Abstract
The crystal structure of cesbronite has been determined using single-crystal X-ray diffraction and supported by electron-microprobe analysis, powder diffraction and Raman spectroscopy. Cesbronite is orthorhombic, space group Cmcm, with a = 2.93172 (16), b = 11.8414 (6), c = 8.6047 (4) Å and V = 298.72 (3) Å3. The chemical formula of cesbronite has been revised to CuII 3TeVIO4(OH)4 from CuII 5(TeIVO3)2(OH)6·2H2O. This change has been accepted by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association, Proposal 17-C. The previously reported oxidation state of tellurium has been shown to be incorrect; the crystal structure, bond valence studies and charge balance clearly show tellurium to be hexavalent. The crystal structure of cesbronite is formed from corrugated sheets of edge-sharing CuO6 and (Cu0.5Te0.5)O6 octahedra. The structure determined here is an average structure that has underlying ordering of Cu and Te at one of the two metal sites, designated as M, which has an occupancy Cu0.5Te0.5. This averaging probably arises from an absence of correlation between adjacent polyhedral sheets, as there are two different hydrogen-bonding configurations linking sheets that are related by a ½a offset. Randomised stacking of these two configurations results in the superposition of Cu and Te and leads to the Cu0.5Te0.5 occupancy of the M site in the average structure. Bond-valence analysis is used to choose the most probable Cu/Te ordering scheme and also to identify protonation sites (OH). The chosen ordering scheme and its associated OH sites are shown to be consistent with the revised chemical formula.
Highlights
Tellurium minerals comprise a surprisingly large portion of the mineralogical record (e.g. Christy, Mills & Kampf, 2016; Christy, Mills, Kampf et al, 2016; Grundler et al, 2008)
Over 150 tellurium minerals have been recorded, and abundant sources of new oxysalt minerals such as those at the Otto Mountain mine in California have only been discovered since the year 2000 (e.g. Kampf, Housley, Mills et al, 2010; Kampf et al, 2016)
It is one of many tellurium oxysalt minerals first recognised from the Moctezuma, Bambolla, Bambollita and Oriental mines of Sonora, Mexico (Gaines, 1970; Grundler et al, 2008)
Summary
Tellurium minerals comprise a surprisingly large portion of the mineralogical record (e.g. Christy, Mills & Kampf, 2016; Christy, Mills, Kampf et al, 2016; Grundler et al, 2008). Secondary tellurium minerals featuring two tellurium oxidation states, +IV and +VI, typically form from alteration of sulfide zones rich in tellurides under conditions of high Eh and pH (Mills et al, 2014; Grundler et al, 2008). Cesbronite is a rare secondary tellurium mineral found in only a few North American localities (Williams, 1974; Roberts et al, 1997). It is one of many tellurium oxysalt minerals first recognised from the Moctezuma, Bambolla, Bambollita and Oriental mines of Sonora, Mexico (Gaines, 1970; Grundler et al, 2008). CaO 0.12 0.07 0.17 0.03 CuO 50.6 49.8 51.8 0.53 ZnO 1.0 0.5 1.5 0.28 TeO3 37.7 36.7 38.6 0.50 PbO 0.8 0.3 1.0 0.20 H2O† 7.80 7.74 7.89 0.05 Total 98.07
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