Jasrouxite, a new Pb-Ag-As-Sb member of the lillianite homologous series from Jas Roux, Hautes-Alpes, France

Abstract

The thallium-rich sulfosalt deposit of Jas Roux, situated in the Pelvoux Massif (Hautes-Alpes de´partement, France), occurs in a Triassic sedimentary series. Jasrouxite belongs to the early lead-containing stages of the Tl–As–Sb period of mineralization. It occurs in a silicified gangue, along with smithite and late realgar. Jarouxite forms dark gray anhedral grains with metallic luster, up to<br/>several millimeters in size, as well as aggregates of grains which range to more than 10 mm in diameter. From their surface, grains of jasrouxite are embayed by a three-phase myrmekite aggregate composed of stibnite, boscardinite and smithite. The mineral is brittle, with irregular fracture; no cleavage or parting was observed. In reflected light, color is off-white. Bireflectance is weak, in off-white tones; anisotropy is distinct. Rare straight twin lamellae occur in otherwise untwinned crystals. The empirical formula, on the basis of 132 apfu derived from the crystal-structure determination, is Cu0.79Ag14.64Pb4.10Tl0.05As15.37Sb24.87S72.18. Calculated density is<br/>4.87 g.cm3. The simplified formula is Ag16Pb4(Sb25As15)40S72; the only notable variation is the partial substitution As$Sb.<br/>Jasrouxite is triclinic, space group P-1. Lattice parameters are a 8.2917(5) A ° , b 19.101(1) A ° , c 19.487(1) A ° , a 89.731(1), b 83.446(1), g 89.944(1), V 3066.1(3) A ° 3, Z ¼ 1 for Ag16Pb4(Sb24As16)40S72.<br/>Jasrouxite is a member of the lillianite homologous series, its order number is 4 and degree of Ag þ (Sb, As)$2 Pb substitution is 36.5 % above the theoretical 100 % substitution expressed as PbAg(Sb,As)3S6. The most important features of the crystal structure are: substantial replacement of lead in selected trigonal prismatic sites by Sb, formation of a string of large volumes for accommodation<br/>of lone electrons of Sb and As in the central portions of galena-like slabs, and high contents of Ag and (Sb,As).