The effect of fictive temperature on Al coordination in high-pressure (10 GPa) sodium aluminosilicate glasses

Typical liquidus temperatures can be over 1000 degrees C greater than the glass transition temperatures for high-pressure aluminosilicate melts so the effect of temperature must be determined if glass data is to be used to approximate the structural speciation present in geologic melts. This study has investigated the effect of fictive temperature (T-f, taken as the temperature where the melt structure is the same as that of the glass) on the percentage of Al-[5] and Al-[6] species in two high-pressure (10 GPa) Na-aluminosilicate glasses (Na3AlSi7O17 and NaAlSi3O8,) where one glass of each composition was quenched from the high-pressure melt while the other was annealed near the glass transition temperature. The Al-27 MAS NMR spectra of the Na3AlSi7O17 samples show that the higher T-f. (quenched) glass contains more high-coordinated Al than the lower T-f (annealed, 475 degrees C) glass. However, the Al-27 spectra of the NaAlSi3O8 samples show the opposite temperature dependency, which in addition to the lack of NBO in this glass, may suggest differing mechanisms for the generation of high-coordinated Al.