Coiraite, (Pb,Sn2+)12.5As3Fe2+Sn4+5S28: a franckeite-type new mineral species from Jujuy Province, NW Argentina

Abstract

AbstractCoiraite, ideally (Pb,Sn2+)12.5As3Fe2+Sn4+S28, occurs as an economically important tin ore in the large Ag-Sn-Zn polymetallic Pirquitas deposit, Jujuy Province, NW-Argentina. The new mineral species is the As derivative of franckeite and belongs to the cylindrite group of complex Pb sulphosalts with incommensurate composite-layered structures. It is a primary mineral, frequently found in colloform textures, and formed from hydrothermal solutions at low temperature. Associated minerals are franckeite, cylindrite, pyrite-marcasite, as well as minor amounts of hocartite, Ag-rich rhodostannite. arsenopyrite and galena. Laminae of coiraite consist of extremely thin bent platy crystals up to 50 urn long. Electron microprobe analysis(n =31) gave an empirical formula Pb11.21As2.99Ag0.13Fe1.10Sn6.13S28.0close to the ideal formula (Pb11.3Sn2+1.2)Σ=12.5As3Fe2+Sn4+S28. Coiraite has two monoclinic sub-cells,Q(pseudotetragonal) andH(pseudohexagonal).Q: a5.84(1) Å,b5.86(1) Å, c 17.32(1) Å, β 94.14(1)°, F 590.05(3) Å3, Z = 4,a:b:c =0.997:1:2.955; H (orthogonal setting):a6.28(1) Å,b3.66(1) Å, c 17.33(1) Å, β 91.46(1)°,V398.01(6) Å3, Z= 2, a∶b∶c =1.716∶1∶4.735. The strongest Debye-Scherrer camera X-ray powder-diffraction lines[din Å,(I), (hkl)]are: 5.78, (20),(QandH003); 4.34, (40),(Q004); 3.46, (30),(Qand H 005); 3.339, (20),(Q104); 2.876, (100),(Qand H 006); 2.068, (60),(Q220).