The crystal structures of the mixed-valence tellurium oxysalts tlapallite, (Ca,Pb)3CaCu6[Te4+3Te6+O12]2(Te4+O3)2(SO4)2·3H2O, and carlfriesite, CaTe4+2Te6+O8

The crystal structure of tlapallite has been determined using single-crystal X-ray diffraction and supported by electron probe microanalysis, powder diffraction and Raman spectroscopy. Tlapallite is trigonal, space group P321, with a = 9.1219(17) angstrom, c = 11.9320(9) A and V = 859.8(3) angstrom(3), and was refined to R-1 = 0.0296 for 786 reflections with I > 2 sigma(I). This study resulted from the discovery of well-crystallised tlapallite at the Wildcat prospect, Utah, USA. The chemical formula of tlapallite has been revised to (Ca,Pb)(3)CaCu6[Te34+Te6+O12](2)(Te4+O3)(2)(SO4)(2)center dot 3H(2)O, or more simply (Ca,Pb)(3)CaCu6[Te84+Te26+O30(SO4)(2)center dot 3H(2)O, from H-6(Ca,Pb)(2)(Cu,Zn)(3)(TeO3)(4)(TeO6)(SO4). The tlapallite structure consists of layers containing distorted Cu2+O6 octahedra, Te6+O6 octahedra and Te4+O4 disphenoids (which together form the new mixed-valence phyllotellurate anion [Te34+Te6+O12](12-)),Te4+O3 trigonal pyramids and CaO8 polyhedra. SO4 tetrahedra, Ca(H2O)(3)O-6 polyhedra and H2O groups fill the space between the layers. Tlapallite is only the second naturally occurring compound containing tellurium in both the 4(+) and 6(+) oxidation states with a known crystal structure, the other being carlfriesite, CaTe24+Te6+O8. Carlfriesite is the predominant secondary tellurium mineral at the Wildcat prospect. We also present an updated structure for carlfriesite, which has been refined to R-1 = 0.0230 for 874 reflections with I > 2 sigma(I). This updated structural refinement improves upon the one reported previously by refining all atoms anisotropically and presenting models of bond valence and Te4+ secondary bonding.