Given that the perovskite form of (Mg, Fe)SiO 3 is ... conclusion is disputed by a recent study on a single ..... E. Ito and D. J. Weidner, Crystal growth of MgSiO 3.
GEOPHYSICAL RESEARCH LETTERS, VOL. !7, NO. 12, PAGES 2089-2092,NOVEMBER 1990
CRYSTAL STRUCTURE ANDTHERMALEXPOSIONOF (Mg,Fe)SiO 3 PEROVSKITE J. B. Parise,Y. Wang,A. Yeganeh-Haeri MineralPhysics Imtitute,StateUniversity of NewYork at StonyBrook D. E. Cox
Departmere of Physics, Brookhaven NationalLaboratory Y. Fei
Geophysical Laboratory, Carnegie Institution of Washington Abstract.High-resolution x-raydiffractiondatawere
collectedfrom 10 to 433K on
presentevidencethat Fe substitutes for Mg in the 8-!2 a homogeneous coordinatedA-site. Unfortunately,the substitutionof
Cr3+forMg2+further complicates thechemist• ofthe polycrystalline specimen of Mg0.9Fe0.1SiO3-perovskite. Rietve!dstructuralrefinementand x-ray absorption system byrequiting substitution ofMg2+forSi4•'.
fluorescence
measurements demonstrate that Fe In this report, we present an investigationof the substitutes for Mg and not Si. The thermalexpansion crystalstructureand thermal expansionbehavior of behavioris anisotropic, with the orthorhombic distortion M.•..9Fe0.1SiO3-perovskite, inorder toaddress questions becoming less with increasing temperature. The arisingfromthepreviousstudies,viz. (1) What is the role volumetric thermalexpansion, whichis dominatedby the of ironin MgSiO3-perovskite; (2) Whatis theaverage
decrease in octahedral tilts,is 1.9x10 '5 K'l between !50 •d
373IC No evidence of conversion to enstatite could
be foundup to 873 K.
thermal expansion coefficient of (Mg,Fe)SiO3-perovskite; and (3) At what temperature does (Mg,Fe)SiO 3perovskite revert,if at all, to itslowerpressure polymorph enstatite?
Introduction
Experimental
Giventhattheperovskite formof (Mg,Fe)SiO 3 is
widelyconsideredto dominatethe earth'slower mantle (Liu, 1975;Ito, 1977;Ito et al., 1984;Knittie andJea.•oz,
1987),its crystalstructureand response to changes in temperature andpressure are of fundamental importance in understanding the physicalandchemicalprocesses of the lowermantle.The thermalexpamionbehaviorof
MgSiO3-perovskite, studied byRossandHazen(1989),is reported to be anisotropic.On the otherhand,Knitfieet
al. (1986) characterized •e
zero-pressure thermal
A polycrystalline specimen ofMg0.9Fe0.•SiO 3 perov-
skitewassynthesized in a 2000-tonuniaxialsplitsphere apparatus (USSA-2000). The cellassembly, similarto that usedbyIto andWeldher(1986),employed Re ratherthan Pt as heatingelements,to avoidreason betweenPt and
Fe in thestarting material, (Mg0.9Feo. x)SiO 3 enstatite. Powder waspacked intothe150-/•m-thick Re capsule and the synthesis performedat 260 kbar and 1973K for ! h.
Microprobe analysis gave a
composition of
Mg0•4)Fe0.106(!)Six.007(2)O3 (average of12analyses).
expansion ofpolycrystalline (Mg,Fe)SiO3-perovskite upto X-raydiffractaon intensity measurements wereper840K usingpowderx-rayphotography. Theyobserved a formedat the X-7A beamline,described by Coxet al back-transformation to the low-pressure phaseenstatite. (198.8), at theNationalSynchrotron LightSource usinga Theyconcluded thatthelatticeparameter ratios(a/b and channel cut Si(1!l) monochromater and Ge(220) c/a) remained constants,suggestingfundamental
analyser. Thewavelengths used were0.69684(4)• fordata collected from10to300Kand0.819430)•from300Kto
differences between thebehaviors of MgSiO 3 and Mgl.xFexSiO 3perovskites.
433K;their•.•alues weredetermined usinganSistan•d (i•=5.430825A). The •tmment wasconfigured in the high-resolution mode(seeCoxet al, 1988). For room examined (Mg,Fe)SiO3-perovskite quenched from500 (294K) measurement, the as-synthesized kbarand2001YK usingextended x-rayabsorptionfrae temperature pellet (0.8 mm in diameter and L5 mm length)was structure (EXAFS)spectroscopy, andconcluded thatFe mounted ona glass fiberwithbeamdefining edits adjusted substitutes forSiin theoctahedral (Bhsite, witha beforeand after the sampleto opfimi• peak-tostructural formula vm'xl!(Mglx,,FexSi 0 (Si•,fev)O v backgrounddiscrimination.Data were collectedin •e conclusion isdisputed bya recent study ona single range10.
