Siderite is an iron carbonate mineral (FeCO3, 48% Fe) belonging to the calcite group (hexagonal crystal system). Magnesium and manganese may substitute for the iron. Siderite crystals are yellow to dark brown in color, rhombohedral in shape, and with curved and striated faces, and also occur in masses. Siderite is commonly found in bedded sedimentary iron ores, including banded iron formations (BIFs, see entry Banded Iron Formations), and in hydrothermal veins. It is also a common diagenetic mineral, forming at shallow burial depths in shales and sandstones where it tends to form concretions. The diagenetic formation of siderite strongly depends on the availability of Fe2+ ions in pore waters (see entry Pore Waters) and is thus a phenomenon of suboxic to anoxic environments (Berner, 1971). Diagenetic siderite precipitation has been linked to microbially mediated reactions, specifically to the release of ferrous (Fe2+) iron due to dissimilatory iron reduction performed by Fe3+-reducin...
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