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Title: 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 {Ca2.88Mn2+ 0.06Mg0.04Fe2+ 0.03}Σ3[Fe3+ 1.29Al0.49Ti4+ 0.17Fe2+ 0.06] Σ2(Si2.89Al0.11) Σ3O12. The Rietveld refinement reduced χ2 = 1.384 and overall R (F2) = 0.0315. The HRPXRD data show that the sample contains three phases. For phase-1, the weight %, unit-cell parameter (Å), distances (Å), and site occupancy factor (sof) are 62.85(7)%, a = 12.000 06(2), average = 2.4196, Fe–O = 1.9882(5), Si–O = 1.6542(6) Å, Ca(sof) = 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 = 2.427, Fe–O = 1.999(1), Si–O = 1.665(1) Å, 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 = 2.424, Fe–O = 1.992(2), Si–O = 1.658(2) Å, Ca(sof) = 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 bymore » the HRPXRD, but the SCD misses the minor phases.« less
Authors:
;  [1]
  1. Calgary
Publication Date:
OSTI Identifier:
1121451
Resource Type:
Journal Article
Resource Relation:
Journal Name: Powder Diffraction; Journal Volume: 29; Journal Issue: 1
Research Org:
Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
Sponsoring Org:
FOREIGN
Country of Publication:
United States
Language:
ENGLISH