Cation ordering of {110} and {211} sectors in grandite garnet from Mali

摘要

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) method The chemical compositions of the {110}_(c) and {211}_(c) sectors were determined as (Ca_(2.96)Mg_(0.04)Mn_(0.04))_(Σ3.04)(Fe_(0.97)Al_(0.95)Ti_(0.11))_(Σ2.03)Si_(2.93)O_(12), and (Ca_(2.91)Mg_(0.05)Mn_(0.03))_(Σ2.99)(Fe_(1.01)Al_(0.90)Ti_(0.14))_(Σ2.05)Si_(2.93)O_(12), respectively. Reflections that violate the extinction rule for the space group Ia 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) ?, 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) ?, 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 R 1 = 3.02 and 3.36%, respectively. There were significant differences in the Fe~(3+) distribution trend in the Y sites between the {110}_(c) and {211}_(c) sectors. The Fe~(3+) occupancies of the eight Y sites in the {110}_(c) sector were 66.9(8), 66.0(8), 63.2(8), 60.0(8), 49.8(8), 49.5(8), 47.8(8), and 46.7(8)%. The octahedral Y sites of {110}_(c) sector can be divided into two groups. In terms of symmetry, Fe~(3+) distribution in the Y site of the {110}_(c) sector appeared to be pseudo–orthorhombic. In contrast, the Fe~(3+) distributions in the eight Y sites of the {211}_(c) sector as follows: 60.0(10), 41.9(10), 62.7(10), 69.1(10), 55.5(10), 53.2(10), 55.7(10), and 43.5(10)%. 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.