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Non-ideal solid-solution in garnet; crystal-structure evidence and modelling

Luciano Ungaretti, Marco Leona, Marcello Merli and Roberta Oberti
Non-ideal solid-solution in garnet; crystal-structure evidence and modelling
European Journal of Mineralogy (December 1995) 7 (6): 1299-1312

Abstract

Crystal-chemical characterization of 281 garnets has allowed distortion of the X, Y and Z polyhedra to be examined as a function of chemical composition, and has clarified the reasons for compositional preferences in nature. Pyralspites and ugrandites are separated by a significant composition gap (1.5-2.1 Ca pfu). Most structural parameters within the pyralspite-grossular solid solutions show non-linear variations; thus, the same X cation shows a different bonding behaviour, depending on the garnet structure (pyralspitic or ugranditic) in which it occurs. The unit-cell edge and the fractional coordinates of the O atom have been modelled along the Pyr-Alm, Alm-Gro and Pyr-Gro joins. Positive excess volumes of mixing (Delta V) are present along all three solid solutions; the Delta V value increases, and its compositional dependence is progressively more symmetric, with increasing size difference between the X cation components. The maximum calculated Delta V of mixing along the Pyr-Gro join ( approximately 7.5 Aa (super 3) per unit cell) is mainly due to the excess volume increase of the XO (sub 8) polyhedron with increasing Ca substitution ( approximately 5.5 Aa (super 3) per unit cell). The YO (sub 6) octahedron also shows an excess volume, but its contribution to the Delta I>V of mixing is just enough to compensate for the negative Delta I>V shown by the ZO (sub 4) tetrahedron ( approximately 0.3 Aa (super 3) per unit cell).


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 7
Serial Issue: 6
Title: Non-ideal solid-solution in garnet; crystal-structure evidence and modelling
Affiliation: Universita di Pavia, Dipartimento di Scienze della Terra, Pavia, Italy
Pages: 1299-1312
Published: 199512
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 35
Accession Number: 1996-014802
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: CNR, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United KingdomTwickenhamUKUnited Kingdom
Update Code: 199606
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