STRUCTURE REFINEMENT OF HYDROUS ANDRADITE, CA3FE1.54MN0.20AL0.26(SIO4)(1.65)(O4H4)(1.35), FROM THE WESSELS MIME, KALAHARI MANGANESE FIELD, SOUTH-AFRICA

Orange-red, strongly zoned hydrogarnets with octahedral forms occur at the Wessels mine, Kalahari manganese field, South Africa, and are probably of late hydrothermal origin. 'Hydroandradites' from this locality show the highest degree of hydration for andradites hitherto observed in nature. The crystal structure of such a highly hydrous andradite, Ca3Fe1.54Mn0.20Al0.26(SiO4)(1.65)(O4H4)(1.35) (a = 12.340(1) Angstrom, space group Ia3d) was refined from single-crystal X-ray data to R = 2.38 %. The structure is composed of disordered micro-domains containing SiO4 and (H4O4) tetrahedral units. The distance between O and the centre of the tetrahedron is 1.762(2) Angstrom representing an average distance between a Si-O bond and a vacancy-O distance of a (H4O4) tetrahedron. Oxygen atoms are strongly smeared out along the Si-O vector caused by a smooth adoption to the local environment. The Fe-O distance of 2.021(2) Angstrom is similar to that in andradite.