Mineral Species: Ankerite and Sphalerite

Chemical Formula: Ca(Fe,Mg,Mn)(CO3)2

Chemical Name: Calcium Iron Magnesium Manganese Carbonate

Mineral Class: Carbonate

Mineral Notes: Ankerite's formula is often written as CaFe(CO3)2 but there is usually a significant amount of magnesium amd manganese substitution for the iron.  The mineral forms in iron rich hydrothermal veins and metamorphic rocks with sulfides and other iron minerals. It is a member of the Dolomite Group and the structure consists of alternating layers of carbonate groups and metal anions. To add complexity, the anion layers alternate between calcium and iron. This alternation has the effect of lowering the symmetry which would otherwise  be the same as Calcite. Ankerite is therefore more dense, opaque and deeply colored than Calcite but not as much as the related Siderite.  

Description: A specimen which consists of a mass of fused Ankerite and Siderite crystals. Both are well-formed and the colours compliment each other rather well, espcially the highly metallic luster of the Sphalerite. In several places the Ankerite can be seen to form exceptionally well-formed flattened rhombohedral crystals.

Location: Eagle Mine, Gilman, Colorado, USA.