Iron(II) carbonate

Iron(II) carbonate, or ferrous carbonate, is a chemical compound with formula FeCO
₃, that occurs naturally as the mineral siderite. At ordinary ambient temperatures, it is a green-brown ionic solid consisting of iron(II) cations Fe²⁺
and carbonate anions CO²⁻
₃. The compound crystallizes in the same motif as calcium carbonate. In this motif, the carbonate dianion is nearly planar. Its three oxygen atoms each bind to two Fe(II) centers, such that the Fe has an octahedral coordination geometry.

Ferrous carbonate can be prepared by reacting solution of the two ions, such as iron(II) chloride and sodium carbonate:

Ferrous carbonate can be prepared also from solutions of an iron(II) salt, such as iron(II) perchlorate, with sodium bicarbonate, releasing carbon dioxide:

Sel and others used this reaction (but with FeCl
₂ instead of Fe(ClO
₄)₂) at 0.2 M to prepare amorphous FeCO
₃.

Care must be taken to exclude oxygen O
₂ from the solutions, because the Fe²⁺
ion is easily oxidized to Fe³⁺
, especially at pH above 6.0.

Ferrous carbonate also forms directly on steel or iron surfaces exposed to solutions of carbon dioxide, forming an "iron carbonate" scale:

The dependency of the solubility in water with temperature was determined by Wei Sun and others to be

where T is the absolute temperature in kelvins, and I is the ionic strength of the liquid.