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

Boralsilite, Al16B6Si2O37, is monoclinic, space group C2/m, with a = 14.767 (1), b = 5.574(1), c = 15.079(1) Angstrom, beta = 91.96(1)degrees, and Z = 2. The structure was solved with direct methods and refined to an unweighte d residual of 0.026 using 1193 observed reflections. The structure is close ly related to those of sillimanite, andalusite, grandidierite, synthetic al uminum berate (Al18B4O33), and werdingite. These structures are all based o n a backbone of chains of edge-sharing AlO6 octahedra arranged parallel to c (congruent to 5.6 Angstrom) and at the vertices and center of a pseudo-te tragonal subcell having a congruent to b congruent to 7.5 Angstrom. In the boralsilite structure, AlO6 octahedral chains are cross-linked by Si2O7 dis ilicate 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 octahed ron vertex of an adjacent chain, thus cross-linking the AlO6 octahedral cha ins. Further linkage is provided through vertex-sharing of AlO5 trigonal bi pyramids. These bipyramids alternate with B or Si polyhedra parallel to AlO 6 octahedral chains to form four kinds of cross-linking chains of polyhedra , with alternate atom pairs ([5])A11-Si, ([5])A12-([4])B2, ([5])A13-([3])B1 , and ([5])A14-([3])B3. The units which cross-link between chains of AlO6 o ctahedra can alternatively be viewed as consisting of Si2O7 dimers, trimers of edge-sharing AlO5 trigonal bipyramids (plus a B triangle and B tetrahed ron), 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 un its 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 pot ential for partial vacancy of some anion and cation sites.