Abstract

The new mineral scainiite, Pb 14 Sb 30 S 54 O 5 , the first lead oxy-sulfosalt, has been found at Buca della Vena mine in the Apuan Alpes, northern Tuscany, Italy. Scainiite occurs as black acicular crystals with bluish metallic lustre, elongated [010], up to 2 mm long and 0.2 mm thick. The mineral is brittle, opaque; VHN 20 = 192 kg/mm 2 , d calc = 5.56 g/cm 3 . In reflected light it shows a low anisotropy and bireflectance, and rare red internal reflections. Its ideal chemical formula, derived from electron-microprobe data and from crystal structure refinement, is Pb 14 Sb 30 S 54 O 5 . Scainiite is monoclinic, space group C2/m, a = 52.00(2), b = 8.148(2), c = 24.31(6) Aa, beta = 104.09(4) degrees , Z = 4. The strongest five lines of the X-ray powder diffraction pattern are [d in Aa(I)(hkl)]: 3.472(100)(14 0 4), 2.956(54) (6 2 4, 12 2 1), 2.052(46)(14 0 8, 24 0 1), 3.041(35)(4 0 8), 2.228(22)(22 0 1, 22 0 6, 8 2 9). Scainiite is a structural derivative of synthetic hexagonal Ba (sub approximately 12) Bi (sub approximately 24) S 48 , a member of cyclically twinned sulfosalt structures (zinkenite group). Scainiite crystallized in small fractures of dolomitic lenses in massive Fe-Ba ore, together with many Pb-Sb sulfosalts. The formation of this unique lead oxy-sulfosalt is related to narrow fO 2 -fS 2 conditions, corresponding to the magnetite-hematite-pyrite buffer at relatively high temperature (>300 degrees C).

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