Compression mechanisms in aluminosilicate melts: Raman and XANES spectroscopy of glasses quenched from pressures up to 10 GPa

Raman and XANES spectroscopy were carried out on a series of glasses of com position 44CaO-12Al(2)O(3)-44SiO(2), formed at pressures up to 10 GPa by is obaric quench from a temperature of 2200 degreesC. The most significant cha nges in the Raman spectrum as a function of the synthesis pressure, or dens ity, of the glass occur in the low-frequency region (300-700 cm(-1)), assoc iated with T-O-T bending vibrations, With increasing density of the glass, the overall intensity at low frequencies decreases relative to the high-fre quency portion of the spectrum. Relative intensities of bands within the lo w-frequency region of the Raman spectrum are also very sensitive to synthes is pressure, whereas there is little evidence that pressure influences Q-sp eciation as the high-frequency region of the spectrum remains virtually unc hanged. With initial compression (V/V-0 = 1-0.96), the severe loss in inten sity near 500 cm(-1) indicates coordination of bridging oxygen atoms to an additional cation, which inhibits the vibrational motion that gives rise to this band normally observed for silicate glasses formed at ambient pressur e. At higher densities (V/V-0 < 0.96), bands in the low-frequency region ar e shifted to higher frequencies, indicative of narrower T-O-T angles. No si gnificant changes are observed in the Si and Ca K-edge XANES spectra with i ncreasing densification of the glass. The Al it-edge spectra also show no s ignificant changes among the lower density glasses (V/V-0 = 1-0.96), but re veal a feature near 1570 eV that dramatically increases in relative intensi ty with increasing densification beyond V/V-0 = 0.96. The observations from both Raman and XANES spectroscopy are consistent with two different compre ssion mechanisms operating in different pressure ranges. At lower pressures , the spectroscopic data are characterized by features that we attribute to the presence of triclusters (OT3 units) in the quenched melt. At higher pr essures, T-O-T angle reduction and also an increase in the average coordina tion number of Al are likely to occur to further reduce the volume of the m elt. The complex response of the structure of aluminosilicate melts to comp ression suggests that their physical properties will also behave complexly as a function of pressure. (C) 2001 Elsevier Science B.V. All rights reserv ed.