Ankerite, Calcium,iron,magnesium,manganese,carbonate, Ca(Fe,Mg,Mn)(CO3)2. Eaglebrook mine, west Wales. Although I have listed this specimen as ankerite, it may well be Ferroan dolomite. Both of these minerals have been recorded at this mine. The true identity can only be discovered by analytical means.The exposed rhomohedral fracture surfaces have a slight curvature, and the colour varies in shades of brown across the specimen.