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) (Ca_2.997Mg_0.003)_Σ3{Al_1.794Fe ³⁺_0.196 Cr ³⁺_0.004 Mn ³⁺_0.003 Ti ⁴⁺_0.002 }_Σ2[(SiO₄)_2.851(O₄H₄)_0.151]_Σ3 and (2) (Ca_2.993Mg_0.007)_Σ3{Al_1.977Fe ³⁺_0.020 Mn ³⁺_0.003 Cr ³⁺_0.001 }_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 (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, 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.