Crystal structure of a birefringent andradite-grossular from Crowsnest Pass, Alberta, Canada

The structure of a birefringent andradite grossular sample was refined using single-crystal X-ray diffraction (SCD) and synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Electron-microprobe results indicate a homogeneous composition of {Ca2.88Mn0.062+Mg0.04Fe0.032+}(Sigma 3) [Fe1.293+Al0.49Ti0.174+Fe0.062+] (Sigma 2)(Si2.89Al0.11) O-Sigma 3(12)" The Rietveld refinement reduced chi(2)= 1.384 and overall R (F-2) = 0.0315. The HRPXRD data show that the sample contains three phases. For phase-1, the weight %, unit-cell parameter (A), distances (A), and site occupancy factor (sof) are 62.85(7)%, a= 12.000 06(2), average < Ca-0 > = 2.4196, Fe-0 = 1.9882(5), Si-0 = 1.6542(6) A, Ca(sop= 0.970(2), Fe(sof)= 0.763(1), and Si(sof)= 0.954(2). The corresponding data for phase-2 are 19.14(9)%, a= 12.049 51(2), average < Ca-0 > = 2.427, Fe-0 = 1.999(1), Si-0 = 1.665(1) A, Ca(sof) = 0.928(4), Fe(sof)= 0.825(3), and Si(sof). 0.964(4). The corresponding data for phase-3 are 18.01(9)%, a= 12.019 68(3), average < Ca-0 > = 2.424, Fe-0 = 1.992(2), Si-0 = 1.658(2) A, Ca(sop= 0.896(5), Fe(sof). 0.754(4), and Si(sof). 0.936(5). The fine-scale coexistence of the three phases causes strain that arises from the unit-cell and bond distances differences, and gives rise to strain-induced birefringence. The results from the SCD are similar to the dominant phase-1 obtained by the HRPXRD, but the SCD misses the minor phases. (c) 2013 International Centre for Diffraction Data.