Chemical composition, structural features and luminescent properties of Cr-bearing spinel from marbles of the Eastern Slope of the Urals

   Research subject and Methods. New data on the impurity composition, Raman spectra and photoluminescence of noble spinel from marbles of the Eastern Slope of the Southern and Middle Urals – the Kuchinsky occurrence (Kochkarsky anticlinorium) and the Alabash occurrence (Murzinsky-Aduysky anticlinorium), as well as the Kukh-i-Lal and Goron deposits (Southwestern Pamir) are presented.

   Results. Energy dispersion microanalysis of the chemical composition shows a high chromium content up to #Cr = Cr/(Cr + Al) ~ 0.2 and a low iron content, as well as a deviation in the ratio of divalent and trivalent cations from the stoichiometric in spinel from the Kuchinsky occurrence. The two-mode frequency behavior has been established by Raman spectroscopy in the entire range of compositions corresponding to spinel-magnesiochromite solid solutions. The tetrahedral sublattice vibration parameters – the frequency of the breathing mode and the width of the bending mode of the MgO₄ groups – are characterized by the highest concentration sensitivity. The reaction of the spinel tetrahedral sublattice is qualitatively similar when the structure is disordered due to (1) the isomorphous substitution VICr³⁺ → ^VIAl³⁺, (2) the thermally induced inversion ^IVMg²⁺ → ^VIAl³⁺, ^VIAl³⁺ → ^IVMg²⁺, (3) the radiation defects; for the analysis of quantitative differences, a diagram "width of the mode of deformation vibrations MgO₄ vs. frequency of the lattice mode T(Mg)" is proposed. Variations in the structure and properties of the Cr³⁺ emission center under disordering have been determined by low-temperature photoluminescence spectroscopy.

   Conclusions. The vibrational properties and photoluminescence of chromium ions are determined by several interrelated factors: (1) impurity composition, (2) nonstoichiometry, (3) structure inversion, (4) vacancy defects. The position and relative intensity of zero-phonon N-lines resulting from the Cr³⁺ emission center distortions are proposed for use as highly sensitive structural probes, in particular, to assess the gemological value of the spinel. The features of the composition, structure, and luminescent properties of the samples of the Kuchinsky occurrence formed at the progressive stage of regional metamorphism under conditions of increasing temperature and pressure are revealed.

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