Dervillite, Ag 2 AsS 2 , from Lengenbach quarry, Binn valley, Switzerland: occurrence and crystal structure L. BINDI 1 *, F. NESTOLA 2 , L. DE BATTISTI 3 AND A. GUASTONI 4 1 Dipartimento di Scienze della Terra, Universita ` degli Studi di Firenze,Via G. La Pira 4, I-50121 Florence, Italy 2 Dipartimento di Geoscienze, Universita ` di Padova, Via Gradenigo 6, I-35131, Padua, Italy 3 FGL (Forschungsgemeinschaft Lengenbach), Via dello Storno 18, I-20147 Milan, Italy 4 Museo di Mineralogia, Universita ` di Padova, Via Giotto 1, I-35122, Padua, Italy [Received 24 September 2013; Accepted 22 October 2013; Associate Editor: A. Christy] ABSTRACT Dervillite, As 2 AsS 2 , 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 ˚ , b = 99.352(8)º, V = 842.0(1) A ˚ 3 and Z = 8. In the crystal structure [R 1 = 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(S 2 As) and As(S 2 AsAg). Such metalloid–metalloid or metal–metalloid bonds account for the apparent charge imbalance observed in the chemical formula. KEYWORDS: dervillite, silver, Lengenbach, Switzerland. Introduction DERVILLITE was described first by Weil (1941) as a natural compound of lead, sulfur and antimony, and possibly also bismuth. A reinvestigation of the original sample by Bari et al. (1983) proved that the chemical data given by Weil (1941) were erroneous, because they were based on chemical tests on material that was apparently not extracted from the dervillite crystals. Unfortunately, given wartime conditions, the study could not be completed. By means of X-ray single-crystal precession photographs and an X-ray powder diffraction study of the holotype, Bari et al. (1983) found that dervillite is monoclinic, space group P2/a, with a = 6.833, b = 12.932, c = 9.638 A ˚ , b = 99.33º, V = 1715.9(7) A ˚ 3 and Z = 8. New electron microprobe analyses indicated the presence of only Ag, As and S, giving rise to the Ag 2 AsS 2 chemical formula. However, a full structural study was never carried out and the dervillite structure remains unknown. In the course of a research project dealing with Ag-rich minerals from the Lengenbach quarry, Binn Valley, Canton Valais, Switzerland (Nestola et al., 2010; Bindi et al., 2010a, 2012a,b, 2013; Guastoni et al., 2012) we discovered a dervillite crystal (Fig. 1) in a sample coming from the so- * E-mail: luca.bindi@unifi.it DOI: 10.1180/minmag.2013.077.8.05 Mineralogical Magazine, December 2013, Vol. 77(8), pp. 3105–3112 # 2013 The Mineralogical Society FIG. 1. SEM-BSE image of dervillite on jordanite.