TY - JOUR
T1 - Effects of hydration on the structure and compressibility of wadsleyite, β-(Mg2SiO4)
AU - Holl, Christopher M.
AU - Smyth, Joseph R.
AU - Jacobsen, Steven D.
AU - Frost, Daniel J.
PY - 2008/4
Y1 - 2008/4
N2 - A suite of pure magnesian wadsleyite (β-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 β = 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, ρ = 3.470(2)-0.046(2) CH2og/cm3 (with CH2o in wt% H2O). Bulk moduli and pressure derivatives of wadsleyite are KTO = 173(5) GPa, K01 = 4.1(15) for 0.005 wt% H2O; KTO = 161(4) GPa, K01 = 5.4(11) for 0.38 wt% H2O; KTO = 158(4) GPa, K01 = 4.2(9) for 1.18 wt% H2O; and KTO = 154(4) GPa, K01 = 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.
AB - A suite of pure magnesian wadsleyite (β-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 β = 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, ρ = 3.470(2)-0.046(2) CH2og/cm3 (with CH2o in wt% H2O). Bulk moduli and pressure derivatives of wadsleyite are KTO = 173(5) GPa, K01 = 4.1(15) for 0.005 wt% H2O; KTO = 161(4) GPa, K01 = 5.4(11) for 0.38 wt% H2O; KTO = 158(4) GPa, K01 = 4.2(9) for 1.18 wt% H2O; and KTO = 154(4) GPa, K01 = 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.
KW - Bulk modulus
KW - Equation of state
KW - Mantle Transition Zone
KW - Nominally anhydrous minerals
KW - Wadsleyite
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U2 - 10.2138/am.2008.2620
DO - 10.2138/am.2008.2620
M3 - Article
AN - SCOPUS:43149119053
SN - 0003-004X
VL - 93
SP - 598
EP - 607
JO - American Mineralogist
JF - American Mineralogist
IS - 4
ER -