A quarter of a century after its synthesis and with >200 papers based on its use, `Co(CO 3 ) 0.5 (OH)·0.11H 2 O' proves to be Co 6 (CO 3 ) 2 (OH) 8 ·H 2 O from synchrotron powder diffraction data.

The successful attempt to solve the crystal structure of Co(CO)(OH)·0.11HO (denoted CCH), based on synchrotron powder diffraction data, leads to a drastic revision of the chemical formula to Co(CO)(OH)·HO [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 [CoO] 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 CoO have, as a result, to be reconsidered.