VIAENEITE, (FE,PB)(4)S8O, A NEW MINERAL WITH MIXED SULFUR VALENCIES FROM ENGIS, BELGIUM

Viaeneite, (Fe,Pb)(4)S8O, forms aggregates up to 4 mm in size. Associa ted minerals are pyrite, marcasite, melnikovite, sphalerite, galena, o ther Zn-Pb compounds with double sulfur valencies, goethite, zincian s iderite, smithsonite and greigite. The mineral has a yellow colour, a distinct reflection pleochroism and is strongly annisotropic. Its R % in air, for lambda = 546 nm, is 23.8-36.8%, VHN200 = 252 kg/mm(2) (ran ge: 123-357), D-meas = 3.8 +/- 0.1 g/cm(3), D-calc = 3.65 g/cm(3). Mic roprobe analyses give (wt.%, average): Fe 42.37, Pb 4.02, Zn 0.05, Ni 0.20, As 0.17 S 49.74, O 3.69, total 100.24. The empirical formula (ba sed on microprobe analyses) is: e3.91Pb0.10Ni0.02)(4.03)(S8.00As0.01)( 8.01)O-1.16. The sulphur is present in two valency combinations: disul phide, S-2(2-), and thiosulphate, S2O32-, in the ratio 11:1. Viaeneite is monoclinic : a = 9.717 +/- 0.008, b = 7.280 +/- 0.006, c = 6.559 /- 0.007(Angstrom) and beta = 95.00 degrees +/- 0.03, V = 462.2 +/- 0. 8 Angstrom(3), Z = 2. The cell is probably primitive. The strongest XR D powder reflections are 3.43(5), 2.709(10), 2.419(8), 2.323(7), 1.920 (6), 1.758(8), 1.595(5), 1.162(5), 1.092(5), 1.035(5), 0.9911(5) and 0 .9576(7)(Angstrom). The name is given in honour of Prof. W. Viaene, Ka tholieke Universiteit Leuven, Belgium.