ALBERT

Description: Roxbyite is triclinic, space group P1̄, with unit-cell dimensions a 13.4051(9), b 13.4090(8), c 15.4852(3) Å, α 90.022(2), β 90.021(2), γ 90.020(3)°. The crystal structure was solved using 12190 independent intensity data with I 〉 4σ(I), out of some 43652 unique reflections measured with a single-crystal diffractometer. There are 58 different Cu atoms and 32 different S atoms in the structure. Refinement using isotropic temperature-factors yielded Rw = 0.079. The number of parameters refined was 372. Because the crystal used is twelve-fold rotationally twinned about a, 12 twin-volume parameters were invoked. The structure was determined after transformation of an initial model obtained for a unit cell four times larger, also with space group P, with a 30.9625, b 30.9727, c 13.4051 Å, α 90.007, β 89.972, γ 119.994° and with a composition Cu228S128. This larger cell, which was determined by the instrumental software during data collection, actually results from complicated multiple twinning of the true cell defined above. The structure of roxbyite is based on a hexagonal-close-packed framework of sulfur atoms with the copper atoms occupying these layers, all having triangular coordination. Other layers sandwiched between the close-packed sulfur layers consist purely of double, or split, layers of Cu atoms. Some of these Cu atoms have two-fold linear coordination, but mostly they have three- and four-fold coordination to the sulfur atoms in the close-packed layers that lie above and below them. The crystal structure of roxbyite bears a strong kinship to those of low chalcocite and djurleite.