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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 (λ and σ 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 Å, and a ≈ 22.8, b ≈ 8.4, c ≈ 54 Å, and space group symmetries F2/d11 and A2/d11, respectively. Only the former polytype (MDO1) could be identified in the analyzed crystals. The MDO 1  structure was solved and refined to R = 6.26% for 4968 reflections with Fo &gt; 4σ(Fo). In the standard C2/c setting of the space group the unit-cell parameters of MDO 1  are a = 8.371(2), b = 45.502(9), c = 27.273(6) Å, β = 98.83(3)°, V = 10265(4) Å 3 , 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, Pb 20 Sn 2 (Bi,As) 22 S 54 Cl 6 : OD-character, polytypism, twinning, and modular description