Siderite

Siderite, also known as “chalybite”, is an iron carbonate mineral that belongs to the calcite group. It can contain a wide variety of impurities, including manganese, magnesium, calcium, zinc, and cobalt. Siderite usually comes in hues of brown with the occasional white and gray specimens. It can occur as rhombohedral crystals with rounded faces and, in rare cases, as scalenohedral crystals. Botryoidal, concretionary, massive, stalactitic, and mammillary are some its more common formations.