The thallium-rich sulfosalt deposit of Jas Roux, situated in the Pelvoux Massif (Hautes-Alpes département, 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 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 4.87 g.cm−3. The simplified formula is Ag16Pb4(Sb25As15)∑40S72; the only notable variation is the partial substitution As↔Sb. Jasrouxite is triclinic, space group P-1. Lattice parameters are a 8.2917(5) Å, b 19.101(1) Å, c 19.487(1) Å, α 89.731(1)°, β 83.446(1)°, γ 89.944(1)°, V 3066.1(3) Å3, Z= 1 for Ag16Pb4(Sb24As16)∑40S72.
Jasrouxite is a member of the lillianite homologous series, its order number is 4 and degree of Ag + (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 of lone electrons of Sb and As in the central portions of galena-like slabs, and high contents of Ag and (Sb,As).