Crystal structure of morimotoite from Ice River, Canada

摘要

The crystal structure of a morimotoite garnet, ideally Ca3(Ti~(4+)Fe~(2+))Si_3O_(12), from the Ice River alkaline complex, British Columbia, Canada was refined by the Rietveld method, space group Ia3-bard, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Electron-microp-robe analysis indicates a homogeneous sample with a formula {Ca_(2.91)Mg_(0.05)Mn_(0.03)~(2+)}_(∑3)[Ti_(1.09)Fe_(0.46)~(3+) Fe_(0.37)~(2+)Mg_(0.08)]_(∑2)(Si_(2.36)Fe_(0.51)~(3+)Al_(0.14))_(∑3)O_(12). The HRPXRD data show a two-phase intergrowth. The reduced X~2 and overall R(F~2) Rietveld refinement values are 1.572 and 0.0544, respectively. The weight percentage, unit-cell parameter (A), distances (A), and site occupancy factors (sofs) for phase-1 are as follows: 76.5(1)%, a = 12.156 98(1) A, average = 2.4383, Ti-O = 2.011(1), Si-O = 1.693(1) A, Ca(sof) = 0.943(2), Ti(sof) = 0.966(2), and Si(sof) = 1.095(3). The corresponding values for phase-2 are 23.5(1)%, a = 12.160 67(2) A, average = 2.452, Ti-O= 1.988(3), Si-O= 1.704(3) A, Ca(sof) = 1.063(7), Ti(sof) = 1.187(7), and Si(sof) = 1.220(8). The two phases cause strain that arises from structural mismatch and gives rise to low optical anisotropy. Because the two phases are structurally quite similar, a refinement using a single-phase model with anisotropic displacement parameters shows no unusual displacement ellipsoid for the O atom that requires a "split O-atom position", as was done in previous studies.