THE CRYSTAL STRUCTURE OF KRIVOVICHEVITE, Pb3[Al(OH)6](SO4)(OH)

Abstract

The crystal structure of krivovichevite, Pb 3 [Al(OH) 6 ](SO 4 )(OH), a new mineral species from hydrothermal deposits of the Mt. Lepkhe–Nelm, Lovozero alkaline massif, Kola Peninsula, in Russia, has been solved from single-crystal X-ray-diffraction data by direct methods and refined to R 1 = 0.034 for 681 unique reflections with | F o | ≥ 4σ F . The mineral is acentric trigonal, space group R 3 c , a 7.742(2), c 32.082(9) A, V 1665.2(7) A 3 , Z = 6. The structure of krivovichevite contains one symmetrically unique Pb position coordinated by four OH groups and three O atoms. The coordination of the Pb 2+ cation is highly asymmetrical owing to the localized 6 s 2 lone pairs of electrons. On one side, the Pb 2+ cation is coordinated by three OH groups that form rather short Pb 2+ –OH bonds (2.28–2.50 A). The resulting Pb(OH) 3 configuration is complemented by one intermediate Pb 2+ –O4 bond (2.76 A) and three longer Pb 2+ –φ bonds (φ: O, OH; 2.81–3.03 A). There is one Al 3+ cation in the structure, coordinated by six OH groups to form isolated Al(OH) 6 3− octahedra. One symmetrically independent S 6+ cation is coordinated by four O 2− anions to form SO 4 2− sulfate tetrahedra. The structure of krivovichevite is based upon layers of Pb 2+ cations and OH − groups ( A ) separated by layers of Al(OH) 6 3− octahedra and SO 4 2− tetrahedra ( B ). The A layer is a hexagonal array of Pb consisting of Pb 3 triangles, with every sixth triangle centered by the OH − group. The sheets are stacked in the sequence ... ABAB ... The stacking sequence of the Pb sheets corresponds to a hexagonal close-packing sequence … ababab ..., whereas the stacking sequence of the Al sulfate sheets is that of a cubic close-packing sequence ... abcabc ... Combination of these two stacking sequence leads to the 12-layer sequence ... A a B a A b B b A a B c A b B a A a B b A b B c ..., where M n ( M = A , B ; n = a , b , c ) corresponds to the M sheet in the n position. It seems likely that krivovichevite represents a metastable ephemeral phase that forms in the early stages of oxidation of galena prior to a transformation to stable plumbojarosite–hinsdalite-like phases.