siderite or chalybite, a mineral, varying in color from brown, green, or gray to black and occurring in nature in massive and crystalline form. A carbonate of iron, FeCO₃, it serves as an iron ore, especially in the British Isles. It is widely distributed, being found also in the United States, Europe, South America, and Australia.

or chalybite

Iron carbonate (FeCO₃), a widespread carbonate mineral that can be 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).

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