PHONON-SPECTRA OF ALKALI FELDSPARS - PHASE-TRANSITIONS AND SOLID-SOLUTIONS

The phonon spectra of IR-active vibrations in alkali feldspars have be en systematically investigated in the spectral range 50-2000 cm(-1) at temperatures between 20 and 900 K. Samples with high degrees of Al-Si order display IR absorption line profiles with spectral line widths t hat are systematically smaller than those for samples with low degrees of Al-Si order. Phonon frequencies depend in a nonlinear manner on th e Na-K content, indicating nonideal mixing properties. The mode Grunei sen parameters are largest for low-frequency lattice modes and Si-O st retching modes; some negative Gruneisen parameters were found at phono n energies below 430 cm(-1). The temperature evolution of the phonon s pectra shows clear indications of the <C2/m-C(1)over bar> transition i n Al-Si disordered Na-rich samples. An anomaly for highly Al-Si ordere d microcline at 270 K is confirmed.