CALCULATION OF THE PHASE BOUNDARIES OF WUSTITE AT HIGH-PRESSURE

The phase boundaries of wustite at high pressure are calculated using a thermodynamic data set describing the Fe-O system. A new thermodynam ic description of wustite is presented, whereas the thermodynamic desc riptions of iron, oxygen, magnetite, and hematite are taken from liter ature. An equation for the Gibbs energy of formation of wustite at amb ient pressure rests on recent calorimetric studies. The description gi ves reasonable values for the eutectoidal point and the phase boundari es of wustite at high temperatures and furthermore explains the format ion of a metastable intermediate two-phase mixture of nearly stoichiom etric wustite and magnetite on heating quenched wustite to around 500 K. Extrapolations to high pressure are made by taking the compositiona l, temperature, and pressure variation of the molar volume of wustite into consideration. In view of the spread in the experimental and esti mated bulk modulus values for wustite, three alternatives regarding th e compositional variation of the bulk modulus are considered. The assu mption of a composition independent bulk modulus (K-0 = 180 or 150 GPa ) results in stoichiometric FeO being the stable phase at high pressur es, whereas the assumption of a composition dependent bulk modulus res ults in a significant widening of the homogeneity region of wustite an d also indicates that FeO disproportionates to iron and nonstoichiomet ric wustite at high pressures. The effect of changes in partial deriva tive K-0/partial derivative T and partial derivative K-0/partial deriv ative P on the calculated phase boundaries is considered. Earlier expe rimental studies and theoretical considerations are discussed as are t he implications of phase transitions in wustite and in the neighboring phases.