Cation ordering in iridescent garnet from Tenkawa village, Nara prefecture, Japan

Abstract

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.