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

• Published in: Contributions to mineralogy and petrology 2015-02, Vol.169 (2), p.1, Article 10
• Main Authors: Antao, Sytle M., Zaman, Mashrur, Gontijo, Vitor L., Camargo, Eric S., Marr, Robert A.
• Format: Article
• Language: English
• Subjects: Anisotropy; Chemistry; Crystal structure; Crystals; Diffraction; Earth and Environmental Science; Earth Sciences; Geology; Mineral Resources; Mineralogy; Original Paper; Petrology; Structure; X-ray diffraction; X-rays
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-015-1104-0

The crystal structure of two zoned birefringent andradite garnets, ideally Ca 3 Fe 2 3+ Si 3 O 12 , from (1) Black Lake, Quebec (QC) and (2) Willsboro, New York (NY), was refined with the Rietveld method, space group I a 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) Ca 3 [Fe 1.92 3+ Al 0.07 Mn 0.01 3+ Mg 0.01 ] Σ2 Si 3 O 12 , Adr 96 Grs 3 and (2) {Ca 2.99 Mg 0.01 Mn 0.01 2+ } Σ3 [Fe 1.52 3+ Al 0.47 Ti 0.01 ] Σ2 (Si 2.98 Al 0.02 ) Σ3 O 12 , Adr 76 Grs 23 . 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 Adr 91 Grs 9 . For the NY sample-2, phase-2a is Adr 76 Grs 23 and phase-2b is Adr 59 Grs 39 . The reduced χ 2 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 Δ a  = 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 Δ a  = 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.