanceis virtually unchangedat 1.625(10)A. Octa- spectively.The correspondingbulk moduli ( I /0) are 1.5( I ), hedra do not compressuniformly in monticellite. In the 1.1(l), and >3 Mbar for the Ml, M2, and T sites,re- Ml polyhedron the longestMl-Ol bonds compress3.00/0, spectively. whereasthe shortestMl-O3 bondscompress only 0.20lo; Hazen and Finger (1982) have derived an empirical M1-O2 bonds of intermediate length have intermediate equation for bulk modulus of a given polyhedron follow- compressionof 1.90/0.The simple pattern of longer bonds ing the approachof Hazen and Prewitt (1977). This equa- being more compressible does not hold completely for tion is the M2 octahedron. The longest M2-Ol bond is most ,, K,: l7.5(2)2./d3lMbar (5) compressible(3.50/o between I bar and 62 kbar), but the short M2-O2 bond is next most compressible (3.00/o), where Ko is the polyhedral bulk modulus, d is the mean comparedto 1.60locompression of the M2-O3 bonds with cation-anion distancein lngstroms, arrdz" is the integral intermediate length. formal charge of the cation. Polyhedral moduli of mon- There are no dramatic changesin oxygen-cation-oxy- ticellite predicted from Equation 5 and the mean cation- gen or cation-oxygen-cation anglesat high pressure.The anion bond distancesin Table 4 are 1.6, l.l, and 7.0 largest systematicchange occurs for the Ml-O2-M2 an- Mbar for the Ml, M2, and T sites, in good agreement gle,which increasesfrom 123.5(3) to 124.6(2)obetween with the measuredvalues. I bar and 62 kbar. The Ml-O2 and M242 bonds are the longestand are among the most compressiblein their DrscussroN respective octahedra; the increase in MI-O2-M2 angle High-pressurevs. high-temperaturestructures may reflect increased Ml-M2 repulsion as these M-O Monticellite volume reductions that occur upon bonds compress.No other angle changesby more than compression are significantly different from those ob- lo over this pressurerange. served on cooling from high temperature (Lager and Quadratic elongation (I) and anglevariance (o2),as de- Meagher, 1978).'1One of the most important differences fined by Robinson et al. (1971),are measuresof polyhe- is the relative volume change of Ml vs. M2. Thermal dral distortion. The quadratic elongation of the Ml and expansionof Ml (t 1.7 x l0 r kbar-') is slightly greater M2 polyhedra in monticellite do not vary significantly than that of M2 (= 1.3 x 10-ekbar '). On cooling,there- with pressure (Table 5), whereas the octahedral-angle fore, Ml contractsmore than M2. Compressionof Ml, variancesdecrease slightly as a function of pressure.This on the other hand, is lessthan that of M2; with increasing pressureeffect is small, however, and the strongereffects pressure,M2 contractsmore than Ml. of temperature (Brown and Prewitt, 1973) and compo- Thesedifferences may be quantified by consideringthe (Robinson sition et al., 1971)appear to overshadowany ratio B/a of polyhedral compressibility to thermal expan- pressurecontribution to polyhedral distortion. sion (i.e., what temperature increaseis required to offset Poiyhedral volumes for the Ml, M2, and T sites(Table a givenpressure increase). For Ml, B/a:16.8 x l0-ol/ program voLcAL, 5) were calculatedusing the written by [5.2 x l0-5] * l3'C/kbar, closeto the averagevalue of L. W. Finger (in Hazen and Finger, 1982). Polyhedral l4C/kbar observedby Hazenand Prewitt (1977)for Mg volumes for monticellite vary linearly with pressureand predominantly are controlled by bond shortening rather ':Note that the linear thermal expansioncoefficients for mon- than polyhedral distortion (Fig. 3). Average compress- ticellite and glaucochroiteil Table 9 of I-agerand Meagher(1978) ibilities (l bar to 62 kbar) for Ml (Mg), M2 (Ca), and T are typesetincorrectly and should be reversed. SHARP ET AL.: HIGH-PRESSURE CRYSTAL CHEMISTRY OF MONTICELLITE 753

Fig. 4. Relativeunit-cell parameters (d/dJ vs. relativeunit- cell volume(V/V) for monticellite.Circles, points, and crosses 2o 60 representthe b, c, and a axes,respectively. Points in the upper P (kbor) rightrepresent high-temperature data, whereas those in thelower left representhigh-pressure data. Fig. 3. Polyhedralvolumes of monticelliteas a functionof pressure.

