Lead-antimony sulfosalts from Tuscany (Italy). XIX. Crystal chemistry of chovanite from two new occurrences in the Apuan Alps and its 8 Å crystal structure

Abstract

AbstractTwo new occurrences of the lead oxy-sulfosalt chovanite have been identified at the Monte Arsiccio and Pollone mines, Apuan Alps, Tuscany, Italy. Chovanite from Monte Arsiccio occurs as black acicular crystals, up to 5 mm long, associated with rouxelite, robinsonite, sphalerite, valentinite,baryte, dolomite, quartz and Ba-rich K-feldspar ('hyalophane') in metadolostone vugs. Its unit-cell parameters are a = 48.38(5), b = 4.11(4), c = 34.18(4) Å, β = 106.26(2)°, V = 6521(64) Å3, space group C2/m. Very weakreflections indicate the doubling of the b parameter. Electron-microprobe data gave (wt.%): Ag 0.28, Tl 1.51, Pb 45.57, Sb 31.00, As 1.09, S 19.73, Se 0.05, Cl 0.02, sum 99.25. On the basis of ∑Me = 58 apfu, its formula is Ag0.30Tl0.86Pb25.56Sb29.59As1.69S71.52Se0.07Cl0.05.Adding one oxygen atom, it is close to the formula TlPb26(Sb,As)31S72O. Chovanite from Pollone occurs as thick black acicular crystals, up to 1 cm long, associated with baryte and quartz. The high-diffraction quality of the available material allowed the solutionand refinement of the 8 Å crystal structure in the space group P21/c, with unit-cell parameters a = 34.052(3), b = 8.2027(7), c = 48.078(4) Å, β = 106.258(4)°, V = 12891.9(19) Å3. The refinement convergedto R1 = 9.14% on the basis of 19,853 observed reflections with Fo > 4σ(Fo). Electron-microprobe data gave (wt.%): Ag 0.12, Tl 0.54, Pb 48.34, Bi 0.20, Sb 26.71, As 3.37, S 20.23, Cl 0.07, sum 99.57. It corresponds to the formulaAg0.13Tl0.30Pb26.94Bi0.10Sb25.33As5.20S72.85Cl0.20, close to the idealized formula Pb28(Sb,As)30S72O, with a single oxygen atom bound to two (Sb/As) atoms alternating witha vacancy along b as in scainiite and in other Pb oxy-sulfosalts. The crystal chemistry of this 8 Å crystal structure is detailed, taking into account its modular description, the (Sb,As)mSn polymerization, its topological derivation from pellouxite,and the oxygen non-stoichiometry.