Raman study of the coordination of aluminum in jadeite melts as a function of pressure

The coordination of Al3 * in glasses of jadeite composition, synthesized at 1450 " and 1550"C and over the pressure range 0.fi)l-40 kbar, has been determined using laser Raman spectroscopy. The Raman spectrum of glass ofjadeite composition at I atm resembles that of SiO2 glass, except that the Si-O stretching bands occur at lower frequencies than in SiO, glass and are weakly polarized in contrast to the depolarized bands in SiO, glass. The shift of the Si-O bands in the jadeite glass to lower frequencies is due to Si(Al)-O coupled modes. The weakly polarized stretching bands indicate that the local ordering in jadeite glass may re-semble that of c-cristobalite. The spectra ofjadeite glasses formed from liquids quenched at high pressure are similar to that of glass formed from liquid quenched at I atm, except that the S(AI)-O stretching bands become strongly polarized with increasing pressure and are shifted to lower frequencies. There is also a decrease in the intensity of the Rayleigh tail. The major spectral differences betweon the l-atm and high-pressure glasses are explained in terms of differences in the sym-metry of the local ordering of the network structure and a systematic decrease in the bond angle at the oxygen atom shared by two adjacent silicon atoms. Overall, the effect ofpressure on the structure of NaAlSirOu melts is to increase the degree of local ordering and to change the local network structure to a coesite-type structure. Al3 * remains tetrahedrally coordinated over the pressure range investigated; there is no support for the hypothesis that a pressure-induced coordination change ofAl3 * from fourfold to sixfold is responsible for the observed decrease in viscosity of melts ofjadeite composition at elevated pressures