Siderite mineral. This mineral consists of iron carbonate (FeCO3) and the ore is mined as a source of iron metal. Similar minerals are rhodochrosite (MnCO3) and magnesite (MgCO3) where the manganese and magnesium substitute for iron in the crystal structure. The crystals formed can be trigonal or hexagonal. Colours can vary widely. This sample contains yellow-orange lentil-shaped crystals on a substratum of quartz with the mineral galenite. This sample, from the Sierra Almagrena, Spain, is 90 millimetres wide.