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Dervillite: crystal structure determination

Posted by Marco E. Ciriotti  
Marco E. Ciriotti December 13, 2013 02:20PM
▪ 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.

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.
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