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ankerite 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 ((CaMg0.75Fe0.25)(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.