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University of Basel, Natural History Museum, Basel, Switzerland
Universitaet Goettingen, Federal Republic of Germany
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.
This record provided courtesy of AGI/GeoRef.
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