Original paper

Structure refinement of (001) disordered gaudefroyite Ca4Mn3+3[(BO3)3(CO3)O3]: Jahn-Teller-distortion in edge-sharing chains of Mn 3+O6 octahedra

Hoffmann, Christina; Armbruster, Thomas; Kunz, Martin

European Journal of Mineralogy Volume 9 Number 1 (1996), p. 7 - 20

40 references

published: Dec 30, 1997
manuscript accepted: Sep 20, 1996
manuscript received: Jul 5, 1996

DOI: 10.1127/ejm/9/1/0007

BibTeX file

ArtNo. ESP147050901002, Price: 29.00 €

Download preview PDF Buy as PDF

Abstract

Abstract The crystal structure of gaudefroyite, Ca4Mn3+3[(BO3)3(CO3)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 O-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

Keywords

gaudefroyitecrystal structureJahn-Teller distortionedge-sharing Mn3+O6 octahedral chains