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

Journal Article · Thu Feb 20 04:00:00 EST 2014 · Powder Diffraction

DOI:https://doi.org/10.1017/S0885715613001255· OSTI ID:1121451

Antao, Sytle M.; Klincker, Allison M. ^[1]

---

1. Calgary

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 {Ca_2.88Mn²⁺ _0.06Mg_0.04Fe²⁺ _0.03}_Σ3[Fe³⁺ _1.29Al_0.49Ti⁴⁺ 0.17Fe²⁺ _0.06] _Σ2(Si_2.89Al_0.11) _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 by the HRPXRD, but the SCD misses the minor phases.

• Research Organization: Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
• Sponsoring Organization: FOREIGN
• OSTI ID: 1121451
• Journal Information: Powder Diffraction, Journal Name: Powder Diffraction Journal Issue: 1 Vol. 29; ISSN 0885-7156
• Country of Publication: United States
• Language: ENGLISH