Mineral Species: Ankerite

Chemical Formula: Ca(Fe,Mg,Mn)(CO3)2

Chemical Name: Calcium Iron Magnesium Manganese Carbonate

Mineral Class: Carbonate

Mineral Notes: Ankerite's simple formula is often given as CaFe(CO3)2 but there is significant substitution of the iron by magnesium and manganese. The mineral forms in iron rich hydrothermal veins and metamorphic rocks with sulfides and other iron minerals. Ankerite is a member of the Dolomite Group and contains alternating layers of CO3 held together by metal ions. The metal layers alternate between Fe and Ca. Ankerite is chemically and physically similar to Dolomite and Siderite, but is usually more dense, opaque and deeply coloured than dolomite; but not as much as siderite. Some authorities believe that Ankerite may be a short-lived intermediate in the transformation between Calcite and Dolomite.  

Description: Clusters of Ankerite crystals surrounding groups of pale green Fluorite crystals and black blades of Sphalerite. The Fluorite is very clear and gemmy which provides a very aesthetic contast for the gently coloured yellow Ankerite. Close inspection shows the latter as short, prismatic crystals .

Location: Smallcleugh Mine, Nenthead, Cumbria, England.