LOW-TEMPERATURE INFRARED SPECTRA BETWEEN 1200 AND 20 CM-1 AND NORMAL-COORDINATE ANALYSIS OF SILICATES WITH OLIVINE STRUCTURE

Infrared spectra of powdered samples of Mg2Sio 4(olivine) and 7-Ca2SiO4 (space group D 2h16, Z+4)were measured at liquid-N2 and liquid-He temperature between 1200 and 20 cm-1. The low-temperature spectra do not show noticeable band sharpening. Using a simple force field with five stretching-type force constants and symmetry-adapted coordinates, a normal-coordinate analysis is carried out for both crystal compounds. It is shown that satisfactory agreement between observed band frequencies and normal frequencies may be obtained through few iteration cycles. A number of features of the conventionally used group-vibration approach are confirmed by the normal-coordinate result, which in turn leads to a complete and fairly de-tailed assignment of the olivine-type infrared absorption spectra.