Suredaite, PbSnS3, a new mineral species, from the Pirquitas Ag-Sn deposit, NW-Argentina: mineralogy and crystal structure

Suredaite, ideally PbSnS3, is a new mineral species from the Pirquitas Ag-S n deposit (Province Jujuy, NW-Argentina). It was observed in symmetrically banded veins in the Oploca district, and is associated with sphalerite, ars enopyrite, pyrite-marcasite, cassiterite, cylindrite, franckeite, hocartite , rhodostannite, and various Ag-Sb and Ag-Bi sulfosalts in minor amounts. S uredaite occurs in layers up to 1 cm in thickness as aggregates of radially arranged tabular-prismatic (single) crystals, has a metallic lustre, and a dark grey streak. VHN50 ranges between 18.2 and 20.6 (mean 19.6) GPa, the Mohs hardness is 2.5-3. It has perfect cleavages parallel to {001}, {101}, and {100}. The measured density varies between 5.54 and 5.88 g/cm(3), D-x w as determined to be 5.615 g/cm(3). In reflected plane-polarised light, it i s white and is not perceptibly bireflectant or pleochroic. It lacks interna l reflections and is weakly anisotropic with metallic blue, mauve to brown rotation tints. Specular reflectance percentages in air and in oil are tabu lated from 400 to 700 nm and compared graphically with those for the type s pecimen of teallite, PbSnS2. Electron microprobe analyses showed suredaite to be chemically inhomogeneous with respect to the compositional variations (in wt%): Pb 42.3-48.5, Ag 0.3-1.1, Fe 0.3-1.0, As 0.2-2.1, Sn 27.7-30.2, S 23.1-24.7. The crystal structure determined from single-crystal X-ray dif fraction data revealed orthorhombic symmetry [space group Pnma, Z = 4, a = 8.8221(3), b = 3.7728(3), c = 14.0076(3) Angstrom; V = 466.73(4) Angstrom(3 )]. The atomic arrangement is isostructural to the NH4CdCl3 structure type which exists in a series of isotypic sulfides and selenide compounds. The s uredaite structure, which is the natural analogue of synthetic PbSnS2, cons ists of columns of double-edge sharing octahedra running parallel to the b axis, which house the Sn atoms. These columns are linked by rods of eightfo ld-coordinated Pb atoms. On the basis of the structure determination, the e mpirically determined idealized formula follows a ([8])(Pb, As,Ag, Sn) ([6] )(Sn,Fe)S-3 stoichiometry. Crystalchemical arguments suggest Ag possibly to occupy interstitial sites according to the alternative formula ([4])(squar e,Ag) ([8])(Pb, As, Sn) ([6])(Sn,Fe) S-3. The name of this new mineral spec ies is in honor of R.J. Sureda Leston, head of the Department of Mineralogy and Eco nomic Geology, University of Salta, Argentina.