Various members of the tourmaline group (schorl, dravite, and elbaite) as well as the products of their gradual oxidation were investigated by EPR spectroscopy in X and Q bands at 293 K and 77 K. In the EPR spectra of the schorl and dravite samples the signals at g ≈ 2 and 4.3 were attributed to clustered and isolated Fe3+ ions, respectively. The EPR spectra of Fe-poor elbaite are dominated by signals at g ≈ 2.5 and 3.5, assigned to Mn2+ ions.
In the schorl and dravite samples, gradually annealed in air above 750 K, the total intensity of the EPR spectrum increased with increasing temperature, due to the oxidation of Fe2+ (d6) to Fe3+ (d5) ions. The Fe3+ ion being a product of thermal oxidation initially occupies sites with g ≈ 4.3 and after heating at temperatures above 1070 K forms clusters with g ≈ 2.0.
In the Fe-poor elbaite the total intensity of the spectrum gradually decreased with the increasing oxidation temperature up to 1150 K, due to the transformation of paramagnetic Mn2+ (d5) into Mn3+ (d4) ions. Simultaneously, the signal of Fe3+ at g ≈ 4.3 became more pronounced. At still higher temperatures (T > 1150 K) the intensity of the signal around g ≈ 2.0 increased indicating further oxidation of Mn3+ to Mn4+ (d3).