Simultaneous refinement of two components of an exsolution intergrowth; crystal structures of the lindstroemite-krupkaite pair

Crystal structures of intergrown lindstromite and krupkaite were determined in two distinct samples by a simultaneous crystal-structure refinement using a modified JANA program package. The exsolution intergrowths have a bulk composition, determined by means of electron microprobe, equal to bd (sub 60.1) and bd (sub 64.1) , respectively. The subscript indicates a percentage of the aikinite component in this bismuthinite derivative. The crystal structure of lindstromite contains two aikinite-like ribbons Cu (sub 2) Pb (sub 2) Bi (sub 2) S (sub 6) and eight krupkaite-like ribbons (ideally CuPbBi (sub 3) S (sub 6) ) in a unit cell. The latter are oversubstituted; the ideally empty tetrahedral sites associated with them refined to the occupancy values of 0.04-0.09 Cu and 0.14-0.18 Cu, respectively, for the two above bulk compositions. The refined phase-compositions are Cu (sub 6.26) Pb (sub 6.26) Bi (sub 13.74) S (sub 30) (bd (sub 62.6) ) associated with CuPbBi (sub 3) S (sub 6) (bd (sub 50) ) for the sample "bd (sub 60) " and Cu (sub 6.64) Pb (sub 6.64) Bi (sub 13.36) S (sub 30) (bd (sub 66.4) ) associated with oversubstituted krupkaite, the composition of which could not been refined, for the sample "bd (sub 64) ". From stoichiometry calculations, the latter is approximately bd (sub 60.) Both latter values may be somewhat overestimated as a result of cross-correlations between the two closely related structures during the refinement.