The effects of pressure and temperature on nonstoichiometric wüstite, FexO: The iron-rich phase boundary
- Cite this article as:
- McCammon, C.A. & Liu, L. Phys Chem Minerals (1984) 10: 106. doi:10.1007/BF00309644
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An experimental study using piston-cylinder, Bridgman anvil and diamond anvil cell techniques was undertaken to study the effect of pressure on the composition of FexO in equilibrium with Fe. At constant temperature the value of x first increases and then decreases with increasing pressure. The rate of change of x is a function of temperature. We have theoretically calculated the variation of the composition of wüstite with pressure and temperature. The initial increase of x with pressure for P<10 GPa occurs because the partial molar volume of FeO in FexO is smaller than the molar volume of Fe, favouring an increase in stoichiometry of FexO. To reproduce the experimentally observed decrease in x above 10GPA, the bulk modulus of FexO must vary strongly with x for x≳0.96, causing a rapid increase in the partial molar volume of Fe in FexO. Continuation of a strong sensitivity of K to x in FexO for x≲0.96, however, leads to absurdly low molar volumes of FexO at high pressure and no equilibrium between Fe and FexO exists. Observations therefore require a reduced sensitivity of K with x for x<0.96, achieved by a negligible variation of K0 with x for x<0.96, or a strong variation of dK/dP with x, or perhaps both.