siderit

Siderit, also known as siderite, is a mineral in the iron carbonate group with the chemical formula FeCO3. It is a carbonate mineral that typically forms pale brown, yellow-brown, or reddish-brown crystals and masses. The name derives from the Greek sideros, meaning iron.

Siderit forms in a variety of settings. It commonly occurs as concretions and nodules in sedimentary rocks, particularly in coal beds and shales, where iron-rich fluids precipitate carbonate during diagenesis. It can also form in hydrothermal veins and in metamorphosed iron-rich rocks. It is frequently associated with calcite, dolomite, and limonite.

Physical properties and identification notes include a hardness of about 3.5 to 4 on the Mohs scale and a specific gravity around 3.9 to 4.0. Siderit exhibits good rhombohedral cleavage and a vitreous to earthy luster. It may form rhombohedral crystals and is readily identified by its color and by effervescence when treated with dilute acids due to the carbonate group.

Economic and practical context remains modest. Siderit has been mined as an iron ore in some deposits, but it is generally not a major source of iron today, largely due to its occurrence as nodules in sedimentary rocks and competition from more concentrated iron ore minerals. It is also of interest to mineral collectors and as a marker mineral in sedimentary environments.