|Origin||Tincroft mine, Illogan, Cornwall|
Siderite is a major source of iron for steel making, because it is easily worked, often free of sulphur and phosphorus, and sometimes manganese-rich. Its colour and higher specific gravity distinguish it from calcite and dolomite, and its rhombohedral cleavage, from sphalerite. It occurs in the Coal Measures as beds and nodules of impure iron carbonate called clay ironstone, which used to be a valuable iron ore. A dark carbonaceous variety is called blackband. Oolitic ironstone is formed most often by the same colloidal processes that form oolitic limestones. Less commonly, iron-rich fluids form siderite as a secondary mineral in calcareous rocks. When extensive, these deposits are economically important. Siderite is also known as chalybite, named after the Chalybes, ancient iron workers who lived by the Black Sea.