Ferrous carbonate, FeCO₃

Ferrous carbonate, FeCO₃, occurs in nature as spathic iron ore, which, however, is not pure, even in the crystalline form, as it contains several other isomorphous carbonates. When crystalline, the mineral is generally known as chalybite or siderite. The crystals belong to the hexagonal system, and are brown in colour. Upon exposure to moisture and air they are gradually converted into hydrated ferric oxide. When heated, they begin to decompose at about 400° C. in air.

In the laboratory ferrous carbonate may be obtained as a white precipitate by adding sodium carbonate to a solution of ferrous sulphate in the absence of air. If air is present, the precipitate rapidly turns green in consequence of oxidation, carbon dioxide is evolved, and ultimately the mass consists almost entirely of ferric hydroxide.

If the air is entirely excluded, the original white precipitate may be washed and dried and kept in a hermetically sealed tube.

Microscopic crystals of ferrous carbonate have been obtained by precipitating the salt from a ferrous solution with sodium hydrogen carbonate, and heating the mixture in closed tubes for twelve to thirty- six hours at 150° C.

The heat of formation of ferrous carbonate is

[FeO] + (CO₂) = [FeCO₃] + 25,200 calories.