As an extension of previously described work on natural pyrrhotites, we have used a magnetic balance to observe magnetite solution-exsolution phenomena and to ascertain the magnitude of the oxygen solubility. Oxygen is soluble in pyrrhotite to the extent of at least 0.37% at 700°C (1.35% magnetite), and magnetite exsolves at lower temperatures. Oxygen moves freely over short distances at temperatures near 300°C, and since metal vacancies are also known to be mobile at low temperatures, natural pyrrhotite cannot be expected to reflect its conditions of formation with any degree of reliability. The activation energy for diffusion of oxygen in pyrrhotite (106 kJ/mol) is only slightly greater than that for vacancy diffusion (86.8 kJ/mol). “Abnormal” pyrrhotite formed by quenching is probably a mixture of two phases and may be stabilized by oxygen. Limited annealing experiments indicate that the presence or absence of oxygen may affect the phase stability.

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