Abstract

Jentschite, TlPbAs 2 SbS 6 , is a new sulphosalt mineral from the famous Lengenbach locality (Binntal, Switzerland). It was discovered in association with numerous other Tl-As-sulphosalts such as hutchinsonite, wallisite-hatchite, edenharterite, bernardite, abundant realgar and orpiment in small cavities in a dolomitic rock of Triassic age which forms part of the Penninic Monte-Leone-nappe. The first specimens consisted of extremely small polysynthetically twinned crystals; a later find yielded less complicated twinned crystals in sizes up to 2 mm length. Single crystal X-ray diffraction studies gave a monoclinic cell with a = 8.121(3), b = 23.969(9), c = 5.847(3) Aa, beta = 107.68(3) degrees , V = 1084.3(5) Aa 3 , Z = 4, space group P2 (sub 1/n) . The strongest lines in the X-ray powder diagram are (d obs in Aa, I obs , hkl): 2.823 (100)(116), 3.587 (86)(221), 2.778 (84)(260), 3.998 (74)(060), 2.670 (58)(301), 3.816 (54)(210). Chemical composition (electron microprobe, mean of 15 analyses, wt.%) is: Tl 23.92, Pb 21.44, As 19.16, Sb 12.53, S 22.42, total 99.47 wt.% from which a simplified formula TlPbAs 2 Sb 1 S 6 can be derived. This formula is very close to that of the (orthorhombic) mineral edenharterite TlPbAs 3 S 6 . From structural and chemical data it was interpreted as the ordered structure of an Sb-bearing relative of edenharterite, a postulation that meanwhile could be confirmed by the complete structure determinations of both, edenharterite and jentschite (Berlepsch, 1996). Jentschite occurs in prismatic crystals up to 2 mm length; it is opaque with a black metallic to submetallic luster, red translucent in thin fragments. Fracture is uneven to conchoidal, the mineral is extremely brittle, the cleavage along (101) is perfect; the streak is dark red (darker than that of edenharterite). Mohs hardness is 2-2 1/2, according to a microhardness VHN (10 g load) of 38-51 kg/mm 2 . The calculated density yields D calc = 5.24 g/cm 3 . The name is for Franz Jentsch (1868-1908) from Binn, a local "Strahler" and for several years head of an early Lengenbach syndicate that exploited the rare Lengenbach sulphosalt minerals at the beginning of this century.

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