Silician Magnetite from the Kamaishi Mine

Abstract

Silician magnetite was found in the copper sulfide ores from the Tengumori ore deposit of the Kamaishi mine, Iwate Prefecture, northeastern Japan. The mineral occurs generally as a zonal intergrowth with normal magnetite and as a narrow rim surrounding normal magnetite sparsely dispersed in chalcopyrite-pyrrhotite-cubanite ores. The optical and compositional aspects of the mineral fairly approximate those of magnetite. In polished section, the mineral is isotropic with brownish gray color and is slightly darker than normal magnetite. The chemical composition of the material, determined using an electron microprobe, is as follows: CaO 0.15, FeO 87.25 (total Fe as FeO), SiO₂ 5.52 wt. percent. Although the crystal structure is not still defined, the optical and compositional data indicate a possibility of a new variety of magnetite with a coupled substitution 2Fe³⁺→←Si⁴⁺+Fe²⁺. In the case, the structural formula is written as Fe²⁺(Fe3+/1.6Si_0.2Fe2/0.2)O₄.