Thermodynamic study of the FeO-MgO-Al2O3-SiO2 system : Data assessment and phase diagram calculation

Abstract: This dissertation consists of assessed thermodynamic properties for phases in the geologically and industrially important system Fe-Mg-Al-Si-O and calculated phase diagrams. Mathematical processing of different kinds of experimental data (phase equilibrium data, calorimetric, volumetric, thermophysical,electrochemical, activity data) is used to derive internally consistent thermodynamic database. The Murnaghan equation of state is used to describe the pressure-volume-temperature (P-V-T) relations in the end members and stoichiometric phases. Thermal expansion, compressibility and pressure derivative of bulk modulus are considered as temperature dependent. Sublattice model is applied to describe solid and liquid solutions and Redlich-Kister model is used for interaction between species on a sublattice.Thermodynamic data for solid phases in the Fe-Si-O system are optimized to reproduce phase relations at temperatures up to 2000 K and pressures between 1 bar and 20 GPa. Parameters for liquid phases in the Fe-Si-O system are assessed from solid-liquid equilibria at 1 bar. Because of lack of high pressure experimental data for liquid, melting relations at high pressures are calculated only in thesubsystems Fe-FeO, SiO2 and Fe2SiO4.Thermodynamic parameters for solid phases in the system FeO-MgO-Al2O3-SiO2 are assessed and subsolidus phase relations are calculated at pressures up to 30 GPa. The calculated phase diagram for pyrolite composition are used to explain seismic discontinuities in the Earth's mantle. The 400 km discontinuity could be attributed to garnet+olivine+β-spinel+hp-clinopyroxene divariant zone. The seismic boundary at 650-680 km could be related to two very narrow fields of ilmenite=perovskite and γ-spinel=perovskite+magnesiowustite transitions. Other seismic discontinuities could be assigned to phase transformations occurring at 450 km (complete dissolution of pyroxene in garnet), at 500-520 km (β-spinel+γ-spinel zone), and at 600 km (γ-spinel+stishovite+ilmenite or garnet+ilmenite zones). Garnet disappears at 730-750 km and the assemblage perovskite+magnesiowustite is stable below.