The effect of non-stoichiometry on the high-temperature behaviour of MgAl2O4 spinel

The equation of state for gamma-Fe2SiO4 was determined by single-crystal X-ray diffraction up to 10.2 GPa at room temperature. The pressure–volume data, measured at 10 different pressures, do not show any evidence of a phase transformation and were fit by a third-order Birch–Murnaghan equation of state. The following coefficients were refined simultaneously: V0 = 559.44(6) A3, KTD = 187.3(1.7) GPa, and K0 = 5.5(4). This result implies that the Mg/Fe substitution along the gamma-Fo–Fa join does not significantly affect the bulk modulus. The sample shows higher K' than other compositions along the Fo–Fa join, which generally have K' about 4. Based on our results we can calculate the sound bulk velocity of gamma-Fe2SiO4 up to the pressures of the Transition Zone. Due to the larger K' of gamma-Fe2SiO4, the difference in bulk sound velocity between the two end-members decreases with increasing pressure.