g=4.3 Isotropic EPR Line in Tourmaline

An isotropic EPR line with g=4.302±0.006 was found in a natural single crystal of tourmaline. This line can be ascribed to the impurity Fe3+, occupying a host site with the axial field parameter D=0 and the rhombic field parameter E ≫ hν. After an inspection of the environments of different hostcation sites, it is believed that Fe3+ ion replacement of the B3+ host site is most likely responsible for this isotropic line.