Calorimetric study of the stability of high pressure phases in the systems CoOSiO 2 and “FeO”SiO 2, and calculation of phase diagrams in MOSiO 2 systems

Abstract

High temperature calorimetric measurements of the enthalpies of solution in molten if2 PbO · B 2O 3 of α- and γ-Fe 2SiO 4 and α-, β-, and γ-Co 2SiO 4 permit the calculation of phase relations at high pressure and temperature. The reported triple point involving α-, β-, and γ-Co 2SiO 4 is confirmed to represent stable equilibrium. The curvature in the α−β phase boundary in Co 2SiO 4 and of an α−γ boundary in Fe 2SiO 4 at high temperature is explained in part by the effects of compressibility and thermal expansion, but better agreement with the observed phase diagram is obtained when one considers the effect of small amounts of cation disorder in the spinel and/or modified spinel phases. The calculated ΔH 0 and ΔS 0 values for the α−β, α−γ, and β−γ transitions show that enthalpy and en changes both vary strongly in the series Mg, Fe, Co, and Ni, and are of equal importance in determining the stability relations. The disproportionation of Fe 2SiO 4 and Co 2SiO 4 spinel to rocksalt plus stishovite is calculated to occur in the 170–190 kbar region; cation disorder and/or changes in wüstite stoichiometry can affect the P− T slope. The calorimetric data for CoSiO 3 and FeSiO 3 are in good agreement with the observed phase boundary for pyroxene formation from olivine and quartz. The decomposition of pyroxene to spinel and stishovite at pressures near the coesite-stishovite transition is predicted in both iron and cobalt systems. The use of calorimetric data, obtained from small samples of high pressure phases, is very useful in predicting equilibrium phase diagrams in the 50–300 kbar range.