Crystal chemistry of birefringent hydrogrossular

Abstract

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 $$ Ia\overline{3} d, $$ and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Electron-microprobe analysis (EMPA) gave bulk compositions as follows: (1) (Ca2.997Mg0.003)Σ3{Al1.794Fe 0.196 3+ Cr 0.004 3+ Mn 0.003 3+ Ti 0.002 4+ }Σ2[(SiO4)2.851(O4H4)0.151]Σ3 and (2) (Ca2.993Mg0.007)Σ3{Al1.977Fe 0.020 3+ Mn 0.003 3+ Cr 0.001 3+ }Σ2[(SiO4)2.272(O4H4)0.730]Σ3. Their crystal structure was modeled well as indicated by the Rietveld refinement statistical indicators where the reduced χ2 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 (A), and O–H bond distance (A) 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, X3Y2Z3O12, for garnet. Phase 1a is near end-member grossular, ideally Ca3Al2Si3O12. The deficiencies of the site occupancy factors (sofs) for the Si (=Z) site indicate that there are significant [O4H4]4− replacing [SiO4]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.