The cation ordering of the {110} c and {211} c growth sectors in birefringent garnet from Kayas, Mali was investigated by means of single-crystal X-ray diffraction (SC-XRD) 3d were observed, and crystal structure models for both the {110} c and {211} c sectors should have triclinic symmetry with the space group I 1. The unit-cell parameters of both growth sectors were a = 11.9739(13), b = 11.9742(14), c = 11.9737(13) Å, α = 90.001(9), β = 90.098(9) and γ = 90.014 (10)°for the {110} c sector; a = 11.952(6), b = 11.952(6), c = 11.957(6) Å, α = 90.06(4), β = 90.01(4) and γ = 89.94(4)°for the {211} c sector. Structure refinements for I 1 models of both sectors were convergent with R1 = 3.02 and 3.36%, respectively. There were significant differences in the Fe 3+ occupancies in the Y sites of the {211} c sector were random and different from that of the {110} c sector. Different cation distributions between the growth sectors in the Mali garnet indicate that cation ordering in the crystal structure was formed during growth. These observations suggest that the differences in the Fe 3+ distribution between the {110} c and {211} c sectors are strongly affected by the growth process in each sector.
The crystal structure of iridescent garnet from Tenkawa village, Nara prefecture, Japan was investigated by means of Single-crystal X-ray diffraction method. The garnet crystal is composed of {110} c and {211} c growth sectors and shows birefringence under the crossed polarizer. The BSE image of the cross section shows very fine lamellae texture consisting of andradite-rich and grossular-rich layers. The average chemical composition was determined as Ca 3.01 (Fe 1.93 Al 0.09 ) Σ2.02 Si 2.99 O 12 . The unit-cell parameters were obtained as follows; a = 12.0540(17), b = 12.0580(13), c = 12.0612(13) Å, α = 89.986(9), β = 89.994(10) and γ = 89.994 (10)°. Some reflections which violate the extinction rule for the space group Ia 3d were observed and indicated that all glide planes are absent. Triclinic symmetry with the space group I 1 should be suitable for the structure refinement. The refinement with a I 1 model was convergent with R1 = 3.19%. The Fe 3+ occupancies of the Y11, Y12, Y13, Y14, Y 21, Y22, Y 23, and Y 24 were determined as 86.9(6), 96.1(6), 90.9(6), 98.0(6), 95.1(6), 91.3(6), 93.7(6), and 99.6(6)%, respectively. The Fe 3+ occupancy of the Y11 site is relatively lower, and that of the Y 24 site is higher. The iridescent garnet from Tenkawa shows very fine lamellae texture consisted of Fe-rich and Al-rich layers, therefore, the refined crystal structure of the Tenkawa garnet should be a superimposed structure of these chemically distinct layers. The Fe-rich layer have almost pure andradite composition, so cation ordering in the Fe-rich layer is negligible. The Al-rich layer should have more obvious ordered cation distribution than the Ferich layer. The ordered cation distribution in the Al-rich layer should mainly contribute the cation ordering in the refined crystal structure of the Tenkawa garnet.
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