Abstract

An untwinned sample of boralsilite from the Larsemann Hills, Antarctica was used for measurement of intensity data. Boralsilite, Al 16 B 6 Si 2 O 37 , is monoclinic, space group C2/m, with a = 14.767(1), b = 5.574(1), c = 15.079(1) Aa, beta = 91.96(1) degrees , 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 (Al 18 B 4 O 33 ), and werdingite. These structures are all based on a backbone of chains of edge-sharing AlO 6 octahedra arranged parallel to c ( congruent to 5.6 Aa) and at the vertices and center of a pseudo-tetragonal subcell having a congruent to b congruent to 7.5 Aa. In the boralsilite structure, AlO 6 octahedral chains are cross-linked by Si 2 O 7 disilicate groups, BO 4 tetrahedra, BO 3 triangles, and AlO 5 trigonal bipyramids. A given BO 4 or SiO 4 tetrahedron or BO 3 triangle shares two vertices with two adjacent AlO 6 octahedra of one chain and a third vertex with an octahedron vertex of an adjacent chain, thus cross-linking the AlO 6 octahedral chains. Further link-age is provided through vertex-sharing of AlO 5 trigonal bipyramids. 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 (super [5]) Al1-Si, (super [5]) Al2- (super [4]) B2, (super [5]) Al3- (super [3]) B1, and (super [5]) Al4- (super [3]) B3. The units which cross-link between chains of AlO 6 octahedra can alternatively be viewed as consisting of Si 2 O 7 dimers, trimers of edge-sharing AlO 5 trigonal bipyramids (plus a B triangle and B tetrahedron), and dimers of edge-sharing AlO 5 trigonal bipyramids (plus B triangles and tetrahedra), Variations on these themes are found in the structures of sillimanite, andalusite, grandidierite, werdingite, mullite, and synthetic Al 18 B 4 O 33 . The interchangeability and variety of the various interchain units appears to result in part from the flexibility produced by the ability of A1 and B to assume a variety of coordinations by oxygen and from the potential for partial vacancy of some anion and cation sites.

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