Dervillite from Lengenbach: occurrence and crystal structure

[Marco E. Ciriotti]

Referenza:
▪ Bindi, L., Nestola, F., de Battisti, L., Guastoni, A. (2013): Dervillite, Ag2AsS2, from Lengenbach quarry, Binn valley, Switzerland: occurrence and crystal structure. Mineralogical Magazine, 77, 3105–3112.

Abstract:
Dervillite, Ag2AsS2, has been found in a sample from the Lengenbach quarry in the Binn Valley, Valais, Switzerland. It occurs as very rare crystals up to 200 mm across on jordanite. Dervillite is opaque with a metallic lustre and shows a dark brown streak. The structure is monoclinic, space group Pc, with a = 9.6155(7), b = 12.9331(8), c = 6.8616(5) A˚, beta = 99.352(8)º, V = 842.0(1) A˚ 3 and Z = 8. In the crystal structure [R1 = 0.060 for 2370 reflections with I > 2s(I)], silver adopts various coordinations extending from quasi linear to quasi tetrahedral whereas arsenic forms very peculiar crystal-chemical environments, such as As(S2As) and As(S2AsAg). Such metalloid–metalloid or metal–metalloid bonds account for the apparent charge imbalance observed in the chemical formula.

[Marco E. Ciriotti]

Un bell'esempio di soluzione di strutture irrisolte che comporta automaticamente la variazione di status da Q a Rd.