THE CRYSTAL STRUCTURE OF GUETTARDITE, PbAsSbS₄, AND THE TWINNITE–GUETTARDITE PROBLEM

Guettardite from the Barika Au–Ag deposit in Azarbaijan Province, western Iran, formed in fractures developed in silica bands situated in massive banded pyrite and barite ores. Fractures host veinlets that contain a number of Ag–As–Sb–Pb-rich sulfosalts, tetrahedrite–tennantite, realgar, pyrite and Au–Ag alloy. The variation in the chemical composition of guettardite is minor: Pb_0.95–0.96Sb_0.96–1.02As_1.03–1.06S_3.99–4.02. The lattice parameters were determined from a single crystal as a 8.527(4), b 7.971(4), c 20.102(10) Å, β 101.814(7)°, space group P2₁/c. The structure of guettardite contains six distinct coordination polyhedra of cations. Atoms Pb1 and Pb2 form slightly skewed tricapped trigonal coordination prisms arranged in a zig-zag layer. Two distinct As and two Sb sites in a chess-board arrangement form MeS₅ pyramids with trapezoidal bases. Guettardite is a homologue of sartorite, ideally PbAs₂S₄, and is the N = 3 member of the sartorite homologous series of sulfosalts. We contend that guettardite and twinnite are configurational polytypes composed of two alternating types of OD layers, formed by different orientations of tightly bonded crankshaft chains in adjacent As–Sb-based OD layers (layer symmetry P1̄) separated by a Pb-based OD layers [layer symmetry Pm2₁(n)]. Whether the observed differences in the Sb:As ratio determine the polytype is still an open question. A complete structural analogy has been found in the pair BaSb₂S₄ – BaSb₂Se₄.