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Aschamalmite (Pb₆Bi₂S₉): crystal structure and ordering scheme for Pb and Bi atoms

¹ Dipartimento di Scienze della Terra, Università degli Studi di Pavia, via Ferrata 1, I-27100 Pavia, Italy
² Centro Grandi Strumenti, Università degli Studi di Pavia, via Bassi 21, I-27100 Pavia, Italy

^* E-mail: callegari{at}crystal.unipv.it

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)°, V = 1 780.8(4) ų, 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 structure-refinement 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.

KEYWORDS: aschamalmite, heyrovskyite, sulphosalts, structure refinement, Susa Valley