THE CRYSTAL CHEMISTRY OF BIREFRINGENT NATURAL UVAROVITES. PART III. APPLICATION OF THE SUPERPOSITION MODEL OF CRYSTAL FIELDS WITH A CHARACTERIZATION OF SYNTHETIC CUBIC UVAROVITE

Abstract

Synthetic, flux-grown uvarovite, Ca3Cr2 [SiO4]3, was investigated by optical methods, electron microprobe analysis, UV-VIS-IR microspectrometry, and luminescence spectroscopy. The crystal structure was refined using single-crystal X-ray CCD diffraction data. Synthetic uvarovite is optically isotropic and crystallizes in the “usual” cubic garnet space group Ia3¯d [a=11.9973 Å, Z=8; 21524 reflections, R1=2.31% for 454 unique data and 18 variables; Cr–O=1.9942(6), Si–O=1.6447(6), Ca–Oa=2.3504(6), Ca–Ob= 2.4971(6) Å]. The structure of Ca3Cr2[SiO4]3 complies with crystal-chemical expectations for ugrandite group garnets in general as well as with predictions drawn from “cubically averaged” data of non-cubic uvarovite–grossular solid solutions (Wildner and Andrut 2001). The electronic absorption spectra of Cr3+ in trigonally distorted octahedra of synthetic uvarovite were analyzed in terms of the superposition model (SM) of crystal fields. The resulting SM and interelectronic repulsion parameters are \(\)=9532 cm−1, \(\)=4650 cm−1, power law exponent t 4=6.7, Racah B35=703 cm−1 at 290 K (reference distance R 0=1.995 Å; fixed power law exponent t 2=3 and spin-orbit parameter ζ=135 cm−1). The interelectronic repulsion parameters Racah B 55=714 cm−1 and C=3165 cm−1 were extracted from spin-forbidden transitions. This set of SM parameters was subsequently applied to previously well-characterized natural uvarovite–grossular solid solutions (Andrut and Wildner 2001a; Wildner and Andrut 2001) using their extrapolated Cr–O bond lengths to calculate the energies of the spin-allowed bands. These results are in very good agreement with the experimentally determined band positions and indicate the applicability of the superposition model to natural 3d N prevailing systems in geosciences. Single-crystal IR absorption spectra of synthetic uvarovite in the region of the OH-stretching vibration exhibit one isotropic absorption band at 3508 cm−1 at ambient conditions, which shifts to 3510 cm−1 at 77 K. This band is caused by structurally incorporated hydroxyl groups via the (O4H4)-hydrogarnet substitution. The water content, calculated using an integral extinction coefficient ɛ=60417 cm−2 l mol−1, is c H2O=33 ppm.