Lead-antimony sulfosalts from Tuscany (Italy). VII. Crystal structure of pellouxite, ~ (Cu,Ag)₂Pb₂₁Sb₂₃S₅₅ClO, an expanded monoclinic derivative of Ba₁₂Bi₂₄S₄₈ hexagonal sub-type (zinkenite group)

Authors: Palvadeau, Pierre; Meerschaut, Alain; Orlandi, Paolo; Moëlo, Yves

Source: European Journal of Mineralogy, Volume 16, Number 5, 2004 , pp. 845-855(11)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

The crystal structure of pellouxite, a natural Pb-Sb-(Cu, Ag) oxy-chloro-sulfide from Tuscany (Italy), has been solved and refined using single-crystal X-ray-diffraction, with a final R = 0.044. The symmetry is monoclinic, space group C 2/ m, with a = 55.824(11), b = 4.0892(8), c = 24.128(5) Å, = 113.14(3)°, V = 5065(2) Å³. The structural formula is close to (Cu_0.68Ag_0.26)Pb_10.44Sb_11.56S₂₇(Cl_0.5S_0.5)O_0.5, with Z = 4, or, ideally (according to ordering revealed by a weak 2b superstructure), (Cu,Ag)₂Pb₂₁Sb₂₃S₅₅ClO, with Z = 4. Among cations, there are 9 pure Pb positions, 10 pure Sb positions, and 3 Pb/Sb and one Cu/Ag pairs. Together with 27 S atoms, there are one Cl/S pair, as well as one isolated half-filled O position. All the minor elements (Cu with Ag, Cl and O) appear essential for the organization of the crystal structure. The Cu/Ag pair is located within a triangular bipyramid (four S and one Cl/S); vacancies on this site (fixed to 0.06 %) have been correlated to an equivalent Sb excess in the neighbouring Pb/Sb pair according to the substitution Pb²⁺ + (Cu, Ag)⁺ rarr

Sb³⁺ + vacancy. The isolated oxygen is strongly bonded to two Sb atoms, like in scainiite and pillaite. The Cl/S pair is situated in a triangular prismatic coordination (4 Pb + 2 Sb). Following the principles of modular analysis, the whole structure is described in two ways: 1 - a combination of three types of 1D and 2D building blocks, each of them organized around Sb 3+ lone-electron-pair micelles, or 2 - a single type of complex rod-layer parallel to (20 Gamma

), organized around columns with pseudo-trigonal symmetry. Pellouxite constitutes the second example of oxy-chloro-sulfosalt after pillaite. It is an expanded monoclinic derivative of synthetic hexagonal Ba₁₂Bi₂₄S₄₈, structurally related to scainiite, Pb₁₄Sb₃₀S₅₄O₅ (topological similarity for about 90 % of the atom positions). It belongs to the zinkenite group of cyclically twinned sulfosalt structures and derivatives.

Keywords: CRYSTAL STRUCTURE; PELLOUXITE; OXY-CHLORO-SULFIDE; LEAD; ANTIMONY; ZINKENITE GROUP

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2004/0016-0845

Publication date: 2004-01-01

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