Silicon-substituted magnetite and accompanying iron oxides and hydroxides from the Kumano mine, Yamaguchi Prefecture, Japan: Reexamination of the so-called maghemite (γ-Fe₂O₃)

Abstract

Iron oxides and hydroxides found in the so-called maghemite-bearing iron oxide ores from the Kumano mine, Yamaguchi Prefecture, Japan have been investigated using polarizing microscopy, reflecting microscopy, powder X-ray diffractometry (XRD), and electron probe microanalysis (EPMA) to reevaluate the previous study carried out by Shibuya (1958).
     The results show that the iron oxide described as maghemite by Shibuya is actually magnetite, showing a compositional zoning of Si with trace amounts of Al and Ca, and that only hematite and goethite are found as alteration products. The compositional zoning of the magnetite may indicate that the silicon-bearing magnetite was formed during reducing conditions.
     The a lattice parameter of a = 8.376(1) Å for the silicon-bearing magnetite is shorter than that of normal magnetite. This contraction in the crystal lattice is probably caused by a coupled substitution: Si⁴⁺ for Fe³⁺ on the tetrahedral sites and a divalent cation (mainly Fe²⁺) for Fe³⁺ on the octahedral sites, with a simple substitution of Al³⁺ for Fe³⁺ on the tetrahedral sites.