Omission solid solution in magnetite: Kenotetrahedral magnetite*

Abstract

Magnetite and sulfur, when heated together for 1 to 30 days days in silica tubes, where vapor is always present, react to form pyrite and iron-deficient magnetite (Fe 3– v O 4 , 0 < v ≤ 1/3) when S/Fe 3 O 4 ≤ 2/3 (at. ratio). For 2/3 < S/Fe 3 O 4 ≤ 7/4, FeSO 4 occurs as an additional phase. In this magnetite a measurable fraction of the vacancies is found to occupy tetrahedral sites in the inverse-spinel structure, thus forming “kenotetrahedral magnetite”. Sharpness of powder lines, absence of superstructure reflections, and difference in cell dimension distinguish this magnetite from the previously known “keno-octahedral” type, in which vacancies occur exclusively on octahedral sites. Total number of vacancies and number of tetrahedral vacancies are given on a nomogram as functions of density and cell dimension. Kenotetrahedral magnetite does not take measurable amounts of sulfur in solid solution. It forms when stoichiometric Fe 3 O 4 is heated in a flow of pure oxygen. Silica-tube experiments conducted over extended periods of time and collapsible-gold-tube experiments at 2 kbar show that “kenotetrahedral magnetite” is metastable. The reaction 3Fe 3 O 4 + 2S → 4Fe 2 O 3 + FeS 2 proceeds through the formation of metastable Fe 3– v O 4 and apparently a stage involving a metastable type of hematite before stable hematite is produced.