Three cubic phases intergrown in a birefringent andradite-grossular garnet and their implications

Abstract

The crystal chemistry across the garnet series is examined, and several systematic trends are reported. The crystal structure of three different cubic phases intergrown in a birefringent near end-member andradite from Namibia was refined by the Rietveld method, space group $Ia\bar{3}d,$ and monochromatic synchrotron high-resolution powder X-ray diffraction data. Electron microprobe results indicate three phases with distinct compositions. The sample is birefringent, indicating that it is not cubic when observed optically. The reduced χ ² and overall R (F ²) Rietveld refinement values are 1.655 and 0.0284, respectively, so the multi-phase refinement is excellent. The composition, weight %, unit-cell parameter (Å), distances (Å), and site-occupancy factors (sofs) are as follows: phase-1, Adr₉₉, 88.5(1)  %, a = 12.06259(1), average 〈Ca–O〉 = 2.4310, Fe–O = 2.0189(4), Si–O = 1.6490(4) Å, Ca(sof) = 0.948(1), Fe(sof) = 0.934(1), and Si(sof) = 0.940(1). For phase-2: Adr₇₁Grs₂₈, 7.1(1) %, a = 12.00361(5), average 〈Ca–O〉 = 2.440, Fe–O = 1.979(3), Si–O = 1.641(3) Å, Ca(sof) = 0.913(5), Fe(sof) = 0.767(4), and Si(sof) = 0.932(5). For phase-3: Grs₇₉Adr₁₇, 4.4(1) %, a = 11.89719(4), average 〈Ca–O〉 = 2.404, Al–O = 1.935(4), Si–O = 1.667(3) Å, Ca(sof) = 0.944(6), Al(sof) = 1.069(7), and Si(sof) = 0.887(5). The dominant phase-1 (89 %; Adr₉₉) is nearly end-member andradite, Ca₃Fe ³⁺₂ Si₃O₁₂, which contains no cation order in the Ca(X) or Fe(Y) sites. The intergrowth of the three cubic phases causes considerable strain in the minor phases-2 and phases-3 that arise from different structural parameters and gives rise to strain-induced birefringence. For comparison, the results for an isotropic, single-phase, grossular–andradite garnet (Grs₇₆Adr₂₁) are also presented. The strain in the minor phases is about 3–5 times more than the unstrained dominant phase-1, or the unstrained single-phase grossular–andradite.