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

Donald R. Peacor, Roland C. Rouse and Edward S. Grew
Crystal structure of boralsilite and its relation to a family of boroaluminosilicates, sillimanite, and andalusite
American Mineralogist (August 1999) 84 (7-8): 1152-1161

Abstract

An untwinned sample of boralsilite from the Larsemann Hills, Antarctica was used for measurement of intensity data. Boralsilite, Al (sub 16) B (sub 6) Si (sub 2) O (sub 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 (sub 18) B (sub 4) O (sub 33) ), and werdingite. These structures are all based on a backbone of chains of edge-sharing AlO (sub 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 (sub 6) octahedral chains are cross-linked by Si (sub 2) O (sub 7) disilicate groups, BO (sub 4) tetrahedra, BO (sub 3) triangles, and AlO (sub 5) trigonal bipyramids. A given BO (sub 4) or SiO (sub 4) tetrahedron or BO (sub 3) triangle shares two vertices with two adjacent AlO (sub 6) octahedra of one chain and a third vertex with an octahedron vertex of an adjacent chain, thus cross-linking the AlO (sub 6) octahedral chains. Further link-age is provided through vertex-sharing of AlO (sub 5) trigonal bipyramids. These bipyramids alternate with B or Si polyhedra parallel to AlO (sub 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 (sub 6) octahedra can alternatively be viewed as consisting of Si (sub 2) O (sub 7) dimers, trimers of edge-sharing AlO (sub 5) trigonal bipyramids (plus a B triangle and B tetrahedron), and dimers of edge-sharing AlO (sub 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 (sub 18) B (sub 4) O (sub 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.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 84
Serial Issue: 7-8
Title: Crystal structure of boralsilite and its relation to a family of boroaluminosilicates, sillimanite, and andalusite
Affiliation: University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States
Pages: 1152-1161
Published: 199908
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 17
Accession Number: 1999-066022
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. tables
S69°30'00" - S69°30'00", E76°19'60" - E76°19'60"
Secondary Affiliation: University of Maine, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 199922
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