Crystal structure of boralsilite and its relation to a family of boroaluminosilicates, sillimanite, and andalusite

Abstract

Boralsilite, Al16B6Si2O37, is monoclinic, space group C2/m, with a = 14.767(1), b = 5.574(1), c = 15.079(1) Å, β = 91.96(1)°, and Z = 2. The structure was solved with direct methods and refined to an unweighted residual of 0.026 using 1193 observed reflections. The structure is closely related to those of sillimanite, andalusite, grandidierite, synthetic aluminum borate (Al18B4O33), and werdingite. These structures are all based on a backbone of chains of edge-sharing AlO6 octahedra arranged parallel to c (κ 5.6 Å) and at the vertices and center of a pseudo-tetragonal subcell having a κ b κ 7.5 Å. In the boralsilite structure, AlO6 octahedral chains are cross-linked by Si2O7 disilicate groups, BO4 tetrahedra, BO3 triangles, and AlO5 trigonal bipyramids. A given BO4 or SiO4 tetrahedron or BO3 triangle shares two vertices with two adjacent AlO6 octahedra of one chain and a third vertex with an octahedron vertex of an adjacent chain, thus cross-linking the AlO6 octahedral chains. Further linkage is provided through vertex-sharing of AlO5 trigonal bipyramids. These bipyramids alternate with B or Si polyhedra parallel to AlO6 octahedral chains to form four kinds of cross-linking chains of polyhedra, with alternate atom pairs ([5])Al1-Si, ([5])Al2-([4])B2, ([5])Al3-([3])B1, and ([5])Al4-([3])B3. The units which cross-link between chains of AlO6 octahedra can alternatively be viewed as consisting of Si2O7 dimers, trimers of edge-sharing AlO5 trigonal bipyramids (plus a B triangle and B tetrahedron), and dimers of edge-sharing AlO5 trigonal bipyramids (plus B triangles and tetrahedra), Variations on these themes are found in the structures of sillimanite, andalusite, grandidierite, werdingite, mullite, and synthetic Al18B4O33. The interchangeability and variety of the various interchain units appears to result in part from the flexibility produced by the ability of Al and B to assume a variety of coordinations by oxygen and from the potential for partial vacancy of some anion and cation sites.