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The crystal structure of vurroite, Pb₂₀Sn₂(Bi, As)₂₂S₅₄Cl₆: OD-character, polytypism, twinning, and modular description

Daniela Pinto^1,*, Elena Bonaccorsi², Tonci Bali-uni³ and Emil Makovicky³

¹ Dipartimento Geomineralogico, University of Bari, Via E. Orabona 4, I-70125 Bari, Italy
² Dipartimento di Scienze della Terra, University of Pisa, Via Santa Maria 53, I-56126 Pisa, Italy
³ Department of Geography and Geology, University of Copenhagen, Østervoldgade 10, DK-1350 Copenhagen K, Denmark

Correspondence: ^* E-mail: d.pinto{at}geomin.uniba.it

The crystal structure of the type specimen of vurroite from Vulcano (Aeolian Islands, Italy) has been solved and refined using single-crystal X-ray diffraction data collected at the Elettra synchrotron facility (Basovizza, Trieste). Vurroite has an OD (order-disorder) structure belonging to the category III of OD structures composed of equivalent layers. The OD-groupoid family ( and partial operations) and MDO structures were derived by means of the application of the OD theory. The two theoretically possible polytypes with maximum degree of order (MDO polytypes) have pseudo-orthorhombic metric, with lattice parameters a 45.6, b 8.4, c 54 A, and a 22.8, b 8.4, c 54 A, and space group symmetries F2/d11 and A2/d¹¹, respectively. Only the former polytype (MDO₁) could be identified in the analyzed crystals. The MDO₁ structure was solved and refined to R = 6.26% for 4968 reflections with F_o > 4 (F_o). In the standard C2/c setting of the space group the unit-cell parameters of MDO₁ are a = 8.371(2), b = 45.502(9), c = 27.273(6) A, β = 98.83(3)°, V = 10265(4) A³, Z = 4. Frequent twinning with (001) as the twin plane, together with the occurrence of disordered domains in the structure, was observed.

The crystal structure of vurroite contains lozenge-shaped composite rods made of coordination polyhedra of Pb and Sn, interconnected into layers parallel to (010) of the standard monoclinic setting. These layers are separated by ribbons of As and Bi, each in distorted octahedral coordination. The ribbons form wavy, discontinuous double layers of PbS archetype. Lone electron pairs of As and Bi are accommodated in the central portions of the PbS-like layers. The structure of vurroite contains building blocks topologically similar to those found in the zinckenite group and in the structure of kirkiite. It can be considered a box-work structure containing the smallest possible pseudo-hexagonal block in the form of a sole octahedron.

Key Words: Vurroite • Pb-Bi-As sulfosalts • crystal structure • OD-theory • polytypism • twinning

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