ankerite

Ankerite is a carbonate mineral in the dolomite group. Its ideal chemical formula is Ca(Fe,Mg)(CO3)2, reflecting a solid solution where iron and magnesium can substitute for calcium in the lattice; manganese may occur as a minor substitute. The color ranges from white and colorless to pale brown or gray, with iron-rich specimens often showing darker hues.

Crystallography and physical properties: Ankerite crystallizes in the trigonal (rhombohedral) system and commonly forms rhombohedral crystals, though it is frequently found as massive, granular material. It has a vitreous to pearly luster, a hardness of about 3.5–4 on the Mohs scale, and a specific gravity of roughly 3.8–3.9. It exhibits cleavage in two directions parallel to rhombohedral planes.

Occurrence and formation: It forms in carbonate rocks, especially iron-bearing dolostones and skarns, and in hydrothermal veins where Fe-rich fluids are present. Ankerite is commonly associated with dolomite, calcite, siderite, and quartz. It has a worldwide distribution and can be found in various geological environments, from sedimentary to hydrothermal settings.

Economic and other notes: Ankerite has limited economic importance as an ore mineral, being of interest mainly for its mineralogical and geochemical implications rather than as a major source of iron or magnesium. It is studied as a component of Fe-rich carbonate systems and as a marker of diagenetic or hydrothermal processes in carbonate rocks.