Spryite, ${\text{Ag}}_{8}\left({{\text{As}}_{0.5}^{3 +} {\text{As}}_{0.5}^{5 +}}\right){\text{S}}_{6}$: structure determination and inferred absence of superionic conduction of the first As³⁺-bearing argyrodite

Abstract

We report data on the composition and crystal structure of the first As³⁺-bearing natural argyrodite, spryite. Spryite has the formula $({{\text{Ag}}_{7.98} {\text{Cu}}_{0.05}})_{\varSigma = 8.03} ({{\text{As}}_{0.31}^{5 +} {\text{Ge}}_{0.36} {\text{As}}_{0.31}^{3 +} {\text{Fe}}_{0.02}^{3 +}})_{\varSigma = 1.00} {\text{S}}_{5.97}$, ideally ${\text{Ag}}_{8} ({{\text{As}}_{0.5}^{3 +} {\text{As}}_{0.5}^{5 +}}){\text{S}}_{6}$. The crystal studied was found in a sample from the Uchucchacua polymetallic deposit, Oyon district, Cajatambo, Lima Department, Peru. The structure was refined in the space group Pna2₁ up to R = 0.0782 using 2099 observed reflections [2σ(I) level]. Spryite is intimately twinned with six twin domains. The structure solution showed that As³⁺ and As⁵⁺ coexist in the new mineral, which represents the first case ever discovered in either a mineral or a synthetic compound belonging to a sulfide/sulfosalt group. High-temperature in situ X-ray diffraction experiments indicated that no phase transitions occur in the temperature range investigated and that the mineral does not show any evidence of fast ionic conduction.