Fe^(2+)-Fe^(3+) interactions in tourmaline

Abstract

The color and spectroscopic properties of ironbearing tourmalines (elbaite, dravite, uvite, schorl) do not vary smoothly with iron concentration. Such behavior has often been ascribed to intervalence charge transfer between Fe^(2+) and Fe^(3+) 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 Fe^(2+) and Fe^(3+) occurs in which the Fe^(2+) bands are intensified without an intense, new absorption band. At low iron concentrations, the intensity of light absorption from Fe^(2+) is about the same for E∥c and E⊥c polarizations, but at high iron concentrations, the intensity of the E⊥c polarization increases more than ten times as much as E∥c. This difference is related to intensification of Fe^(2+) absorption by adjacent Fe^(3+). Extrapolations indicate that pairs of Fe^(2+)-Fe^(3+) have Fe^(2+) absorption intensity ∼200 times as great as isolated Fe^(2+). Enhanced Fe^(2+) absorption bands are recognized in tourmaline by their intensity increase at 78 K of up to 50%. Enhancement of Fe^(2+) absorption intensity provides a severe limitration on the accuracy of determinations of Fe^(2+) concentration and site occupancy by optical spectroscopic methods. Details of the assignment of tourmaline spectra in the optical region are reconsidered.