Fe²⁺-Fe³⁺ Interactions in Tourmaline

Abstract

The color and spectroscopic properties of iron-bearing tourmalines (elbaite, dravite, uvite, schorl) do not vary smoothly with iron concentration. Such behavior has often been ascribed to intervalence charge transfer between Fe²⁺ and Fe³⁺ which produces a new, intense absorption band in the visible portion of the spectrum. In the case of tourmaline, an entirely different manifestation of the interaction between Fe²⁺ and Fe³⁺ occurs in which the Fe²⁺ bands are intensified without an intense, new absorption band. At low iron concentrations, the intensity of light absorption from Fe²⁺ is about the same foe E\\c and E perpendicular c polarizations, but at high iron concentrations, the intensity of the E  perpendicular c polarization increases more that ten times a much as E\\c. This difference is related to intensification of Fe²⁺ absorption by adjacent Fe³⁺. Extrapolations indicate that pairs of Fe²⁺-Fe³⁺ have Fe²⁺ absorption intensity ~200 times a great as isolated Fe²⁺. Enhanced Fe2+ absorption bands are recognized in tourmaline by their intensity increase at 78 K of up to 50%. Enhancement of Fe²⁺ absorption intensity provides a severe limitation on the accuracy of determinations of Fe²⁺ concentration and site occupancy by optical spectroscopic methods. Details of the assignment of tourmaline spectra in the optical region are reconsidered.