IS NEAR-ENDMEMBER BIREFRINGENT GROSSULAR NON-CUBIC? NEW EVIDENCE FROM SYNCHROTRON DIFFRACTION

Abstract

The crystal structures of three birefringent (samples 1 and 2 from Quebec and sample 3 from Tanzania) and one isotropic grossular (sample 4 from Afghanistan) were refined with the Rietveld method, cubic space group Ia 3 d, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Electron microprobe (EMPA) results indicate that the garnets are homogeneous with compositions as follows: (1) {Ca2.92Mn2+0.05Fe2+0.01}∑2.98[Al1.91Fe3+0.09Ti4+0.02]∑2.02(Si2.96Al0.04) ∑3O12, Grs94Adr3; (2) {Ca2.99}[Al1.65Cr3+0.22Fe3+0.05Ti4+0.05Mn3+0.04]∑2.01(Si2.94Al0.06)∑3O12, Grs82Uv11; (3) {Ca2.95Mg0.04Mn2+0.01} ∑3[Al1.04Fe3+0.86Ti4+0.07Mn3+0.01Mg0.01]∑1.99(Si2.94Al0.06)∑3O12, Grs51Adr43; and (4) {Ca2.95Mn2+0.01Fe2+0.01Mg0.01} ∑2.98[Al1.85Fe3+0.16Ti4+0.01]∑2.02(Si2.97Al0.03)∑3O12, Grs91Adr6. Samples 1 and 4 are near-endmember grossular. Two different cubic phases occur in the three birefringent samples, and a single cubic phase occurs in the isotropic sample 4. Their crystal structures are modelled well, as indicated by their Rietveld refinement reduced χ2 and corresponding overall R ( F 2) values for the four samples as follows: (1) 1.078 and 0.0368; (2) 1.081 and 0.0434; (3) 1.902 and 0.0361; and (4) 1.235 and 0.0491. The dominant phase has the following unit-cell parameters (A) and weight fractions (%) for the four samples: (1) a = 11.85140(1), 83.0(2); (2) a = 11.87707(2), 71.3(1); (3) a = 11.94161(1), 91.95(2); and (4) a = 11.85584(1) A, 100%. Their bond distances and site occupancy factors ( sofs ) are as follows: (1) = 2.4017, Al–O = 1.9269(4), Si–O = 1.6506(4) A; Ca( sof ) = 0.964(1), Al( sof ) = 0.954(1), and Si( sof ) = 0.917(1); (2) = 2.4057, Al–O = 1.9371(7), Si–O = 1.6508(6) A; Ca( sof ) = 0.953(1), Al( sof ) = 1.046(2), and Si( sof ) = 0.916(1); (3) = 2.4174, Al–O = 1.9675(4), Si–O = 1.6444(4) A; Ca( sof ) = 0.946(1), Al( sof ) = 1.355(2), and Si( sof ) = 0.932(1); and (4) = 2.4060, Al–O = 1.9292(4), Si–O = 1.6473(4) A; Ca( sof ) = 0.964(1), Al( sof ) = 0.994(1), and Si( sof ) = 0.925(1). The sofs are similar to those obtained by EMPA. The two cubic phases intergrown in the three birefringent samples cause strain that arise from mismatch of unit-cell and bond distances and give rise to strain-induced birefringence. Sample 4 is an isotropic single-phase grossular.