Cation ordering, twinning, and pseudo-symmetry in silicate garnet: The study of a birefringent garnet with orthorhombic structure

The crystal structure of a birefringent garnet (similar to Adr(53)Grs(47)) that occurs as a late-stage rim on andradite from Stanley Butte, Graham County, Arizona is analyzed and refined using single-crystal XRD. The structure has an orthorhombic I 2/a 1 2/d (unconventional setting for Fddd) space group symmetry, with unit-cell parameters of a = b = 11.966(3) angstrom, c = 11.964(3) angstrom, alpha = beta = 90?, gamma = 90.29(2), V = 1713.0(7) angstrom(3), Z = 8. The orthorhombic garnet displays very high birefringence (delta similar to 0.021) produced by the strong Fe-Al ordering in the octahedral sites, with Fe occupancies of 0.804 and 0.221 in Y-1 and Y-2 sites, respectively. Difraction peaks (such as 101 and 103) violating the Ia (3) over bard symmetry of cubic garnet are obvious even in powder XRD pattern. The homogenization temperatures of the fluid inclusions suggest that the low-crystallization temperature is responsible for the ordered orthorhombic structure. The strong ordering state of the structure and the sharp boundaries in the chemical zoning in the crystal (between similar to Adr(53)Grs(47) and similar to Adr(100)) indicate the orthorhombic intermediate grandite garnet is a thermodynamically stable phase at low temperature, separated by wide miscibility gaps from the pure end-members (grossular and andradite) with cubic structures. Most of the previously reported triclinic garnet structures are likely artifacts produced by pseudo-merohedral twinning of less-ordered orthorhombic structure, as indicated by the characteristic pairing pattern of different Y-sites with the same occupancies.