THE CRYSTAL STRUCTURE OF KIRKIITE, PB10BI3AS3S19

Abstract

The crystal structure of kirkiite has been solved using single-crystal data (MoKα X-ray diffraction, CCD area detector) to the conventional R-factor R1 = 0.069. It crystallizes in space group P21/m, with a 8.621(4), b 26.03(1), c 8.810(4) Å, β 119.21(1)° and Z = 2. A crystal-structure determination and chemical analysis resulted in comparable formulae. Pb10 Bi2.16 As3.84S19 and Pb10.08 Bi2.55 Sb0.13 As2.91S19, respectively, which are close to the ideal formula Pb10Bi3 As3S19. The crystal lattice shows a pseudohexagonal symmetry, which is the cause of common twinning in this mineral. The main twin-law has (201̄) as a twin plane, with [100] as the twin axis, owing to the orthorhombic distortion of the pseudohexagonal lattice in accordance with the alignment of lone-electron-pair micelles in rows parallel to (001). The twin mechanism explains the formation of observed twin-lamellae with (201̄) as a contact plane. The crystal structure of kirkiite can be described as (010) slabs of octahedra, three octahedra thick and related mutually by a reflection plane situated in the intervening prismatic layer. In another interpretation, it is composed of slabs based on a transitional PbS-SnS archetype, with tightly bonded layers parallel to (083) of kirkiite; the slabs are unit-cell-twinned on (010) reflection planes. The structure contains one split As position, and two additional sites that could accommodate both As and Bi. The As,Bi distribution over these two sites is determined by the trapezoidal distortion of the half-octahedral coordination environment inside the tightly bonded double layers of the PbS-SnS archetype slabs. Owing to the stoichiometry requirements in this structure, Bi must also substitute for 1/11 of the Pb sites. Bond-valence calculations and the volumes of coordination polyhedra show it to be ordered in two of the Pb sites. Kirkiite and jordanite Pb28As12S46 are a pair of homologues. The general formula of a potential homologous series is Pb8N-4Me3+12S8N+14.