The crystal structures of grossular and spessartine between 100 and 600 K and the crystal chemistry of grossular-spessartine solid solutions.

Rodehorst, U., Geiger, C. A. and Armbruster, T. (2002) The crystal structures of grossular and spessartine between 100 and 600 K and the crystal chemistry of grossular-spessartine solid solutions. American Mineralogist, 87 (4). pp. 542-549.

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Abstract

Spessartine (Mn3Al2Si3O12)-grossular (Ca3Al2Si3O12) solid solutions were synthesized at high pressures and temperatures. Compositionally homogeneous garnets are obtained by crystallizing solid-solution glasses prepared from oxides. The unit-cell parameter. a, for the different solid solutions was determined by X-ray powder diffraction methods and the results give positive deviations from ideal volumes of mixing that can be described with a symmetric mixing model with W-v = 0.80 (+/-0.04) cm(3)/mol. The degree of non-ideality is a function of the difference in size between the Ca2+ and Mn2+ cations and is consistent with the range of those observed for the other aluminosilicate garnet binary joins. The crystal structures of synthetic grossular and spessartine were collected at 50 K intervals between 103 K and 498/648 K using single-crystal X-ray diffraction methods. The rotation of the rigid SiO4 tetrahedra changes slightly by 0.3(1)degrees for grossular and 0.2(1)degrees for spessartine between 103 and 648 K. The volume expansions of the polyhedra were calculated and their distortions in grossular and spessartine were analyzed as a function of temperature. The linear thermal expansion coefficients of the Al-O and two X-O bond were also calculated for almandine, pyrope, grossular, and spessartine. The thermal expansion of spessartine is similar to that of grossular. In terms of polyhedral distortion and bond-valence values, spessartine has the most ideal structure of the aluminosilicate garnets. This could explain its large P-T stability field and the ease of synthesis at low pressures.

Document Type: Article
Keywords: x-ray-diffraction aluminosilicate garnets mixing properties pyrope almandine volumes strain
Research affiliation: Kiel University
ISSN: 0003-004X
Date Deposited: 02 Feb 2012 07:07
Last Modified: 23 Sep 2019 23:44
URI: http://oceanrep.geomar.de/id/eprint/16255

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