Crystal structure, XANES and charge distribution investigation of krennerite and sylvanite: analysis of Au-Te and Te-Te bonds in Au1-xAgxTe2 group minerals

The structure refinement and XANES study of two gold-silver-tellurides [Au1+xAgxTe2, krennerite (x = 0.11-0.13) and sylvanite (x = 0.29-0.31)] are presented and the structures are compared with the prototype structure of calaverite (x = 0.08-0.10). Whereas the latter is well known for being incommensurately modulated at ambient conditions, neither krennerite nor sylvanite present any modulation. This is attributed to the presence of relatively strong Te-Te bonds (bond distances < 2.9 angstrom) in the two minerals, which are absent in calaverite (bond distances > 3.2 angstrom). In both tellurides, trivalent gold occurs in slightly distorted square planar coordination, whereas monovalent gold, partly substituted by monovalent silver, presents a 2+2+2 coordination, corresponding to distorted rhombic bipyramids. The differentiation between bonding and non-bonding contacts is obtained by computation of the Effective Coordination Number (ECoN). The CHARge DIstribution (CHARDI) analysis is satisfactory for both tellurides but suggests that the Te-Te bond in the [Te-3](2-) anion is not entirely homopolar. Both tellurides can therefore be described as Madelung-type compounds, despite the presence of Te-Te in both structures.

Automated summary

This study presents the structure refinement and XANES study of krennerite and sylvanite, two gold-silver-tellurides, and compares them with the prototype structure of calaverite. It is found that krennerite and sylvanite do not exhibit any modulation, unlike calaverite, which is attributed to the presence of relatively strong Te-Te bonds in the former minerals.