TY - JOUR
T1 - The crystal structure of guettardite, PbAsSbS4, and the twinnite-guettardite problem
AU - Makovicky, Emil
AU - Topa, Dan
AU - Tajedin, Husein
AU - Rastad, Ebrahim
AU - Yaghubpur, Abdolmajid
PY - 2012/4
Y1 - 2012/4
N2 - 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), β 7.971(4), c 20.102(10) Å, β 101.814(7)°, space group P 21/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 PbAs2S 4, and is the N = × 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 P1) separated by a Pb-based OD layers [layer symmetry Pm 21(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 BaSb 2S 4 - BaSb 2Se 4.
AB - 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), β 7.971(4), c 20.102(10) Å, β 101.814(7)°, space group P 21/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 PbAs2S 4, and is the N = × 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 P1) separated by a Pb-based OD layers [layer symmetry Pm 21(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 BaSb 2S 4 - BaSb 2Se 4.
U2 - 10.3749/canmin.50.2.253
DO - 10.3749/canmin.50.2.253
M3 - Journal article
SN - 0008-4476
VL - 50
SP - 253
EP - 265
JO - Canadian Mineralogist
JF - Canadian Mineralogist
IS - 2
ER -