Siderite, Iron carbonate, FeCO3. Cafartha mine, Nr Staylittle, Central Wales Orefield. This brecciated material is mostly siderite with small pieces of host rock, and a small amount of calcite. The siderite, which is an iron-carbonate is very pale brown in colour, with a slight curvature on exposed surfaces. There are no actual crystals on any of the specimens.