Two cubic phases in kimzeyite garnet from the type locality Magnet Cove, Arkansas

The crystal structure of an optically anisotropic kimzeyite garnet from Magnet Cove, Arkansas, USA, where it was first discovered, was refined with the Rietveld method, cubic space group, Ia\overline 3 d, and monochromatic [λ = 0.41422 (2) Å] synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. The Rietveld refinement reduced χ² and overall R(F²) values are 1.840 and 0.0647, respectively. The sample, with the general garnet formula ^[8]X₃^[6]Y₂^[4]Z₃^[4]O₁₂, contains an intergrowth of two cubic phases that occur initially as oscillatory growth zoning, and patchy intergrowths arise later from fluid-enhanced dissolution and re-precipitation. The two compositions obtained with electron-probe microanalyses (EPMA) are Ca_3.00(Zr_1.31Ti⁴⁺_0.46Fe³⁺_0.22Mn³⁺_0.01)_∑2[Al_0.76Fe³⁺_1.01Si_1.23]_∑3O₁₂ for phase 1a and Ca_2.99(Zr_1.48Ti⁴⁺_0.37Fe³⁺_0.15)_∑2[Al_0.87Fe³⁺_0.98Si_1.15]_∑3O₁₂ for phase 1b. The weight percentage, unit-cell parameter (Å), distances (Å), and site occupancy factors (s.o.f.s) for phase 1a are as follows: 42.6 (2)%, a = 12.46553 (3) Å, average 〈X-O〉 = 2.482, Y-O = 2.059 (2), Z-O = 1.761 (2) Å, Ca (X s.o.f.) = 0.960 (4), Zr (Y s.o.f.) = 0.809 (3), and Fe (Z s.o.f.) = 0.623 (2). The corresponding values for phase 1b are 57.4 (2)%, a = 12.47691 (2) Å, average 〈X-O〉 = 2.482, Y-O = 2.062 (1), Z-O = 1.762 (1) Å, Ca (X s.o.f.) = 0.957 (3), Zr (Y s.o.f.) = 0.828 (2) and Fe (Z s.o.f.) = 0.617 (2). The main structural differences between the two phases are in the unit-cell parameter, Δa = 0.01138 Å, Y(s.o.f.), and Y-O distance. Structural mismatch between the two cubic phases in a crystal gives rise to strain-induced optical anisotropy.