The non-existence of iron-rich magnetites

Abstract

While magnetite is known to be iron deficient at high temperature, five publications have independently claimed that iron-rich magnetites Fe_3+X O₄ have been observed in the range 250 to 620‡ C. The related experiments are based upon reduction of magnetite, goethite, maghemite, hematite or iron ore, and for the one of them, upon ferrous hydroxide decomposition. Another article reports an unsuccessful attempt to produce the iron-rich magnetites through hematite reduction at 535‡ C, but this temperature is considerably higher than almost all of the reduction temperatures used where iron-rich magnetite has been detected. We have therefore carried out an extensive series of experiments in order to prepare iron-rich magnetite through reductions as near as possible to the published experimental conditions. Through thermogravimetric analysis, chemical analysis, Mössbauer spectrometry and magnetic measurements, we have shown that no excess iron magnetite has been obtained. A possible explanation could be, in some cases at least, the fact that OH groups remain in the material due to preparation from solutions and annealing at moderate temperatures.