Siderite is a mineral composed of iron carbonate FeCO3. 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.
Siderite is commonly found in hydrothermal veins, and is associated with barite, fluorite, galena, and others. It may also be deposited by sedimentary processes.