siderite, iron carbonate (FeCO3), a widespread mineral that is an ore of iron. The mineral commonly occurs in thin beds with shales, clay, or coal seams (as sedimentary deposits) and in hydrothermal metallic veins (as gangue, or waste rock). Manganese (Mn), magnesium (Mg), and calcium generally substitute in part for iron; siderite forms a complete solid-solution (chemical replacement) series with magnesite (MgCO3), another with rhodochrosite (MnCO3), and an incomplete one with ferroan calcite. For detailed physical properties, see carbonate mineral (table).