Monticellite may be considered as alternating layers of octahedra in oxides and silicates. For M2, the value of Ml octahedra and M2-Si polyhedra perpendicular to c. p/a:19.5 x l0al/14.2 x l0 'l :22C/kbar, compared In this case,because Ml is the most expansiblepolyhe- to the averagevalue of I 8'C/kbar recorded by Hazen and dron, the greatestexpansion will be in the Ml octahedral Prewitt (1977). Note that the Si tetrahedron undergoes plane perpendicular to c. Expansion parallel to layering little, if any, volume changewith either pressureor tem- in the Ml octahedral plane will be limited by the adja- perature increases. cent, more rigid M2-Si polyhedral layers. The predicted The important differencesin high-pressureversus high- efect of pressureand temperature on monticellite, from temperatureresponse of the Ml and M2 cation polyhedra the above crystallographicconsiderations, is observedin are manifestin unit-cell behavior.The D-axislength, which the high-pressureand high-temperatureexperiments. is closely tied to M2 volume (Hazen, 1987),is almost Other olivine structuresmeasured as a function of pres- twice ascompressible as a or c. However, the c-axislength, sure and temperature, in contrast to monticellite, have which is more dependenton the sizeof M I , is much more an inverse pressure-temperaturerelationship. The change expansible than a or D. A plot of relative unit-cell edge oflattice constantswith increasingpressure can be offset lengthsversus relative unit-cell volume (Fig. a) illustrates by increasing temperature. However, in each of these the significantdifferences between unit-cell expansionand minerals (forsterite, fayalite, and chrysoberyl), Ml and compression. This "noninverse" behavior may be ex- M2 have nearly identical volumes, and the compress- plained by considering the compressibilities and expan- ibility and the expansivity of the Ml and M2 sites are sivities of the Ml and M2 octahedra in relation to the thus nearly identical. Neither the Ml nor M2 octahedral olivine structure. planeswill be favoredduring compressionor expansion. The only unique plane of M2 octahedra in olivine is For monticellite, on the other hand, there is no condition perpendicular to the b axis. Monticellite may, therefore, of combined high pressureand high temperature for which be consideredas a layered structure with alternating lay- the lattice parametersare the sameas at room conditions. ers of M2 octahedra and Ml and Si polyhedra perpen- This fact may, in part, explain the limited stability field dicular to b. A common feature of layered structures is of monticellite as a function of pressureand temperature. that compressibility and expansivity tend to be greatest The noninverse effect ofpressure and temperature on perpendicular to layering (Hazen and Finger, 1985). In the lattice parameters of monticellite might be used to monticellite, the majority of compressionis expectedto estimate the P-T conditions of crystallization. Solid-in- take place in the highly compressible M2 octahedral clusion piezothermometry is a method of determining planes, perpendicular to the layering. Compression par- P- I conditions during crystallization by examining a bir- allel to layering in the M2 octahedralplane will be limited efringent halo in a host crystal causedby the entrapment by the adjacent, less compressibleMl and Si polyhedral of a solid inclusion.Adams et al. (1975)determined P-I plane. conditions for zero-strain birefringence in garnet that In contrast, there is only one unique plane of Ml