Unravelling the luminescence spectrum of garnet grossular var. tsavorite: the role of chromium (III), manganese (II) and misattribution of vanadium (II)

Abstract

Gem quality garnet var. tsavorite from Tanzania is investigated by means of SEM-EDS, XPS and fluorescence spectroscopy to decipher its luminescence spectrum. The EDS analysis confirms that the main constituents of the tsavorite samples from Tanzania are those of grossularia (i.e. Ca, Al, Si, O), with V, Mn, Cr, Ti and Mg as minor but characteristic elements (particularly V, Mn and Cr) of the tsavorite variety. The XPS analysis shows that the oxidation state of Mn is +2 and that of V is +3. The luminescence of tsavorite in the yellow region (591 nm) is originated by Mn2+ in dodecahedral coordination (D2 point symmetry) while the luminescence in the red region (690-750 nm) is originated by Cr3+ in octahedral coordination (Oh point symmetry) in a strong crystal field. Contrary to what is often reported in the literature, this study demonstrates that V2+ (or other 3d transition metal ions as well) is not responsible for the red emissions of tsavorite.