ankerite

Ankerite is a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral carbonates with formula: Ca(Fe,Mg,Mn)(CO₃)₂. In composition it is closely related to dolomite, but differs from this in having magnesium replaced by varying amounts of iron(II) and manganese. Forms series with dolomite and kutnohorite. The crystallographic and physical characters resemble those of dolomite and siderite. The angle between the perfect rhombohedral cleavages is 73° 48', the hardness is 3.5 to 4, and the specific gravity is 2.9 to 3.1. The color is white, grey or reddish to yellowish brown.