Evidence for absorption by exchange-coupled Fe2+-Fe3+ pairs in the near infra-red spectra of minerals

Reduction of Fe³⁺ to Fe²⁺ by heating in hydrogen reduces the absorbance of the bands at 9,000 and 13,800 cm^?1 in the E ⊥ c spectrum of tourmaline, and the 9,000 and 11,000 cm^?1 bands in the E ∥ (001) spectrum of biotite. This behaviour is consistent with the presumed d-d origin of these bands (which seems well established) only if they gain much of their intensity from exchange-coupling with neighbouring Fe³⁺ ions. Intensification of spin-forbidden bands in sapphire by Fe³⁺-Fe³⁺ exchange-coupling was recognized by Ferguson and Fielding (1971, 1972), but exchange-coupling has not previously been thought to intensify spin-allowed d-d bands. Spin-allowed exchange-coupled bands resulting from Fe²⁺-Fe³⁺ pairs have features in common with both normal single ion d-d bands, which they resemble in energy, width and pressure dependence, and Fe²⁺+Fe³⁺→Fe³⁺+Fe²⁺ charge transfer bands, which they resemble in temperature-, heat treatment-, composition-, and polarization-dependence. Distinction between normal d-d, charge transfer, and pair d-d absorptions is thus complicated, and criteria for assigning these bands are discussed. Spin-allowed exchange-coupled pair bands should be sought in the spectra of transition metal clusters (trimers and polymers as well as pairs may be involved) whenever geometry favours their origin. It is possible that the bands near 10,000 and 11,500 cm^?1 in blue sapphire, and at about 5,000 cm^?1 in titanian garnets are of this type, but many other examples are likely to occur. Exchange-coupling may involve ions other than Fe³⁺ (e.g., Mn²⁺, also d ⁵), although Fe²⁺-Fe²⁺ coupling is unlikely to be important at laboratory temperatures.