THE PLUMOSE BOULANGERITE FROM BOTTINO, APUAN ALPS, ITALY: CRYSTAL STRUCTURE, OD CHARACTER AND TWINNING

Abstract

We deal with a single-crystal X-ray diffraction study of boulangerite from a plumose sample from the Bottino mine, Apuan Alps, Italy. Chemical composition of the analyzed sample is Pb 4.89 (Sb 4.08 As 0.04 Bi 0.01 ) ∑4.13 (S 10.98 Se 0.02 ) ∑11.00 . The crystal structure was investigated and evaluated in terms of the order–disorder (OD) theory. Boulangerite belongs to the subcategory Ia of OD structures composed of equivalent layers with symmetry P ( n )2 1 m . Two polytypes with maximum degree of order (MDO) are possible: MDO1 with space group P 12 1 / a 1 and unit-cell parameters a 1 ≈ 21.61, b 1 ≈ 23.54, c 1 ≈ 8.05 A, β 1 ≈ 100.7°; MDO2 with space-group P 2 1 / n 11 and unit-cell parameters a 2 ≈ 21.24, b 2 ≈ 23.54, c 2 ≈ 8.05 A, α 2 ≈ 90°. Single-crystal X-ray diffraction patterns (Mo K α, CCD detector) show strong reflections pointing to an orthorhombic substructure (the “family structure” in the OD terminology) and additional weaker reflections that correspond to the polytype MDO 1 . Frequent twinning with (100) as the twin plane was observed. The MDO1 structure was refined on a twinned crystal to R = 0.062 for 8495 reflections with F o > 4σ( F o ). Unit-cell parameters are a 21.554(4), b 23.454(4), c 8.079(2) A, β 100.76(1)°. The structure of boulangerite is composed of rods of SnS archetype six atomic layers thick and three pyramids wide. The central portion of these thick rods is characterized by ribbons of coordination pyramids of Pb and Sb atoms alternating along [001]; the marginal portions contain ribbons of coordination pyramids primarily occupied by Sb. In the refined structure, there are 18 independent cation sites: 10 are pure lead sites, six are pure antimony sites, two mixed positions split into two close sites occupied by Pb and Sb, respectively. On the basis of an OD interpretation, we assess the relationships between monoclinic and orthorhombic structures reported in the literature for boulangerite.