The coordination of Al3+ in glasses of jadeite composition, synthesized at 1450° and 1550°C and over the pressure range 0.001-40 kbar, has been determined using laser Raman spectroscopy. The Raman spectrum of glass of jadeite composition at 1 atm resembles that of SiO2 glass, except that the Si-О stretching bands occur at lower frequencies than in SiO2 glass and are weakly polarized in contrast to the depolarized bands in SiO2 glass. The shift of the Si-О 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 resemble that of α-cristobalite.

The spectra of jadeite glasses formed from liquids quenched at high pressure are similar to that of glass formed from liquid quenched at 1 atm, except that the Si(Al)-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 between the 1-atm and high-pressure glasses are explained in terms of differences in the symmetry 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 of pressure on the structure of NaAlSi2O6 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 of Al3+ from fourfold to sixfold is responsible for the observed decrease in viscosity of melts of jadeite composition at elevated pressures.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.