Study of the nanosiderite from ferruginous quartzites of Kursk magnetic anomaly by transmission electron microscopy

Resumo

Biogenic siderite consisting of equally crystallographically oriented disklike nanoparticles 5–20 nm in size has been found (using transmission electron microscopy) in oxidized ferruginous quartzites (jaspilites) of the Lebedinsky field of the Kursk magnetic anomaly. Based on microdiffraction data and highresolution images, lowering of the siderite structure symmetry from \(R\overline 3 c\) to \(R\overline 3 \) has been established for the first time. A siderite structure model is proposed to explain this fact. Within this model, vacancies formed as a result of oxidation of some part of Fe²⁺ cations to the Fe³⁺ state are ordered in one of two nonequivalent octahedral sites. Identical crystallographic orientation and nanoparticle morphology have been established for coexisting siderite and hematite. It is suggested that the revealed specific features of nanosiderite are related to its biogenic origin.