DANTOPAITE, Ag5Bi13S22, THE 6P NATURAL MEMBER OF THE PAVONITE HOMOLOGOUS SERIES, FROM ERZWIES, AUSTRIA

Abstract

Dantopaite, Ag 5 Bi 13 S 22 , is the 6 P natural member of the pavonite homologous series. It is a very rare mineral in a quartz vein hosted by Variscan metagranitoids at the abandoned mining district at Erzwies, Gasteiner Valley, province of Salzburg, Austria. The associated sulfosalts are lillianite–gustavite, heyrovskyite, eskimoite, vikingite, ourayite, bismuthinite, krupkaite, benjaminite, pavonite, and cosalite. Traces of pyrite, tetradymite and native gold also are present. The new species occurs as isolated grains intimately intergrown with pavonite or benjaminite or both. The grain size does not exceed 250–300 μm. Dantopaite has a greyish white color and is opaque. It has a metallic luster and a grey streak. Its hardness (VHN 50 ) is ~190 kg/mm 2 (3½ on the Mohs scale). In plane-polarized light, and compared to galena, it is weakly bireflectant, and distinctly pleochroic in shades of grey. The anisotropy is distinct in air and oil, and the rotation tints change from greyish blue to brownish grey. The reflectances (in air) are tabulated. The average result of electron-microprobe analysis is: Cu 1.68, Ag 11.56, Pb 4.64, Bi 63.82, Te 0.34, S 17.52, total 99.57 wt.%, which gives Cu 1.06 Ag 4.24 , Pb 0.9 Bi 12.23 S 21.89 Te 0.11 . The ideal formula is Ag 5 Bi 13 S 22 . Dantopaite is monoclinic with a 13.380(3), b 4.0492(9), c 18.690(4) A, V 975.8(4) A 3 , space group C 2/ m and Z = 1. The calculated density is 6.74 g/cm 3 . The strongest eight lines in the (calculated) powder-diffraction pattern [ d in A(I)( hkl )] are: 3.578(58)(205), 3.452(90)(112), 3.331(36)(401), 3.301(40)(204), 2.861(100)(313), 2.225(24)(117), 2.025(25)(020) and 2.013(24)(512). The crystal structure, based on 1051 observed reflections ( R 1 = 7.3%), is that of N = 6 pavonite homologue, consisting of thick slabs with a width of six octahedra on a diagonal, in alternation with thin slabs composed of foreshortened octahedra and pairs of Bi coordination pyramids. The central columns of the thin structural slabs are populated by octahedrally coordinated Ag sites and tetrahedrally coordinated Cu1 and Cu2 sites; all of them are only partly occupied. In the thick slabs, with distorted octahedral bismuth sites, silver partly replaces bismuth in the centrally placed octahedra. The name honors Dr. Dan Topa (University of Salzburg), ore mineralogist and crystallographer.