Abstract
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.
Highlights
Several studies have documented birefringent garnets with lamellar or oscillatory features, which were referred to as “chemical zoning” instead of separate phases, e.g., [1,2,3,4]
This study examines the crystal chemistry of five optically anisotropic uvarovite samples from different localities
Uvarovite garnets have cubic symmetry and can exist as a single-phase or an intergrowth of a few cubic uvarovite phases, which occurs as epitaxial growth features (“chemical zoning”) instead of exsolution (e.g., Figure 3b,c)
Summary
Several studies have documented birefringent garnets with lamellar or oscillatory features, which were referred to as “chemical zoning” instead of separate phases, e.g., [1,2,3,4]. Such birefringent garnets appear to contain a few cubic garnet phases that grow as oscillatory zoning or by re-sorption and re-precipitation that gives rise to patchy features, e.g., [5,6,7]. Several reasons were given for the birefringence, but the main one appears to be cation orders in the X and Y sites that cause a symmetry reduction, e.g., [1,10,14,15]. Other suggested reasons for the birefringence in garnet were discussed and are not repeated here [16]
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