Electron paramagnetic resonance study of iridium in forsterite

Electron paramagnetic resonance of Ir2+ in forsterite is studied at Q-band frequency and room temperature. There are four equivalent spectra superimposed along the three crystallographic axes. The individual spectrum consists of four hyperfine lines of approximately equal intensity separated from each other by 42 G; one axis of the g tensor is near the c axis. Ir2+ is certainly substituted for Mg2+. Because of the fourfold degeneracy of the EPR spectrum, it may be suggested that iridium occurs at M-1. Taking into account that one eigenvector of the g tensor is nearly parallel to c, it seems also possible that the substitution takes place at the M-2 position. In this case, the lattice relaxation of the crystal structure around Ir2+ at M-2 must break the point symmetry m at M-2.