Iron(II) carbonate, or ferrous carbonate, is a chemical compound with formula FeCO3, 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.

Preparation

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

FeCl2 + Na2CO3 → FeCO3 + 2NaCl

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

Fe(ClO4)2 + 2NaHCO3 → FeCO3 + 2NaClO4 + CO2 + H2O

Sel and others used this reaction (but with FeCl2 instead of Fe(ClO4)2) at 0.2 M to prepare amorphous FeCO3.

Care must be taken to exclude oxygen O2 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:

Fe + CO2 + H2O → FeCO3 + H2

Properties

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

log ⁡ K s p = − 59.3498 − 0.041377 T − 2.1963 / T + 24.5724 log ⁡ T + 2.518 I − 0.657 I , {\displaystyle \log K_{\mathit {sp}}=-59.3498-0.041377T-2.1963/T+24.5724\log T+2.518{\sqrt {I}}-0.657I,}

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

Iron carbonate decomposes at about 500–600 °C (773–873 K).

Uses

Ferrous carbonate has been used as an iron dietary supplement to treat anemia. It is noted to have very poor bioavailability in cats and dogs.

Toxicity

Ferrous carbonate is slightly toxic; the probable oral lethal dose is between 0.5 and 5 g/kg (between 35 and 350 g for a 70 kg person).

Iron(III) carbonate

Unlike iron(II) carbonate, iron(III) carbonate is rarely encountered. Attempts to produce iron(III) carbonate by the reaction of aqueous ferric ions and carbonate ions result in the production of iron(III) oxide with the release of carbon dioxide or bicarbonate. It is instead produced by the heating of iron(III) oxide at 2300 °C under 33 GPa of carbon dioxide. Iron(III) carbonate is unstable at atmospheric pressure, decomposing back into iron(III) oxide and carbon dioxide.