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 int ensity and lattice parameter data attest to the violation of the cubic garn et space group Ia (3) over bard and the symmetry reduction to subgroups wit h triclinic (I (1) over bar), monoclinic (I2/a), or at most orthorhombic sy mmetry (Fddd). Careful structure refinements starting in space group I (1) over bar reveal that partial long-range Cr3+/Al ordering on the octahedral sites is the most prominent noncubic feature. For each crystal, a nearly pe rfect linear correlation of the individual octahedral size with its Cr occu pancy is observed. Considering the dependence on the bulk Cr mole fraction, the individual octahedral size in non-cubic uvarovite-grossular solid solu tions is represented by < Cr/Al-O > (Angstrom) = 1.9247 + 0.0147 X-Cr(bulk) + 0.0534 X-Cr(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 s ymmetry. Within the range of these low water contents, the refined Si-O bon d length, averaged over crystallographically different tetrahedra, correlat es with the total integral OH absorption coefficient (xi through the equati on much less than Si-O much greater than (Angstrom) = 1,6455 + 1.0074 . 10( -7)alpha (i)(cm(-2)). Although the Si and Ca atoms occupy general positions in I (1) over bar, they deviate little from their respective special posit ions in Ia (3) over bard. Consistent with the respective angular lattice distortions, the refined Cr3 +/Al site distribution pattern is definitely triclinic in the Saranka sampl e, somewhat less pronounced triclinic (pseudomonoclinic) in the Veselovsk s ample, and distinctively pseudo-orthorhombic or orthorhombic in all four sa mples from the Saranov locality. Considering crystal chemical evidence, thr ee of the pseudo-orthorhombic Saranov samples with elevated water content a ppear 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 beha vior along the uvarovite-grossular join. Evidence for and against the catio n ordering models postulated in the literature are discussed.