A mineral (FeCO₃) with the same space group and hexagonal crystal system as calcite (CaCO₃). Siderite has a gray, tan, brown, dark brown, or red color, has rhombohedral cleavages, and occasionally may show rhombohedral crystal terminations. It may display curved crystal faces like dolomite (CaMg(CO₃)₂), but more commonly is found as massive, compact, or earthy masses.