THE CRYSTAL-STRUCTURE OF VIKINGITE

The crystal structure of vikingite from La Roche Balue, France, has been refined using 507 independent reflections to an R value of 0.08. An electron-microprobe analysis of vikingite yields Pb9.82Cd0.27Ag3.91Bi11.98S30.43; the structure determination gave Pb10.96Ag3.52Bi11.52S30 for the crystal studied. Cell parameters are a 13.60, b 4.11, c 25.25 angstrom, beta 95.60-degrees, space group C2/m. Trigonal prismatic sites Me 3 on planes of unit-cell twinning are fully occupied by Pb, octahedral sites Me 6 and Me 7 in the central portions of the thicker slabs are primarily Pb as well; the semimarginal octahedral sites Me 1 and Me 5 are occupied by Bi: substitution takes place in the marginal octahedral sites Me 2 (0.48 Ag + 0.52 Bi) and Me 4 (0.38 Ag + 0.62 (Bi + Pb)). The Me 4 to Me 7 sites are situated in the thicker, 7 octahedra wide slabs, similar to those in Ag - Bi substituted heyrovskyite; Me 1 and Me 2 in the thinner, 4 octahedra wide slabs, similar to those in gustavite. Departures from the PbS archetype are small, with the largest departures occurring in marginal and semi-marginal sites. The composition of the thinner slabs is PbAgBi3S6, that of thicker slabs Pb4.48Ag0.76Bi2.76S9.