EDGE-SHARING MN2+O4 TETRAHEDRA IN THE STRUCTURE OF AKATOREITE, MN2+9AL2SI8O24(OH)8

The crystal structure of akatoreite, Mn92+Al2Si8O24(OH)8, a 8.337(2), b 10.367(2), c 7.629(1) angstrom, alpha 104.46(1), beta 93.81(2), gamma 104.18(1)degrees, V 613.2(2) angstrom3, P1BAR, Z = 1, has been solved by direct methods and refined to an R index of 2.9% for 2691 observed reflections measured with MoKalpha radiation. There are five independent Mn positions. Four of these are octahedrally coordinated by O2- and OH- anions, and the mean bond-lengths at these sites show the Mn to be entirely in the divalent state. The fifth Mn position is tetrahedrally coordinated by four O2- anions, and the mean bond-length shows the Mn to be in the divalent state. Adjacent (MnO4) tetrahedra share an edge to form an [Mn2O6] dimer. There is one Al position, coordinated by six anions in an octahedral arrangement, and both site-scattering refinement and mean bond-length show no substitution of Fe3+, or Mn3+ for Al at this site. There are four distinct Si positions, all of which are tetrahedrally coordinated; one of the silicate tetrahedra is an acid silicate group: SiO3(OH). The four Si tetrahedra form a linear [Si4O12(OH)] cluster, and akatoreite is thus a sorosilicate. The (Mnphi6) and (Alphi6) octahedra (phi: unspecified anion) form edge-sharing strips of octahedra, three octahedra wide, that extend along the a direction. These strips are cross-linked into sheets by the [Mn2O6] tetrahedral dimers, which share edges with the peripheral octahedra of adjacent strips. The resultant sheets are linked into a complex heteropolyhedral framework by the sorosilicate fragments and by a hydrogen-bond network. Relationships to other sorosilicates are reviewed.