Order-disorder, polytypes and twinning in the crystal structure of vurroite

Vurroite, ideally Pb20Sn2(Bi,As)22S54Cl6, is a complex mineral type, where the two minor chemical components, Sn and Cl, act as essential constituents together with Pb, Bi, As and S 1, 2. X-ray single crystal data on vurroite strongly indicate an orthorhombic Fcentred symmetry 1, 3. In this study the crystal structure of vurroite is interpreted as an OD structure belonging to the category III of OD structures composed of equivalent layers 4. The application of the OD procedures allowed the derivation of the OD-groupoid family (λ and σ operations), as well as the MDO (Maximum Degree of Order) structures. The layer symmetry (λ) is A(2)mm, the interlayer symmetry (σ) consists of a glide plane n1/2,1/2 and two-fold screw axes parallel to 010 and 001 with the translation components ¼ b and ¼ c, respectively. For this OD family two MDO polytypes exists. The former has monoclinic symmetry, C12/c1, whereas the latter is monoclinic, P12/c1. The OD treatment of the crystal structure of vurroite allowed to prove that the true symmetry of this mineral is monoclinic and that the apparent orthorhombic symmetry observed for the X-ray pattern of the measured crystal is due to a twinning phenomenon. Prof. S. Merlino is gratefully acknowledged for his contribution on the OD interpretation of the structure of vurroite. 1 Pinto D., PhD thesis, University of Bari, 2004, 165. 2 Garavelli A.,Mozgova N.N., Orlandi P., Bonaccorsi E., Pinto D., Borodaev Y., Canadian Mineralogist, 2005, in press. 3 Pinto D., Balić-Žunić T., Bonaccorsi E., Makovicky E., 26th Nordic Geological Winter Meeting, Uppsala, Sweden,2004, 106-107, abstract. 4 Dornberger-Schiff K., Abh. Dtsch. Akad. Wiss. Berlin. Kl. Chem. Geol. Biol., 1964, 3, 1-107