Journal of Earth and Planetary Materials
Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian
IMPACT FACTOR 2018: 2.631
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The crystal chemistry of birefringent natural uvarovites: Part II. Single-crystal X-ray structures
The crystal structures of six birefringent uvarovite-grossular garnets from three localities (Saranov. Veselovsk, and Saranka. Ural Mountains. Russia) were investigated using single-crystal X-ray CCD diffraction data. The intensity and lattice parameter data attest to the violation of the cubic garnet space group Ia3̅d and the symmetry reduction to subgroups with triclinic (I1̅), monoclinic (I2/a), or at most orthorhombic symmetry (Fddd). Careful structure refinements starting in space group /I reveal that partial long-range Cr3+/Al ordering on the octahedral sites is the most prominent non- cubic feature. For each crystal, a nearly perfect linear correlation of the individual octahedral size with its Cr occupancy is observed. Considering the dependence on the bulk Cr mole fraction, the individual octahedral size in non-cubic uvarovite-grossular solid solutions is represented by <Cr/Al- 0> (Å) = 1.9247 + 0.0147 XCr(bulk) + 0.0534 Xcr(individual). These uvarovites also structurally incorporate traces of hydrous component (<1 wt% H2O) as O4H4 “hydrogarnet” substitution in a non-cubic way. thus leading to further subtle deviations from cubic symmetry. Within the range of these low water contents, the refined Si-O bond length, averaged over crystallographically different tetrahedra. correlates with the total integral OH absorption coefficient αi through the equation ≪ Si-0≫ (Å) = 1.6455 + 1.0074·10-7 αi(cm-2). Although the Si and Ca atoms occupy general positions in I1̅. they deviate little from their respective special positions in la3̅d.
Consistent with the respective angular lattice distortions, the refined Cr3+/Al site distribution pattern is definitely triclinic in the Saranka sample, somewhat less pronounced triclinic (pseudomonoclinic) in the Veselovsk sample, and distinctively pseudo-orthorhombic or orthorhombic in all four samples from the Saranov locality.
Considering crystal chemical evidence, three of the pseudo-orthorhombic Saranov samples with elevated water content appear to have triclinic symmetry as well, while one low-water uvarovite is classified as orthorhombic. These results are in good agreement with those of optical and UV-VIS-IR spectroscopic investigations reported in Part I of the present study (Andrut and Wildner 2001).
Previous structure data for birefringent garnets reported in the literature as non-cubic are compared and discussed. Special emphasis is also drawn to structural and crystal chemical details exhibiting a non-ideal mixing behavior along the uvarovite-grossular join. Evidence for and against the cation ordering models postulated in the literature are discussed.