Spryite, Ag8(As3+0.5As5+0.5)S6: structure determination and inferred absence of superionic conduction of the first As3+‑bearing argyrodite

We report data on the composition and crystal structure of the first As3+-bearing natural argyrodite, spryite. Spryite has the formula (Ag7.98Cu0.05)sum=8.03(As5+0.31Ge0.36As3+0.31Fe3+0.02)]=1.00S5.97, ideally Ag8(As3+0.5As5+0.5)S6. 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.