A rare silver mineral, dervillite (ideally Ag2AsS2), has been found in specimens from the famous Jáchymov mining district, Czech Republic. It occurs as very rare long-prismatic crystals up to 0.4 mm across in association with proustite, bismuth and native silver in the thin arsenic veinlets within the Trojická vein (Svornost mine). Dervillite is monoclinic, space group Pc, with a = 9.6375(3), b = 12.9462(4), c = 6.8497(2) Å, β = 99.510(2)° and V = 842.88(2) Å3 (Z = 8). The new structure refinement, R1 = 2.94% for 18767 reflections with [I > 3σ(I)] and wR2 = 7.93% for all 20050 reflections, provided a better fit to the data compared to earlier studies, revealing that silver (8 symmetrically independent atomic sites), which adopts various coordinations (from quasi-linear to tetrahedral) in the structure of dervillite vibrates non-harmonically at room temperature. The Gram-Charlier development, describing the atomic displacement parameters of silver atoms, was used to model their non-harmonic behaviour. A discussion on the use of the approach to the data with limited quality is also provided.
