Siderite is a mineral composed of iron carbonate FeCO3. It takes its name from the Greek word sideros, “iron”. It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Both magnesium and manganese commonly substitute for the iron.
Its crystals belong to the hexagonal system, and are rhombohedral in shape, typically with curved and striated faces. It also occurs in masses. Color ranges from yellow to dark brown or black, the latter being due to the presence of manganese (sometimes called manganosiderite).
Siderite is commonly found in hydrothermal veins, and is associated with barite, fluorite, galena, and others. It is also a common diagenetic mineral in shales and sandstones, where it sometimes forms concretions. In sedimentary rocks, siderite commonly forms at shallow burial depths and its elemental composition is often related to the depositional environment of the enclosing sediments (Mozley, 1989). In addition, a number of recent studies have used the oxygen isotopic composition of sphaerosiderite (a type associated with soils) as a proxy for the isotopic composition of meteoric water shortly after deposition (e.g., Ludvigson et al., 1998).