The effect of oxidation and reduction on thermal expansion of magnetite from 298 to 1173 K at different vacuum conditions

Three thermal expansion curves of natural and synthetic magnetite (Fe₃O₄) have been measured under different vacuum conditions (10⁻⁴ and 10⁻⁶ mbar). The different behavior in thermal expansion depends on vacuum level, which controls the partial oxidation of the sample. If the vacuum is poor, that is in mildly oxidizing conditions, the thermal expansion curve presents a discontinuity at 875 K and the samples oxidizes. In non-oxidizing conditions the discontinuity is not present and the thermal expansion coefficient is markedly smaller. The experimental curves indicate that virtually all thermal expansion data on magnetite in the literature were measured on oxidized or partially oxidized samples.