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

Abstract

The crystal structure of tlapallite has been determined using single-crystal X-ray diffraction and supported by electron probe micro-analysis, powder diffraction and Raman spectroscopy. Tlapallite is trigonal, space group P321, with a = 9.1219(17) Å, c = 11.9320(9) Å and V = 859.8(3) Å3, and was refined to R1 = 0.0296 for 786 reflections with I > 2σ(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)3CaCu6[Te4+3Te6+O12]2(Te4+O3)2(SO4)2·3H2O, or more simply (Ca,Pb)3CaCu6Te4+8Te6+2O30(SO4)2·3H2O, from H6(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 [Te4+3Te6+O12]12−), Te4+O3 trigonal pyramids and CaO8 polyhedra. SO4 tetrahedra, Ca(H2O)3O6 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, CaTe4+2Te6+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 R1 = 0.0230 for 874 reflections with I > 2σ(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.

The discreditation of oboyerite and a note on the crystal structure of plumbotellurite

“…Following the discreditations of ‘girdite’ and ‘oboyerite’, the number of remaining valid mixed-valence Te oxysalt minerals is just four. Carlfriesite and tlapallite have known crystal structures (Missen et al ., 2019), but tlalocite (Williams, 1975) and yecoraite (Williams and Cesbron, 1985) do not. The latter two minerals will be the subject of future study to determine whether they are truly mixed-valence Te minerals and not mixtures.…”

Hagstromite, Pb8Cu2+(Te6+O6)2(CO3)Cl4, a new lead–tellurium oxysalt mineral from Otto Mountain, California, USA

“…The sharp bands at 1041 cm –1 (hagstromite), 1038 cm –1 (agaite) and 1056 cm –1 (thorneite) are in the shift range where the strongest carbonate band typically occurs. Tellurates have been shown previously to have the components of their Te–O stretching modes in the 850 to 600 cm –1 region (Blasse and Hordijk, 1972; Frost, 2009; Frost and Keeffe, 2009; Kampf et al ., 2013b; Frikha et al ., 2017; Missen et al ., 2019). Of the several hagstromite spectra recorded on different crystals in random orientations, all but one were featureless between 3900 and 1700 cm –1 .…”