Coiraite, (Pb,Sn²⁺)_12.5As₃Fe²⁺Sn⁴⁺₅S₂₈: a franckeite-type new mineral species from Jujuy Province, NW Argentina

Abstract

Coiraite, ideally (Pb,Sn 2+ ) 12.5 As 3 Fe 2+ Sn 4+ 5 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 mm 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 2+ 1.2 ) S=12.5 As 3 Fe 2+ Sn 4+ 5 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 , b 94.14(1)º, V 590.05(3) Å 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 , b 91.46(1)º, 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

Merelaniite, Mo4Pb4VSbS15, a New Molybdenum-Essential Member of the Cylindrite Group, from the Merelani Tanzanite Deposit, Lelatema Mountains, Manyara Region, Tanzania

“…As the two Q and H subcells correspond to centered pseudo-tetragonal and pseudo-hexagonal lattices, and because we measured a c-axis of about 12 Å (see above), merelaniite should contain [2(M2S2)] and [1(M′2S4)] atoms, respectively [24]. For the whole structure, considering one unit H subcell, which contains [1(M′2S4)] atoms, for the same volume, the Q part will correspond to ([2(M2S2)] × SH/SQ) atoms.…”

“…As S H /S Q ≈ 0.50 (0.496 and 0.493 from single-crystal and powder data, respectively), the structural formula of merelaniite can be written as: [2(M 2 S 2 )] 0.50 ·[(M 2 S 4 )], which is consistent with the electron microprobe results yielding a metal to sulfur ratio (M:S) of 2:3. Recalculation of the formula from the electron microprobe results on the basis of two metal atoms, or, alternatively, on the basis of five atoms (two cations + three anions) and following the crystal-chemical preference of the elements usually reported in minerals and synthetic compounds belonging to the cylindrite group [24], and also assuming V and W are trivalent and tetravalent, respectively, gives Interestingly, as mentioned in the chemistry section, we identified a bimodal composition to some of the whiskers, which might also be observed optically ( Figure 6). After characterizing both compositions (from a chemical and structural point of view) it is clear there are strong As-enrichments that cause the bimodality.…”