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

Abstract

Coiraite, ideally (Pb,Sn 2+ ) 12.5 As 3 Fe 2+ Sn 5 4+ S 28 , 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 μm long. Electron microprobe analysis (n = 31) gave an empirical formula Pb 11.21 As 2.99 Ag 0.13 Fe 1.10 Sn 6.13 S 28.0 , close to the ideal formula (Pb 11.3 Sn 1.2 2+ ) ∑=12.5 As 3 Fe 2+ Sn 5 4+ S 28 . Coiraite has two monoclinic sub-cells, Q (pseudotetragonal) and H (pseudohexagonal). Q: a 5.84(1) Å, b 5.86(1) Å, c 17.32(1) Å, β 94.14(1)°, V 590.05(3) A 3 , Z = 4, a:b:c = 0.997:1:2.955; H (orthogonal setting): a 6.28(1) Å, b 3.66(1) Å, c 17.33(1) Å, β 91.46(1)°, V 398.01(6) Å 3 , Z = 2, a:b:c = 1.716:1:4.735. The strongest Debye-Scherrer camera X-ray powder-diffraction lines [d in Å, (I), (hkl)] are: 5.78, (20), (Q and H 003); 4.34, (40), (Q 004); 3.46, (30), (Q and H 005); 3.339, (20), (Q 104); 2.876, (100), (Q and H 006); 2.068, (60), (Q 220).