Detection of structural varieties of red gem spinels from Ratnapura, Sabaragamuwa province of Sri Lanka
A new instrumental method for spectrographical detection of particularities of crystal fields around optically active ions, such as the trivalent chromium ion, was applied to an investigation of red magnesium-spinels from the Ratnapura gem gravels; also from Kangaiyam in India and some other sources for comparison. The method is characterized by analysis of luminescence spectra in the red and near infrared ranges excited by a concentrated flux of blue radiation, and analysed with a spectrograph of considerable optical power. Spectrograms are enregistered on photographic material of high resolving power; they reveal multiplet structures and intensity-distributions of component lines. Slight variations in spectral structure are significant for the detection of structural varieties of minerals. Systematic investigation of the spinels revealed the existence of a hitherto unknown variety of red gem spinel, provisionally named spineloïd. Its properties are described. Numerical data on the resolved luminescence spectra, in the wavelength range between 6500 Å and 7600 Å, are given for the normal and extraordinary varieties. Other physical properties of both varieties are reviewed and compared. The results of the investigation are discussed, and evaluation of data leads to a tentative conclusion regarding the nature of the spineloïd structure. The spineloïd is a stoechiometric or near stoechiometric spinel (MgO.Al2O3) in which Cr3+ ions appear in pairs in regular polyhedral configurations; the spectrum is modified by the presence of manganese ions of different valencies.
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Lagerwey, A.A.F. (1974). Detection of structural varieties of red gem spinels from Ratnapura, Sabaragamuwa province of Sri Lanka. Scripta Geologica, 27, 1–27.