Contribution
Structure refinement of hydrous andradite, Ca3Fe1.54Mn0.20Al0.26(SiO4)1.65(O4H4)1.35, from the Wessels mine, Kalahari manganese field, South Africa
Armbruster, Thomas
European Journal of Mineralogy Volume 7 Number 5 (1995), p. 1221 - 1226
21 références bibliographiques
publié: Oct 5, 1995
manuscrit accepté: Jul 3, 1995
manuscrit reçu: Jun 21, 1995
DOI: 10.1127/ejm/7/5/1221
Abstract
Abstract 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) Å, 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) Å 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) A is similar to that in andradite.
Mots-clefs
hydrogarnet • andradite • crystal structure • disorder • Kalahari manganese field