oc- showed birefringent halos around qtrartz at room condi- tahedra, and this plane is perpendicular to the c axis. tions. Piezothermometry and piezobarometry could pos- 754 SHARP ET AL.: HIGH-PRESSURE CRYSTAL CHEMISTRY OF MONTICELLITE sibly be used for monticellite. Becausemonticellite does 3CaMgSiO.: CarMgSi,O,+ MgrSiOo. (6) not expand and compress inversely, there should be a monticellite merwinite forstente unique pressureand temperature at which a piezometric halo around a solid inclusion would vanish. We have This reaction was reversedat l0 kbar and -ll75"C examined a number of grains of CascadeSlide monticel- (Yoder, 1968)and has a slopeof -90'C/kbar (Sharpet lite, which crystallized at 750 + 30"C and 7.4 + I kbar al., 1986). Monticellite is not stable above 25 kbar at (Valley and Essene,1980). Strong birefringent halos are room temperature (Yoder, 1968), and therefore, all represent around opaque inclusions in some parts of the finements over 25 kbar were made on a metastablephase. grains. The pressureand temperature of inclusion equil- The reducedmonticellite stability range,when compared ibration could be determined by optical examination of to forsterite, is thought to be related to the larger Ca cat- thesehalos as a function of P and Zin a heateddiamond- ion entering the M2 site. There are, however, no obvious anvil pressurecell. This technique has prospectsnot only structural discontinuities near 25 kbar; all structural pa- for pressure and temperature estimates, but also as a rameters vary regularly from I bar to 62 kbar. measureof the degreeof recrystallization during cooling The breakdown of monticellite by Reaction 6 involves in samplesthat do not show the piezobirefringence. the conversionofCa in 6-fold coordinationto 8-, 9- and lO-fold coordination in merwinite (Moore and Araki, pressure Comparisonwith forsterite at high 1972\.The mean Ca-O bond distance for monticellite at The Ml sitesin both forsterite and monticellite contain 25 kbar is 2.35 A. This distancerepresents the lower limit Mg in octahedralcoordination. TheseMl octahedrahave of mean Ca-O bonds in minerals with Ca in 6-fold co- nearlyidentical size (2.10 vs. 2.13A;, Uutt modulus(1.4 ordination. If this distanceis a limiting factor in control- vs. 1.5 Mbar) and thermalexpansion (1.6 vs. 1.7'C ') ling the stability of monticellite, then with increasing for forsterite and monticellite, respectively(Hazen, 1976; temperature, increased pressure would be necessaryto Lager and Meagher,1978; Kudoh and Tak6uchi, 1985). compressthe M2 octahedrato the point where the mean The tetrahedral Si sitesare also similar in thesetwo com- M2-O distanceis 2.35 A. However, the upper stability pounds,with averageSi-O distancesof 1.635 A, near- of monticellite has a negative slope in P-T space,in con- zero thermal expansion,and high bulk modulus. trast to what would be expectedfrom the above reason- The principal differenceslie in the M2 sites,which con- ing. Unlike monticellite, the forsterite-fayalite olivines tain Mg in forsterite and Ca in monticellite. The Ca-bear- have a positive P-7 stability limit. The large difference ing polyhedron of monticellite, with mean Ca-O of 2.37 in octahedralvolumes for monticellite may contribute to A and volume of 16.6 A', is significantly larger than the the negative slope for the upper stability of monticellite. forsterite M2, with mean Mg-O of 2.13 A and volume The polyhedral compressibilities and expansivities in 13.5 43. Hazen and Prewitt (1977) demonstratedthat monticellite are in good