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 C2/m, with a = 55.824(11), b = 4.0892(8), c = 24.128(5) Å, β = 113.14(3)°, V = 5065(2) Å3. The structural formula is close to (Cu0.68Ag0.26)Pb10.44Sb11.56S27(Cl0.5S0.5)O0.5, 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)+ → 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 1 D and 2D building blocks, each of them organized around Sb3+ lone-electron-pair micelles, or 2 - a single type of complex rod-layer parallel to (201̄), organized around columns with pseudo-trigonal symmetry. Pellouxite constitutes the second example of oxychloro-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.
