Crystal chemistry of birefringent hydrogrossular

Crystal structure refinements of two fine-grained, massive, birefringent hydrogarnet samples from South Africa [1. green “jade” and 2. pink “jade”] were carried out with the Rietveld method, cubic space group I a 3 ¯ d , and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Electron-microprobe analysis (EMPA) gave bulk compositions as follows: (1) (Ca 2.997 Mg 0.003 ) Σ3 {Al 1.794 Fe 0.196 3+ Cr 0.004 3+ Mn 0.003 3+ Ti 0.002 4+ } Σ2 [(SiO 4 ) 2.851 (O 4 H 4 ) 0.151 ] Σ3 and (2) (Ca 2.993 Mg 0.007 ) Σ3 {Al 1.977 Fe 0.020 3+ Mn 0.003 3+ Cr 0.001 3+ } Σ2 [(SiO 4 ) 2.272 (O 4 H 4 ) 0.730 ] Σ3 . Their crystal structure was modeled well as indicated by the Rietveld refinement statistical indicators where the reduced χ 2 and overall R ( F 2 ) values are 1.133 and 0.0467, respectively, for sample 1 and 1.308 and 0.0342 for sample 2. Two cubic phases are contained in each sample. For phase 1a in sample 1, the weight fraction (%), unit-cell parameter (Å), and O–H bond distance (Å) are as follows: 74.4(1), a = 11.88874(4), and O–H = 0.98(9); the corresponding data for phase 1b are 25.6(1), a = 11.9280(5), and O–H = 0.91(9). For phase 2a in sample 2, the corresponding data are 52.0(1), a = 12.0591(1), and O–H = 0.90(6); the corresponding data for phase 2b are 48.0(1), a = 11.9340(2), and O–H = 0.90(7). The anisotropic displacement ellipsoids for the O atoms show no unusual features and are not elongated along the “Si–O” bond direction, which is written as Z–O, because of the general formula, X 3 Y 2 Z 3 O 12 , for garnet. Phase 1a is near end-member grossular, ideally Ca 3 Al 2 Si 3 O 12 . The deficiencies of the site occupancy factors ( sofs ) for the Si (=Z) site indicate that there are significant [O 4 H 4 ] 4− replacing [SiO 4 ] 4− . The Z–O distance is large in phase 1b, phases 2a, and 2b compared to a typical Z–O distance in anhydrous grossular or phase 1a. The H atoms occur in different environments around the vacant Z site in the two samples, and they may also bond to the O atoms surrounding the X and Y sites, if they contain vacancies as indicated by the refinement sofs . Two cubic phases are intergrown in each sample and cause strain that arise from structural mismatch and give rise to strain-induced birefringence in hydrogrossular.