The nature of color of gem-quality diaspore (“zultanites”) from Turkey

Abstract

Two samples of natural gem-quality diaspore (“zultanite”) from Turkey and, for comparison, one sample of high-chromium diaspore from Saranovskoye chromite deposit in northern Urals, were studied by optical absorption spectroscopy. In the polarized spectra of “zultanites” there are two broad bands of Cr³⁺, caused by electronic spin-allowed transitions ⁴A_2g → ⁴T_2g and ⁴A_2g → ⁴T_1g, split by a low-symmetry crystal field of Cr³⁺ in the distorted octahedral structural position of Al³⁺. The spectral positions of the bands, especially of the former one, are close to that in Cr³⁺-bearing chrysoberyl (alexandrite), that causes a weak alexandrite-effect in “zultanites”. A few much weaker bands may be assigned to the spin-forbidden electronic transitions of Fe³⁺, also substituting Al³⁺ ions in the octahedral positions of the diaspore structure. The origin of a series of relatively narrow absorption bands in near UV-range, appearing on the background of a strong high-energy absorption edge, is not quite clear. A strong pleochroic absorption edge in the IR range is undoubtedly caused by O–H vibrations of hydroxyl groups in the structure of diaspore (α-AlOOH).