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 Ca3Fe23+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[Fe1.923+Al0.07Mn0.013+Mg0.01]Σ2Si3O12, Adr96Grs3 and (2) {Ca2.99Mg0.01Mn0.012+}Σ3[Fe1.523+Al0.47Ti0.01]Σ2(Si2.98Al0.02)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 (F2) 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) A, and those for phase-1b are 33.0(1) % and a = 12.0535(1) A with Δa = 0.007 A. For the NY sample, the corresponding values for phase-2a are 81.3(1) % and a = 12.00456(1) A, and those for phase-2b are 18.7(1) % and a = 11.96687(1) A with Δa = 0.038 A. 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.