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_1.93Al_0.09)_Σ2.02Si_2.99O₁₂. 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 Ia3d were observed and indicated that all glide planes are absent. Triclinic symmetry with the space group I1 should be suitable for the structure refinement. The refinement with a I1 model was convergent with R1 = 3.19%. The Fe³⁺ occupancies of the Y11, Y12, Y13, Y14, Y21, Y22, Y23, and Y24 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 Fe³⁺ occupancy of the Y11 site is relatively lower, and that of the Y24 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 Fe–rich 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.