agreementwith those predicted polyhedral bulk modulus is inversely proportional to from the empirical model of Hazen and Finger (1982). polyhedral volume. The 20o/olarger volume of the Ca site When considerationof polyhedral variations is combined in monticellite is thus reflected in its 200/0greater bulk with crystal-structureconstraints, the origin of monticel- modulus (1.35 vs. l.l Mbar). Hazen and Prewitt also lite anisotropic expansionand compression,as well as the observed that polyhedral thermal expansion is usually noninverserelationship betweenthem, may be explained. dependenton Pauling bond strength(cation chargedivid- It is crucial, therefore, to view expansion and compres- ed by coordination number) and is independent ofpoly- sion on both the interatomic and interpolyhedral level to hedral volume. Thus, M2 octahedrain both monticellite understand and quantify the pressure-temperaturebe- and forsterite are predicted to have the same thermal havior of minerals. expansivity. Lager and Meagher (1978), however, ob- servedmonticellite M2 expansionto be 1.3 x l0 5oC I, Acruowr-nncMENTS 'C which is lessthan the 1.6 x l0 5 ' value observedfor We gratefully acknowledgeconstructive reviews by E J. Esseneand C. forsterite M2 (Hazen, 1976). It is possible that the Ca- T Prewitt. We also thank J. W. Valley and G. E. Brown for providing filled M2 site of monticellite is approaching a structural the specimensofmonticellite and 7-CarSiOo,respectively. This research part National ScienceFoundation grants EAR83- limit that restricts further was supported in by expansion of the already large 19209and EAR84-19982. site. Both monticellite and forsterite exhibit significant RnrnnnNcBs compressional anisotropy. Axial compression ratios for monticellite(a:b:c : 1.0:1.9: l. I ) comparewith forsterite Adams, H G., Lewis, H.C., and Rosenfeld,J.L. (1975)Solid inclusion values (1.0:2.0:1.6).The significantlygreater compress- piezothermometryII: Geometric basis, calibration for the association quartz-gamet,and application to some pelitic schists.American Min- ibility of b compared to a is a consequenceof the poly- eralogist,60, 58,t-598 hedral linkages between rigid Si tetrahedra and more Barnett,J D., Block, S., and Piermarini,G J (1973)An optical fluores- compressibleoctahedra (Hazen, I 987). cence system for quantitative pressuremeasurement in the diamond anvil cell. Review ofScientific Instruments. 44. 1-9. Stability field of monticellite Birle, JD., Gibbs, G.V., Moore, PB, and Smith, J.V. (1968) Crystal structure of natural olivines. American Mineralogist, 53, 807-824. Monticellite breaks down to forsterite and merwinite Bragg,W.L., and Brown, G.B. (1926) Die Struktur des Olivins. Zeit- by the reaction schrift {iir Kristallographie, 63, 538-556 SHARP ET AL.: HIGH-PRESSURE CRYSTAL CHEMISTRY OF MONTICELLITE 755

Brown, G.E, Jr. (1970)Crystal chemistry of the olivines.Ph.D. thesis, peaksin slep-scan-measuredBragg reflections Acta Crystallographica, Virginia PolytechnicInstitute, Blacksburg,Virginia A30. 580-584. -(1982) Olivines and silicate spinels. MineralogicalSociety of Lumpkin, G.R, Ribbe, P.H, and Lumpkin, N.E (1983)Compositlon, AmericaReviews in Mineralogy,5, 275-381. order-disorderand lattice parametersof olivines: Determinative meth- Brown, GE., Jr., and Prewitt, C.T. (1973) High-temperaturecrystal ods for Mg-Mn and Mg-Ca silicate olivines. American Mineralogist, chemistryof hortonolite AmericanMineralogist, 58, 577-587. 