The first single-crystal structure refinement of aschamalmite (Pb6Bi2S9) from Susa Valley (Piedmont, Italy) is reported. The mineral is monoclinic, C2/m, a = 13.719(1) Å, b = 4.132(1) Å, c = 31.419(3) Å, β = 90.94(1)º, V = 1 780.8(4) A ˚3, Z = 4. The Pb6Bi2S9 compound crystallizes also in an orthorhombic form as heyrovskyite (Cmcm) and our study is focused on understanding the reason leading to a change in symmetry. The aschamalmite structure forms because of ordering between Pb and Bi on the margins of the two octahedral layers that are symmetrically equivalent in heyrovskyite. The two alternate set of octahedral slabs are not related by a crystallographic mirror plane and the symmetry decreases to monoclinic. The cation ordering couples opposite sequences of Pb and Bi octahedra at the margins of slabs. In particular,the succession [Me4A]Bi-[Me5A]Pb-[Me4A]Bi-[Me5A]Pb faced to the series [Me4B]Pb- [Me5B]Bi-[Me4B]Pb-[Me5B]Bi occurs in about 70% of the unit-cells of the crystal,while the contrary sequence ([Me4A]Pb-[Me5A]Bi-[Me4A]Pb-[Me5A]Bi faced to [Me4B]Bi-[Me5B]Pb-[Me4B]Bi-[Me5B]Pb) occurs in the remaining unit-cells. The marginal octahedra have ideal populations (a.p.f.u.): [Me4A]1.40Bi+0.60Pb, [Me4B]1.40Pb+0.60Bi, [Me5A]1.40Pb+0.60Bi, [Me5B]1.40Bi+0.60Pb,in agreement with our structurerefinement results.

The probable site populations for pure heyrovskyite have been proposed,as well as the reasons that prevent the formation of a completely ordered monoclinic phase.

Sulphosalts in the system CuSbS2-CuBiS2 (chalcostibite-emplectite) form a complete solid solution series. Seven compositions with the general formula Cu(SbxBi1–x)S2 have been synthesized using dry methods at 310°C. All members of the series are orthorhombic (space group Pnma) and show smoothly increasing a and b cell parameters with substitution of Bi for Sb; the c cell parameter increases up to 50% CuBiS2 substitution and then becomes constant. DSC experiments on CuBiS2 show an endothermic heat effect (2.45 kJ/mol.) at 472°C due to the breakdown reaction to Cu3BiS3 (wittichenite) plus Bi2S3 (bismuthinite). With the addition of 10% CuSbS2 to CuBiS2, the decomposition temperature increases and the endothermic peak is broadened but the energy remains essentially the same (2.53 kJ/mol.). No evidence of this decomposition was observed when the amount of the CuSbS2 component was >30%. The local structure and co-ordination of Cu in the samples were studied by EXAFS analysis of the Cu-K edge but no significant variation occurs in the local Cu environment. The Debye-Waller factor for the first shell of S atoms surrounding Cu in end member CuSbS2 tends to be slightly smaller than for the intermediate solid solutions, suggesting that the tetrahedral Cu environments in the intermediate composition samples is somewhat more disordered than in the end-member. The low expansion characteristics along c appear to be controlled by the linkages between the (CuS3 + BiS2) sheets perpendicular to c being relatively inflexible.