Écrinsite, AgTl3Pb4As11Sb9S36, a new thallium-rich homeotype of baumhauerite from the Jas Roux sulphosalt deposit, Parc national des Écrins, Hautes-Alpes, France

Abstract

Ecrinsite, ideally AgTl 3 Pb 4 As 11 Sb 9 S 36 , is a new thallium sulphosalt species found in the Jas Roux As–Sb–Pb–Tl–Hg–Ag deposit, Parc National des Ecrins, Departement des Hautes-Alpes, France. Associated minerals in four different samples are jasrouxite, stibnite, smithite, guettardite, chabourneite, pierrotite and As-bearing zinkenite. Ecrinsite is opaque with metallic lustre. It is brittle without any discernible cleavage and with conchoidal fracture. In reflected light ecrinsite is white, pleochroism is not discernible. Internal reflections are absent. In crossed polarisers, anisotropism is distinct, with rotation tints in shades of grey. The reflectance data (%, air) are: 37.3, 38.6 at 470 nm, 35.2, 36.7 at 546 nm, 34.0, 35.5 at 589 nm and 32.0, 33.3 at 650 nm. Mohs hardness is 3–3½, microhardness VHN 25 is in the range 175–201, with a mean value of 189 kg mm −2 . Average results of 20 electron-microprobe analyses for the structurally investigated grain are (in wt%): Ag 2.03(10), Cu 0.02(1), Tl 14.57(20), Pb 16.23(32), Sb 23.97(25), As 17.87(17), S 25.20(15), total 99.88 (15), corresponding to Ag 0.87 Cu 0.02 Tl 3.28 Pb 3.61 Sb 9.06 As10.98S36.19 (on the basis of 28 Me + 36S = 64 apfu ). The simplified formula, AgTl 3 Pb 4 Sb 9 As 11 S 36 , is in accordance with the results of the crystal-structure analysis and may be derived from the ideal baumhauerite formula, Pb 12 As 16 S 36 , by substitution of Sb for As and [(Tl,Ag) + + (As,Sb) 3+ ] ↔ 2Pb 2+ . The density, 4.96 g cm −3 , was calculated using the ideal formula. Ecrinsite has a triclinic cell, space group P 1 ¯ , with a = 8.080(2), b = 8.533(2), c = 22.613(4) A, α = 90.23(3)°, β = 97.17 (3)°, γ = 90.83(3)°, V = 1546.7(6) A 3 , and Z = 1. The strongest five lines in the (calculated) powder-diffraction pattern are [ d in A( I )( h k l )]: 4:14(68) ( 10 5 ¯ ) , 3.72(92)(105), 3:56(100) ( 10 6 ¯ ) , 3:53(80) ( 1 2 ¯ 2 ) and 3.48(72)(122). Ecrinsite is a new member of the sartorite homologous series with N 1,2 = 3, 4, i.e. , N = 3.5. Four other members share the same N value as ecrinsite: baumhauerite, argentobaumhauerite, boscardinite and bernarlottiite. By comparison to the closely related boscardinite (ideally Tl 2 Pb 8 Sb 14 As 4 S 36 ), ecrinsite is characterised by both an excess of As over Sb and a distinctly higher substitution of [(Tl,Ag) + +(As,Sb) 3+ ] ↔ 2Pb 2+ . Baumhauerite and bernarlottiite are basically unsubstituted members, whereas argentobaumhauerite has a Ag substitution, without Tl.