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.^[5] 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.^[6]

iron(II) carbonate

Names
Other names ferrous carbonate
Identifiers
CAS Number	563-71-3 Y
3D model (JSmol)	Interactive image
ChemSpider	10774 N
ECHA InfoCard	100.008.418
E number	E505 (acidity regulators, ...)
PubChem CID	11248
UNII	MZ3Q72U52O Y
CompTox Dashboard (EPA)	DTXSID8052225
InChI InChI=1S/CH2O3.Fe/c2-1(3)4;/h(H2,2,3,4);/q;+2/p-2 Key: RAQDACVRFCEPDA-UHFFFAOYSA-L
SMILES C(=O)([O-])[O-].[Fe+2]
Properties
Chemical formula	FeCO3
Molar mass	115.854 g/mol
Appearance	white powder or crystals
Density	3.9 g/cm3[1]
Melting point	decomposes
Solubility in water	0.0067 g/L;[2] Ksp = 1.28 × 10−11 [3]
Solubility product (Ksp)	3.13×10−11[4]
Magnetic susceptibility (χ)	+11,300·10−6 cm3/mol
Structure
Crystal structure	Hexagonal scalenohedral / Trigonal (32/m) Space group: R 3c, a = 4.6916 Å, c = 15.3796 Å
Coordination geometry	6
Related compounds
Other anions	iron(II) sulfate
Other cations	copper(II) carbonate, zinc carbonate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Preparation

Ferrous carbonate can be prepared by reacting solution of the two ions, such as iron(II) chloride and sodium carbonate:^[5]

FeCl
₂ + Na
₂CO
₃ → FeCO
₃ + 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:^[7]

Fe(ClO
₄)₂ + 2NaHCO
₃ → FeCO
₃ + 2NaClO
₄ + CO
₂ + H
₂O

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.^[8]

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.^[7]

Ferrous carbonate also forms directly on steel or iron surfaces exposed to solutions of carbon dioxide, forming an "iron carbonate" scale:^[3]

Fe + CO
₂ + H
₂O → FeCO
₃ + H
₂

Properties

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

${\displaystyle \log K_{\mathit {sp}}=-59.3498-0.041377T-2.1963/T+24.5724\log T+2.518{\sqrt {I}}-0.657I,}$

where T is the absolute temperature in kelvins, and I is the ionic strength of the liquid.^[3]

Iron carbonate decomposes at about 500–600 °C (773–873 K).^[9]

Uses

Ferrous carbonate has been used as an iron dietary supplement to treat anemia.^[10] It is noted to have very poor bioavailability in cats and dogs.^[11]

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).^[12]

Iron(III) carbonate

Unlike iron(II) carbonate, iron(III) carbonate has not been isolated. 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.^[13]