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

• Published in: Acta crystallographica Section B, Structural science, crystal engineering and materials Structural science, crystal engineering and materials, 2016-12, Vol.72 (6), p.846-854
• Main Authors: Antao, Sytle M., Cruickshank, Laura A.
• Format: Article
• Language: English
• Subjects: andradite; Anisotropy; Chemical precipitation; Crystal structure; Crystallography; Crystals; Diffraction; garnet; Garnets; kimzeyite; MATERIALS SCIENCE; optical anisotropy; Phases; Rietveld refinements; synchrotron high-resolution powder X-ray diffraction; Synchrotrons; Two phase; two-phase intergrowth; X-ray diffraction
• ISSN: 2052-5206; 2052-5192; 2052-5206
• DOI: 10.1107/S2052520616014700

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, , 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 Ca3.00(Zr1.31Ti4+0.46Fe3+0.22Mn3+0.01)∑2[Al0.76Fe3+1.01Si1.23]∑3O12 for phase 1a and Ca2.99(Zr1.48Ti4+0.37Fe3+0.15)∑2[Al0.87Fe3+0.98Si1.15]∑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. Two cubic phases in a kimzeyite crystal create optical anisotropy that results from strain originating from structural mismatch. Two different mechanisms, namely epitaxial growth and exsolution on cooling, give rise to the two phases.