HARD MODE INFRARED-SPECTROSCOPY OF CATION ORDERING AND SUBSTITUTION IN A CHAIN SILICATE

Infrared powder-absorption spectra of natural ordered (P2/n) and disor dered (C2/c) as well as experimentally disordered pyroxenes lying in t he binary join augite-jadeite were recorded at room temperature in the frequency region 70-1400 cm(-1). Frequency shifts and linewidth varia tions as a function of composition and degree of order were examined i n particular detail over the spectral ranges 100-200 cm(-1) and 610-80 0 cm(-1). Phonon signals at low frequencies are very sensitive to both changes in composition and degree of order, and are indicative of a n on-ideal mixing behaviour for the C2/c solid solution, Phonons at high frequencies depend on the average composition of the samples and not on local configurational changes related to the order-disorder transit ion, A linear correlation between the short-range order parameter obta ined by IR analysis and the long-range order parameter obtained by str uctural refinements of ?C-ray single crystal diffraction data was foun d. Substantial line broadening in C2/c samples with intermediate compo sitions is interpreted in terms of local structural heterogeneities.