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X and Q band EPR studies of paramagnetic centres in natural and heated tourmaline

Joanna Babiska¹, Krystyna Dyrek^2,*, Adam Pieczka¹ and Zbigniew Sojka^2,3

¹ AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Al. Mickiewicza 30, 30-059 Krakow, Poland
² Jagiellonian University, Faculty of Chemistry, ul. Ingardena 3, 30-060 Krakow, Poland
³ Regional Laboratory of Physicochemical Analyses and Structural Research, ul. Ingardena 3, 30-060 Krakow, Poland

^* Corresponding author, e-mail: dyrek{at}chemia.uj.edu.pl

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).

Key-words: tourmaline, EPR spectroscopy, simulation, thermal oxidation, paramagnetic centres: Fe²⁺, Fe³⁺, Mn²⁺, Mn³⁺, Mn⁴⁺, V⁴⁺ ions.