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The crystal structure of a birefringent garnet (~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) Å,  c  = 11.964(3) Å, α = β = 90°, γ = 90.29(2)°,  V  = 1713.0(7) Å 3 ,  Z  = 8. The orthorhombic garnet displays very high birefringence (δ ~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 3̅ d  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 ~Adr 53 Grs 47  and ~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.

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