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 Mg₂SiO₄ (olivine) and γ‐Ca₂SiO₄ (space group $D_{{\text{2h}}^{16}}\text{, Z\hspace{0.17em}+\hspace{0.17em}4}$ ) 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 detailed assignment of the olivine‐type infrared absorption spectra.