siderita

Siderita is a mineral composed of iron carbonate, with the chemical formula FeCO3. It belongs to the calcite group of minerals, which are characterized by a rhombohedral crystal structure. Siderita is often found in sedimentary rocks, particularly in shales, sandstones, and limestones, and can also occur in hydrothermal veins and some metamorphic rocks. It is a common ore of iron, although not as economically important as hematite or magnetite.

The color of siderita can vary widely, ranging from pale yellow and brown to reddish-brown and gray, and occasionally black. Its streak is white to pale brown. Siderite typically has a Mohs hardness of 3.5 to 4.5 and a specific gravity of around 3.96. It is often found in botryoidal (grape-like) or concretionary forms, as well as in distinct rhombohedral crystals. The mineral is relatively soft and can be scratched with a knife.

Siderita plays a role in geological processes. Its presence can indicate reducing conditions in sedimentary environments. When heated, siderita decomposes to form iron oxides, releasing carbon dioxide. This property has been utilized historically in the production of iron, where siderite was smelted in blast furnaces. While less common today, it can still be found in some industrial applications and is of interest to mineral collectors.