Siderite, Iron carbonate, FeCO3. Aberdaunant mine, Nr. Llanidloes, Central Wales Orefield. This specimen is brecciated material, composed of calcite, host rock, and siderite, more properly known as iron-carbonate. The bulk of the specimen is siderite, with just enough of it's slightly curved rhombohedral nature to allow identification. An old fashioned name for siderite is chalybite, and there are records of chalybeate springs in central wales.