Crystal Chemistry of Birefringent Uvarovite Solid Solutions

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, $Ia\overline{3}d$ . 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.