Siderite is an iron carbonate, composition FeCO3. It is a member of the Carbonate mineral group. Both Mn2+ and Mg2+ substitute for Fe and a solid solution exists between siderite, rhodochrosite (MnCO3) and magnesite (MgCO3).

Distribution

Siderite occurs as clay ironstone, an impure mixture of siderite with clay minerals. It may form concretions or concentric layers. It may also occur as black-band ore, which can form extensive stratified beds admixed with shales, other carbonates, and coal. Siderite is also formed as iron rich waters replace Ca in limestone. Finally, it is a common vein mineral associated with various metallic ores, including the Precambrian banded iron formations.

Importance in Soil Environments

Siderite may occur in soils in small quantities, particularly in fens receiving iron-rich waters.

Graf, D.L. 1961. Crystallographic tables for the rhombohedral carbonates. American Mineralogist 46:1283-1316.