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The effects of pressure and temperature on nonstoichiometric wüstite, FexO: The iron-rich phase boundary

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Abstract

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 Fe x O 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 Fe x O is smaller than the molar volume of Fe, favouring an increase in stoichiometry of Fe x O. To reproduce the experimentally observed decrease in x above 10GPA, the bulk modulus of Fe x O must vary strongly with x for x≳0.96, causing a rapid increase in the partial molar volume of Fe in Fe x O. Continuation of a strong sensitivity of K to x in Fe x O for x≲0.96, however, leads to absurdly low molar volumes of Fe x O at high pressure and no equilibrium between Fe and Fe x O exists. Observations therefore require a reduced sensitivity of K with x for x<0.96, achieved by a negligible variation of K 0 with x for x<0.96, or a strong variation of dK/dP with x, or perhaps both.

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McCammon, C.A., Liu, Lg. The effects of pressure and temperature on nonstoichiometric wüstite, FexO: The iron-rich phase boundary. Phys Chem Minerals 10, 106–113 (1984). https://doi.org/10.1007/BF00309644

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  • DOI: https://doi.org/10.1007/BF00309644

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