The crystal structure of plagionite, Pb5Sb8S17, the second member in the homologous series Pb3+2nSb8S15+2n

Abstract

Plagionite, Pb 5 Sb 8 S 17 , is monoclinic, space group C 2/ c , with a = 13.4857(8), b = 11.8656(4), c = 19.9834(7) Å, β = 107.168(4)°, ϱ meas = 5.54 g cm −3 and ϱ cal = 5.55 g cm −3 for Z = 4. The mineral is the second member of an homologous series Pb 3+2 n Sb 8 S 15+2 n which differ from other lead sulfantimonides in having tabular rather than acicular habits. Intensity data were recorded with an equi-inclination counter diffractometer. The structure was solved with the symbolic addition procedure and refined, by least-squares techniques, to R = 18.2% for 2302 observable reflections. The asymmetric unit contains 16 atoms. Two Pb atoms are coordinated by six and seven S atoms, respectively, in octahedral-like configuration; a third Pb atom has an irregular eightfold coordination which may be described either as a square antiprism, or as a trigonal prism with neighbors along two face normals. Three of four independent Sb atoms have square pyramidal coordination ; a fourth forms a trigonal pyramidal group. Of nine independent sulfur atoms, four have square pyramidal coordination, three have distorted tetrahedral coordination and two have threefold coordination. The structure is composed of slabs of PbS-like structure which are repeated by b . Stacks of such slabs exist parallel to (112) and (1[unk]2) alternately along c and extend indefinitely along [[unk]10] and [110] respectively. It is postulated that structures of other members in the homologous series contain similar slabs which differ in the width of the PbS-like unit.