Lead-antimony sulfosalts from Tuscany (Italy). I. Scainiite, Pb14Sb30S54O5, the first Pb-Sb oxy-sulfosalt, from Buca della Vena mine

The new mineral scainiite, Pb14Sb30S54O5, the first lead oxy-sulfosalt, has been found at Buca della Vena mine in the Apuan Alpes, northern Tuscany, I taly. Scainiite occurs as black acicular crystals with bluish metallic lust re, elongated [010], up to 2 mm long and 0.2 mm thick. The mineral is britt le, opaque; VHN20 = 192 kg/mm(2), d(calc.) = 5.56 g/cm(3). In reflected lig ht it shows a low anisotropy and bireflectance, and rare red internal refle ctions. Its ideal chemical formula, derived from electron-microprobe data a nd from crystal structure refinement, is Pb14Sb30S54O5. Scainiite is monocl inic, space group C2/m, a = 52.00(2), b = 8.148(2), c = 24.31(6) Angstrom, beta = 104.09(4)degrees, Z = 4. The strongest five lines of the X-ray powde r diffraction pattern are [d in Angstrom(I)(hkl)]: 3.472(100)(<(14)over bar > 0 4), 2.956(54) (6 2 4, <(12)over bar> 2 1), 2.052(46)(<(14)under bar> 0 8, <(24)under bar> 0 1), 3.041(35)((4) over bar 0 8), 2.228(22)(<(22)under bar> 0 1, 22 0 6, 8 2 9). Scainiite is a structural derivative of synthetic hexagonal Ba-12Bi-24S48, a member of cyclically twinned sulfosalt structur es (zinkenite group). Scainiite crystallized in small fractures of dolomiti c lenses in massive Fe-Ba ore, together with many Pb-Sb sulfosalts. The for mation of this unique lead oxy-sulfosalt is related to narrow fO(2) - fS(2) conditions, corresponding to the magnetite-hematite-pyrite buffer at relat ively high temperature (>300 degrees C).