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Open AccessArticle

Crystal Chemistry of Birefringent Uvarovite Solid Solutions

Department of Geoscience, University of Calgary, Calgary, AB T2N 1N4, Canada
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Author to whom correspondence should be addressed.
Minerals 2019, 9(7), 395; https://doi.org/10.3390/min9070395
Received: 28 May 2019 / Revised: 17 June 2019 / Accepted: 27 June 2019 / Published: 28 June 2019
The crystal chemistry of five optically anisotropic uvarovite samples from different localities (California, Finland, Russia, and Switzerland) were studied with electron-probe microanalysis (EPMA) and the Rietveld method. Monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data were used, and Rietveld refinement was carried out with the cubic space group, I a 3 ¯ d . The general formula for garnet is [8]X3[6]Y2[4]Z3[4]O12. Uvarovite has the ideal formula, Ca3Cr2Si3O12, which may be written as Ca3{Cr,Al,Fe}Σ2[Si3O12] because of solid solutions. HRPXRD traces show multiple cubic garnet phases in each sample that has a heterogeneous chemical composition. The optical and back-scattered electron (BSE) images and elemental maps contain lamellar and concentric zoning as well as patchy intergrowths. With increasing a unit-cell parameter for uvarovite solid solutions, the Z–O distance remains constant, and the average <X–O> distance increases slightly in response to the Cr3+ ⇔ Al3+ cation substitution in the Y site. The Y–O distance increases most because Cr3+ (radius = 0.615 Å) is larger than Al3+ (radius = 0.545 Å) cations. The Fe3+ (radius = 0.645 Å) cation is also involved in this substitution. Structural mismatch between the cubic garnet phases in the samples gives rise to strain-induced optical anisotropy. View Full-Text
Keywords: uvarovite; grossular; andradite; garnet; optical anisotropy; intergrowths of multiple cubic phases; Rietveld refinements; Synchrotron high-resolution powder X-ray diffraction (HRPXRD); structure; chemistry uvarovite; grossular; andradite; garnet; optical anisotropy; intergrowths of multiple cubic phases; Rietveld refinements; Synchrotron high-resolution powder X-ray diffraction (HRPXRD); structure; chemistry
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Antao, S.M.; Salvador, J.J. Crystal Chemistry of Birefringent Uvarovite Solid Solutions. Minerals 2019, 9, 395.

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