Convergence was noted in the infrared spectra ofsanidine and albite under pressure (up to 40 kbar). Sanidine spectra showed new bands while certain bands were lost in albite spectra. The new, pressure-induced bands in sanidine spectra are related to changes in the position of the alkali ion in the structure as it influences the oxygens of the tetrahedral network. Both pure Si-O and Si,Al-O related stretch modes are affected. Deformation of the coordination polyhedra of the alkali ions and change in the symmetry of the site, through tilting of the Si, Al tetrahedra, are responsible for the observed changes in infrared spectra under pressure. Small positive shifts of the bands for stretch and bending modes show that the tetrahedra are themselves little deformed by these pressures.
It is deduced that site ordering of Al and Si ions is not the principle reason for band multiplicity in the 1000 and 750 cm-1 regions of feldspar spectra. The similarity of spectra for the two feldspars (mono- and triclinic) at 40 kbar suggests that the two structures converge at high pressures and low temperatures.