Abstract:
The crystal structure of vasilyevite, ideally (Hg22+10 O6 I3 Br2 Cl (CO3), triclinic, a 9.344(2), b 10.653(2), c 18.265(4) Å, α 93.262(5), β 90.548(4), Υy 115.422(4)°, V 1638.3(9) Å3, space group P1̄, Z = 2, D(calc) = 9.57 g cm-3, has been solved by direct methods and refined to an R1, index of 8.9% for 3400 unique reflections measured with MoKα X-radiation on a four-circle diffractometer equipped with a CCD detector. There are twenty unique Hg sites that are associated as pairs of sites with pair separations of 2.533 to 2.593 Å; this association of Hg atoms is typical of Hg+ cations pairing to form (Hg2)2+ dimers. Each (Hg2)2+ dimer is extended at each end by bonding to axial O atoms with Hg-O distances in the range 2.00 to 2.32 Å, with one dimer showing a terminal I atom with a very short Hg-I distance of 2.75 Å. These [φ-Hg-Hg-φ] tetramers form near-linear arrangements that are surrounded by several meridional anions, O atoms at distances from 2.44 to 2.85 Å, mixed (Cl,Br) sites at distances from 2.89 to 3.78 Å. and I atoms at distances from 3.11 to 4.02 Å. Bonds to axial anions make angles of <23° with the Hg-Hg axis, involving the Hg atoms to which they are bonded; bonds to meridional anions make angles of <31° with the perpendicular to the Hg-Hg axis, involving the Hg atoms to which they are bonded. There is one site, labeled C,S, that is surrounded by a triangle of O atoms at a mean distance of 1.25 Å: the presence of S in the chemical composition and the refined scattering at the C,S site indicates that this site is occupied by both (CO3) and S2- in the ratio 0.85 : 0.15. There are three distinct I sites occupied solely by I- anions that are [8]- or [9]-coordinated by Hg cations. There are three X sites that are occupied by 0.73 Br + 0.27 Cl, 0.69 Br + 0.31 Cl, and 0.22 Br + 0.78 Cl and are [7]- and [8]-coordinated by Hg cations. The fundamental building block of the vasilyevite structure is the strongly bonded [φ-Hg-Hg-φ] tetramer. These groups link together at their anion vertices to form a complex interpenetrating framework of two discrete three- and four-connected nets. The halogen atoms and the (CO3) and S2- anions occur in the interstices of the two nets and serve to stabilize the nets via additional weak linkages to Hg atoms.
