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)8·H2O from synchrotron powder diffraction data

The successful attempt to solve the crystal structure of Co(CO 3 ) 0.5 (OH)·0.11H 2 O (denoted CCH ), based on synchrotron powder diffraction data, leads to a drastic revision of the chemical formula to Co 6 (CO 3 ) 2 (OH) 8 ·H 2 O [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 6 ] 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 Co 3 O 4 have, as a result, to be reconsidered.