Mineral, Ca(Mg,Fe)(CO₃)₂; sp. gr. 2.9–3.2; hardness 3.5–4.0; trigonal; yellowish-brown, sometimes white, yellow, or grey; white streak; vitreous lustre; crystals can be rhombohedral, but ankerite also occurs massive and granular; cleavage rhombohedral, perfect {1011}, faint {0221}; occurs as a gangue mineral with iron ores, and as fillings associated with coal seams, and in an environment similar to that of dolomite. It is a form of ferroan dolomite where ferrous iron substitutes for Mg in a solid solution series from dolomite (CaMg(CO₃)₂) to ankerite ((CaMg_0.75Fe_0.25)(CO₃)₂). Ankerite often forms as a cement by diagenetic reaction (see diagenesis) from ferroan calcite at burial depths of about 2.5 km. It is named after the Austrian mineralogist M. J. Anker. See also carbonatite.