Assignment of the main 548 and 474 cm -1 bands was carried out on the basis of the observed trends in the shift of bands with substitution of M 2+ or M 3+ . A comparison of the line at ~548 cm -1 for the HT80 and the Fe HT80 samples shows a shift to lower wavenumbers with increasing iron content, Figure , which suggests that Fe 3+ affects this normal mode. Thus, the band at 548 cm -1 is unlikely to be assigned to Mg-O-Mg stretching only, as in that case little or no shift would be observed with increasing M 3+ content. If the band would be assigned to M 3+ -O-M 3+ stretching, no shift should be observed when substituting Mg 2+