Summary: Nickel carbonate occurs in nature as a hexahydrate (hellyerite) and as a hydroxyhydrate (zaratite), but the anhydrous species is only known as an artificial product. All of the compounds show anomalous properties that have led to confusion in the literature.
Only the calcite structure was found for NiCO3 ; hydrothermal conditions appear to be essential for its synthesis, and a variation in cell size was found with changes in duration or temperature of synthesis, or both.
Hellyerite probably is [Ni(H2O)6]CO3, which would explain its ease of decomposition and rarity in nature. Zaratite is not a single mineral, but a composite of amorphous and fibrous components. The conditions under which NiCO3 might be expected to occur are discussed. Nickel bicarbonate formed in the reactions to produce NiCO3 ; its X-ray diffraction pattern, indicating cubic symmetry, analysis, and optical properties are given.
The behaviour of nickel in these compounds is best explained in terms of crystal field theory.