The Nature of the Mn(III) Color Centers in Elbaite Tourmalines

Abstract

The characteristic red color of many natural tourmalines is due to the presence of Mn­(III) cations substituting for aluminum and lithium. These sites originate as Mn­(II) and are oxidized by natural γ-irradiation over geologic time as they sit in the Earth’s crust. Presented here is a thorough analysis of the spin-allowed and spin-forbidden transitions which give rise to the color of these gemstones. Ligand field analysis, supplemented by time-dependent density functional theory, was used to correct the historical assignments of the symmetry-allowed transitions in the polarized UV–visible absorption spectrum. Heat-induced reduction of the oxidized manganese sites provided a probe of the relationship between the spin-allowed and spin-forbidden bands. Notably, the intensity of the spin-forbidden transition was highly dependent on the neighboring ions in the Y-site. Simulations and modeling showed that increased intensity was observed only when two Mn­(III) ions occupied adjacent substitutions in the Y-site via a proposed exchange-coupling mechanism.