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Effects of hydration on the structure and compressibility of wadsleyite



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American Mineralogist Volume 93 pages 598 607 2008 Effects of hydration on the structure and compressibility of wadsleyite Mg2SiO4 Christopher M Holl 1 Joseph R Smyth 1 Steven D Jacobsen 2 and Daniel J Frost3 2 1 Department of Geological Sciences University of Colorado Boulder Colorado 80309 U S A Department of Earth and Planetary Sciences Northwestern University Evanston Illinois 60208 U S A 3 Bayerisches Geoinstitut Universit t Bayreuth Bayreuth 95440 Germany Abstract A suite of pure magnesian wadsleyite Mg2SiO4 samples containing 0 005 0 38 1 18 and 1 66 wt H2O was studied by single crystal X ray diffraction to determine the effects of hydration on cation ordering and crystal symmetry Separate compressibility experiments were carried out to 9 6 GPa to determine the effects of hydration on isothermal equations of state Crystal structure refinements at ambient conditions show cation vacancies order onto the M3 site The most hydrous sample 1 6 wt H2O displayed monoclinic symmetry with 90 090 7 whereas the samples with lower content were statistically orthorhombic The density of wadsleyite decreases with increasing water content at STP according to the empirical relation 3 470 2 0 046 2 CH2O g cm3 with CH2O in wt H2O Bulk moduli and pressure derivatives of wadsleyite are KT0 173 5 GPa K0 4 1 15 for 0 005 wt H2O KT0 161 4 GPa K0 5 4 11 for 0 38 wt H2O KT0 158 4 GPa K0 4 2 9 for 1 18 wt H2O and KT0 154 4 GPa K0 4 9 11 for 1 66 wt H2O Variation of the bulk modulus of wadsleyite with water content is non linear which may be attributable to softening of the structure by ordering of vacancies onto two non equivalent M3 sites M3a and M3b and an accompanying dilution of orthorhombic symmetry Keywords Wadsleyite bulk modulus equation of state nominally anhydrous minerals mantle Transition Zone Introduction Wadsleyite Mg2SiO4 is the stable polymorph of olivine in the mantle Transition Zone from 410 to 525 km depth The olivine wadsleyite transition of Mg2SiO4 is thought to



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