A quarter of a century after its synthesis and with 200 papers based on its use, `Co(CO3)0.5(OH)0.11H2O proves to be Co6(CO3)2(OH)8H2O from synchrotron powder diffraction data

The successful attempt to solve the crystal structure of Co(CO3)0.5(OH) 0.11H2O (denoted CCH), based on synchrotron powder diffraction data, leads to a drastic revision of the chemical formula to Co6(CO3)2(OH)8 H2O [hexacobalt(II) bis(carbonate) octahydroxide monohydrate] and to a hexagonal cell instead of the orthorhombic cell suggested previously [Porta et al. (1992). J. Chem. Soc. Faraday Trans. 88, 311–319]. This results in a new structure-type related to malachite involving infinite chains of [CoO6] octahedra sharing edges along a short c axis, delimiting tunnels having a three-branched star section. All reports discussing cobalt hydroxycarbonates (CCH) without any structural knowledge and especially its topotactic decomposition into Co3O4 have, as a result, to be reconsidered.