68, tt74-tt82. Hamilton, W.C. (1974) Angle settingsfor four-circle difftactometers.In Merrill, L., and Bassett,W.A. (1974)Miniature diamond anvil pressure Internationaltables for X-ray crystallography,4, 273J84. Kynoch Press, cell for single crystal X-ray diffraction studies. Review of Scientific Birmingham, England. Instrumenls, 45, 290-294 Hazen, R.M. (1976) Effects of temperature and pressureon the crystal Moore, P.B., and Araki, T. (1972) Atomic arrangement of merwinite, structureof forsterite American Mineralogist, 6 l, | 280- 1293 Ca,Mg[SiOJ,, an unusual dense-packedstructure of geophysicalinter- -(1985) Comparative crystal chemistry and the polyhedral ap- est.American Mineralogist, 57, 1355-1374. proach MineralogrcalSociety of America Reviews of Mineralogy, 14, Onken, H. (1965) Verfeinerung der Kristallstruktur von Monticellite. 317-346. TschermaksMineralogische und PetrographischeMitteilungen, 10, 34- -(1987) High-pressurecrystal chemistry of chrysoberyl,AlBeOo: 44 Insights on the origin ofolivine elasticanisotropy. Physicsand Chem- Robinson,K., Gibbs,G.V., and Ribbe,P.H. (1971)Quadratic elongation: istryof Minerals,14,13-20. A quantitative measureof distortion in coordination polyhedra. Sci- Hazen, R M, and Finger, L.W. (1980) Crystal structure of forsterite at ence,172, 56'7-570. 40 kbar. CamegieInstitution ofWashingtonYear Book 79,364-367 Sharp, 2.D., Essene,E.J , Anovitz, L M., Metz, G.W., Westrum, E.F., -(1982) Comparativecrystal chemistry. Wiley, New York. Jr, Hemingway,B.S., and Valley,J.W. (1986)The heat capacityof a -(1985) Crystalsat high pressure.Scientjfic American, 252, 110- natural monticellite and phaseequilibria in the systemCaO-MgO-SiOr- 1t7. CO,. Geochimicaet CosmochimicaActa, 50, 1475-1484. - ( 1987)High-temperature crystal chemistry of phenakite (BerSiO) Smyth, J.R. (1975) High temperaturecrystal chemistry of fayalite Amer- and chrysoberyl(BeAlrO). Physicsand Chemistry of Minerals,in press ican Mineralogist,60, 1092-1097. Hazen,R.M., and Prewitt,C T. (1977)Efects of temperatureand pressure Smyth, J.R., and Hazen, R.M. (1973) The crystal structuresof forsterite on interatomic distancesin oxygen-basedminerals American Miner- and hortonolite at severaltemperatures up to 900'C. Amencan Min- alogist,62, 309-3I 5. eralogist,58,588-593 King, H E., and Finger, L.W (1979) Diffracted beam crystal centering Swanson,D.K., Weidner,D.J., Prewitt,C.T., and Kandelin,J.J. (1985) and its application to high-pressurecrystallography. Joumal ofApplied Single-crystalcompression of l-Mg'SiOo (abs). EOS,66, 370. Crystallography,12, 3'74-37 L valley, J.W., and Essene,EJ. (1980)Akermanite in the CascadeSlide Kudoh, Y, and Takeda,H. (1986)Single-crystal X-ray diffractionstudy xenolith and its significancefor regionalmetamorphism in the Adiron- on the bond compressibilityoffayalite, FerSiO4,and rutile, TiO, under dacks.Contributions to Mineralogy and Petrology,'74, 143-152. high pressurePhisica, 139 and 1408, 333-336. Yoder, H.S., Jr (1968)Akermanite and relatedmelilite-bearing assem- Kudoh, Y, and Tak6uchr,Y. (1985)The crystal structureof forsterite blages.Camegie Institution of WashingtonYear Book 66,471-411. Mg,SiOounder high pressureup to 149 kbar Zeitschrift iiir Kristallo- Zachariasen,W H. (1967)A generaltheory ofX-ray diffraction in crystals. graphie,l7l,29l-302. Acta Crystallographica,23, 55 8-564. Lager,G A., and Meagher,E.P. (1978) High-temperaturestructural study of six olivines. American Mineralogist, 63,365-377. MeNuscnrpr REcETvEDDpcel,rsen 16, 1986 Lehmann, M S , and Larsen, F.K. (1974) A, method for location of the M.q.NuscmprAccEprED Aprtl 3. 1987