Effects of Fe/Mg on the compressibility of synthetic wadsleyite: β-(Mg_1-xFe_x)₂SiO₄ (x≤0.25)

Abstract

Four crystals of synthetic wadsleyite, β-(Mg,Fe)₂SiO₄, were mounted together in one diamond-anvil cell for the determination of unit-cell parameters as a function of pressure. The Fe/(Fe+Mg) are 0.00, 0.08, 0.16, and 0.25 (the most iron-rich stable composition). Unit-cell refinements were made at 12 pressures up to 4.5 GPa. No phase transitions were observed and all crystals remained dimensionally orthorhombic. Of the three axes, c is the most compressible (0.000239(3) GPa^-1), whereas compressibilities of a and b are both about 30% less. The Fe content has no systematic effect on volume or linear compressibilities. Bulk moduli, based on a Birch-Murnaghan equation of state (K′ assumed to be 4.00) are 160(3), 169(3), 164(2), and 165(3) GPa for the four crystals in order of increasing Fe. Substitution of Fe for Mg, therefore, does not appear to have a systematic effect on bulk modulus. Other factors, especially Fe³⁺/Fe²⁺ and other deviations from the strict Mg₂SiO₄-Fe₂SiO₄ binary, may have a greater influence on compressibility.