STRUCTURAL MECHANISMS OF SOLID-SOLUTION AND CATION ORDERING IN AUGITE-JADEITE PYROXENES - II - A MICROSCOPIC PERSPECTIVE

Infrared spectra of ordered (P2/n) and disordered (C2/c) pyroxenes bel onging to the join augite-jadeite were investigated at room temperatur e in the range 70-1400 cm(-1). The spectra change as a function of com position and the phase transition P2/n --> C2/c produces an increase i n band widths. The autocorrelation function quantified changes in line widths due to these effects. Phonons at low frequencies an very sensi tive to both changes in composition and degree of order and are indica tive of a non-ideal mixing behavior for the C2/c solid solution. Phono ns at high frequencies depend on the average composition of the sample s and not on local configurational changes related to the order-disord er transition. High degrees of local heterogeneity were suggested by t he Delta(corr) values of the C2/c omphacites at intermediate compositi ons. This effect is most evident in the low frequency region of the sp ectra, corresponding to modes involving, primarily, the M cations. The changes in frequency due to ordering, Delta omega, and the change in line widths delta(Delta(corr))(100-200) for the spectral region 100-20 0 cm(-1) and delta(Delta(corr))(210-800) for the range 210-800 cm(-1) have been used to characterize the state of local order at different c ompositions. The spectra yield information related to the local struct ural states of augite-jadeite pyroxenes, down to a unit-cell length sc ale, which may then be compared with the average structure determined by X-ray diffraction studies of the same samples.