Siderite, Iron carbonate, FeCO3. This mineral is fairly common throughout the Central Wales orefield. This specimen is from Geufron mine, on the edge of the Hafren forest, near Llanidloes. It is composed of closely packed crystals, with very few that are exposed to view. Those that are, as in the inset picture, have a slight curvature on the crystal faces, which helps to identify the mineral. The Geufron mine waste heaps are a good source for copper secondary minerals.