Structure refinement of (001) disordered gaudefroyite Ca₄Mn₃³⁺[(BO₃)₃(CO ₃)O₃]: Jahn-Teller-distortion in edge-sharing chains of Mn³⁺O₆ octahedra

The crystal structure of gaudefroyite, Ca₄Mn₃³⁺[(BO₃)₃(CO ₃)O₃], space group P6₃, (a = 10.589(1), c = 5.891(1) Å, Z = 2) from the N'chwaning II mine in the Kalahari manganese field, South Africa, was refined from single-crystal X-ray diffraction data as disordered in space group P6₃/m. The structural framework of gaudefroyite is composed of infinite chains formed by trans-trans connected edge-sharing Mn³⁺O₆ octahedra which are crosslinked by triangular BO₃ groups, forming two different types of channels. This framework arrangement, as well as the distribution of the Ca²⁺ cations in the structural channels, obeys the higher space-group symmetry P6₃/m. In contrast, CO₃ groups situated in the centre of the wide structural channel locally violate the mirror-plane perpendicular to c. However, averaged over the whole structure carbonate groups exhibit positional disorder governed by the centro-symmetry of the framework. Crystal structures characterized by chains of edge-sharing Mn³⁺O₆ octahedra, as observed for gaudefroyite, were retrieved from the Inorganic Crystal Structure Database to test whether the Jahn-Teller distortion shows a consistent pattern within edge-sharing octahedral chains. Nine structures with trans-trans connected edge-sharing Mn³⁺O₆ octahedra were found. Seven compounds with the octahedral chains linked by various anion groups show a regular zig-zag arrangement of the longest Q-Mn³⁺-O vector where two long vectors meet at one corner of a common edge. Exceptions are manganite, MnOOH, and mozartite, CaMn(OH)SiO₄. It is suggested that the orientation of the Jahn-Teller distortion is dictated by the structural topology rather than that the distortion follows a common orientation within trans-trans edge-sharing chains. Bond valence calculations of the gaudefroyite structure confirm the strong influence of the structural topology upon the Jahn-Teller arrangement within the octahedral chains.