Crystal structure, XANES and charge distribution investigation of krennerite and sylvanite: analysis of Au - Te and Te - Te bonds in Au_1-xAg_xTe₂group minerals

The structure refinement and XANES study of two gold-silver-tellurides [Au_1+xAg _xTe₂, 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 Å) in the two minerals, which are absent in calaverite (bond distances > 3.2 Å). 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 [Te3]^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.