Ankerite
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Ankerite is a calcium, iron, magnesium, manganese carbonate mineral of the group of rhombohedral carbonates with the chemical formula Ca(CO₃)₂. In composition it is closely related to dolomite, but differs from this in having magnesium replaced by... Wikipedia
Cleavage: Perfect on
Color: Brown, yellow, white
Crystal class: Rhombohedral (3); H–M symbol: (3)
Crystal habit: Chrystals rhombohedral with curved faces; columnar, stalactitic, granular, massive
Crystal system: Trigonal
Formula (repeating unit): Ca(Fe,Mg,Mn)(CO3)2
IMA symbol: Ank