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Abstract
Infrared powder-absorption spectra of natural ordered (P2/n) and disordered (C2/c) as well as experimentally disordered pyroxenes lying in the binary join augite-jadeite were recorded at room temperature in the frequency region 70–1400cm−1. Frequency shifts and linewidth variations as a function of composition and degree of order were examined in particular detail over the spectral ranges 100–200cm−1 and 610–800 cm−1. Phonon signals at low frequencies are very sensitive to both changes in composition and degree of order, and are indicative of a non-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 transition. A linear correlation between the short-range order parameter obtained by IR analysis and the long-range order parameter obtained by structural refinements of X-ray single crystal diffraction data was found. Substantial line broadening in C2/c samples with intermediate compositions is interpreted in terms of local structural heterogeneities.