Suseinargiuite, (Na0.5Bi0.5)M-0.04, the Na-Bi analogue of vvulfenite, from Su Seinargiu, Sardinia, Italy

摘要

The new mineral species suseinargiuite, (Na0.5Bi0.5)MoO4, has been discovered in the Mo-Bi occurrence of Su Seinargiu, Sarroch, Cagliari, Sardinia, Italy. It occurs as hemispherical aggregates of very small acicular crystals, up to some micrometres in length. It is colourless, with a pearly to adamantine lustre. In the studied specimens, suseinargiuite is associated with wulfenite. Micro-Raman spectra were collected in the region between 100 and 2000 cm(-1). The following bands have been observed (in cm(-1)): 131, 188, 319, similar to 376, similar to 772, and 876. Electron microprobe data collected on a chemically zoned grain gave (outer and inner zone, respectively -in wt): MoO3 49.03, Bi2O3 42.97, PbO 2.89, Na2O 3.69, total 98.58, and MoO3 45.59, Bi2O3 34.47, PbO 12.04, Na2O 3.03, total 95.13. On the basis of 4 O atoms per formula unit, the chemical formulae are (Na0.35Bi0.54Pb0.04)(Sigma=0.93)Mo0.99O4 and (Na0.31Bi0.46Pb0.17)(Sigma=0.94)Mo0.99O4, respectively. Main diffraction lines are d in angstrom (relative intensity) hkl: 3.146 (100) 112, 2.912 (13) 004, 2.652 (18) 200, 1.964 (34) 204, 1.875 (15) 220, 1.728 (19) 116, and 1.616 (28) 312 and 132. Unit-cell parameters, refined from the powder X-ray diffraction data in a tetragonal setting, space group I4(1)/a, are a = 5.296(1), c = 11.673(2) angstrom, V = 327.4(1) angstrom(3), Z = 4. Owing to the lack of suitable crystals and the very small amount of available material, the crystal structure of suseinargiuite was not solved. However, X-ray powder diffraction data, micro-Raman spectra, as well as chemical analysis, show the close correspondence between suseinargiuite with synthetic (Na0.5Bi0.5)MoO4, which displays a scheelite-type structure.