Refinement of the crystal structure of aminoffite

Aminoffite, (Ca2.95Mn0.08Pb0.01)(BeOH)(2)(Si2.95As0.03)O-10, is tetragonal, P4(2)/n, a 9.809(1), c 9.844(2) Angstrom, V 947.3(3) Angstrom(3), Z = 12. The structure was refined to an R index of 2.9% based on 1265 observed [\F-o\ > 5sigmaF] reflections measured with a four-circle single-crystal diffractometer and MoKalpha X-radiation. There are two Si sites and one Be site in the structure of aminoffite, each coordinated by four anions in a tetrahedral arrangement. There are two distinct Ca sites, coordinated by six O-atoms, and one and two (011) groups, respectively, with <Ca-O> distances of 2.467 and 2.436 Angstrom. The single H-atom is part of an (OH) group that is part of the (Bephi(4)) (phi: unspecified anion) tetrahedron. The (SiO4) groups link to form linear [Si3O10] trimers that are linked by (Bephi(4)) groups to form a sheet of tetrahedra parallel to (001). These sheets stack in the [001] direction and are linked together by interstitial Ca atoms and by hydrogen bonding. Details of the sheet topologies in Be minerals with two-dimensional structural units are examined; (Bephi(4)) tetrahedra show a marked preference for trivalent vertices in two-dimensional nets. Bond-valence considerations show that such (Bephi(4)) tetrahedra must have a monovalent anion [usually (OH)] at one vertex, and hence most such structures have acid {Be-3(OH)} groups.