Optical anisotropy, zoning, and coexistence of two cubic phases in andradites from Quebec and New York

Abstract

The crystal structure of two zoned birefringent andradite garnets, ideally Ca₃Fe₂ ³⁺Si₃O₁₂, from (1) Black Lake, Quebec (QC) and (2) Willsboro, New York (NY), was refined with the Rietveld method, space group $Ia\overline{3} d$, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Electron probe microanalyzer results gave bulk or average compositions as follows: (1) Ca₃[Fe ³⁺_1.92 Al_0.07Mn ³⁺_0.01 Mg_0.01]_Σ2Si₃O₁₂, Adr₉₆Grs₃ and (2) {Ca_2.99Mg_0.01Mn ²⁺_0.01 }_Σ3[Fe ³⁺_1.52 Al_0.47Ti_0.01]_Σ2(Si_2.98Al_0.02)_Σ3O₁₂, Adr₇₆Grs₂₃. The HRPXRD data show the coexistence of two cubic phases as intergrowths in both samples. Phase-1a of the QC sample-1 has a composition of near-end-member andradite, and phase-1b is Adr₉₁Grs₉. For the NY sample-2, phase-2a is Adr₇₆Grs₂₃ and phase-2b is Adr₅₉Grs₃₉. The reduced χ² and overall R (F ²) Rietveld refinement values are 1.074 and 0.0268 for the QC sample, and 1.172 and 0.0333 for the NY sample. For the QC sample, the weight percentage and unit-cell parameter for phase-1a are 67.0(1) % and a = 12.06077(1) Å, and those for phase-1b are 33.0(1) % and a = 12.0535(1) Å with Δa = 0.007 Å. For the NY sample, the corresponding values for phase-2a are 81.3(1) % and a = 12.00456(1) Å, and those for phase-2b are 18.7(1) % and a = 11.96687(1) Å with Δa = 0.038 Å. The two cubic phases are intergrown and cause strain that arises from structural mismatch and gives rise to strain-induced birefringence. The above two samples are compared to andradite samples from Arizona and Magnet Cove.