Annealing effect in raman scattering of various types of tourmalines

Raman scattering studies of an annealing effect in spectra of seven natural tourmalines are presented. It was found that tourmalines containing Fe ions when annealed in air showed Raman features significantly different from those of the Fe-free minerals. Furthermore, tourmalines containing the Fe ions behaved differently depending on whether they were annealed in air or in argon. Our analysis indicated that these differences were mainly due to the Fe ions present in the tourmalines. When Fe-tourmalines were annealed in air, the Fe ions were oxidized into Fe ions, and the oxidation strongly modified the crystal structure, even in tourmalines with a very low Fe concentration (0.20 wt.%). Our finding may be helpful in studying the oxidation of other minerals which contain the Fe ions.