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Two cubic phases in kimzeyite garnet from the type locality Magnet Cove, Arkansas
Author(s) -
Antao Sytle M.,
Cruickshank Laura A.
Publication year - 2016
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s2052520616014700
Subject(s) - electron microprobe , crystallography , rietveld refinement , materials science , phase (matter) , analytical chemistry (journal) , formula unit , crystal structure , tetragonal crystal system , mineralogy , chemistry , organic chemistry , chromatography
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 χ 2 and overall R ( F 2 ) values are 1.840 and 0.0647, respectively. The sample, with the general garnet formula [8] X 3 [6] Y 2 [4] Z 3 [4] O 12 , 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 1.31 Ti 4+ 0.46 Fe 3+ 0.22 Mn 3+ 0.01 ) ∑2 [Al 0.76 Fe 3+ 1.01 Si 1.23 ] ∑3 O 12 for phase 1 a and Ca 2.99 (Zr 1.48 Ti 4+ 0.37 Fe 3+ 0.15 ) ∑2 [Al 0.87 Fe 3+ 0.98 Si 1.15 ] ∑3 O 12 for phase 1 b . The weight percentage, unit‐cell parameter (Å), distances (Å), and site occupancy factors (s.o.f.s) for phase 1 a 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 1 b 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, Δ a = 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.