Ankerite

carbonate

kutnohorite

Formula: Ca(Fe2+,Mg)(CO3)2
Forms a series with dolomite and with kutnohorite
Specific gravity: 3.121
Hardness: 3½ to 4
Streak: white
Colour: brown, white to grey, yellowish-brown, greenish
Solubility: Moderately soluble in hydrochloric and sulphuric acid
Environments:

Carbonatites
Hydrothermal environments

Ankerite is a common but not essential constituent of dolostone.

Alteration

ankerite-dolomite and quartz to augite and CO2
Ca(Mg,Fe)(CO3)2 + 2SiO2 → Ca(Mg,Fe)Si2O6 + 2CO2 (DHZ 2A p384)

ankerite-dolomite and quartz to diopside-hedenbergite and CO2
Ca(Fe,Mg)(CO3)2 + 2SiO2 = Ca(Fe,Mg)Si2O6 + 2CO2 (DHZ 2A p274)

calcite, Fe2+ and Mg2+ to ankerite and Ca2+
4CaCO3 + Fe2+ + Mg2+ = 2Ca(Mg0.5Fe0.5)(CO3)2+ 2Ca2+
Ankerite is believed to be formed from calcite hydrothermally according to the above reaction (DHZ 5B p228)

Common impurities: Mn

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