Thermodynamic properties of K₂Si₄O₉ and Na₂Si₄O₉ glasses quenched from melts at high pressures and high temperatures

抄録

Calorimetric study was performed to measure relative enthalpies of alkali tetrasilicate glasses quenched from melts at various pressures. The high-pressure K₂Si₄O₉ and Na₂Si₄O₉ glasses were obtained by quenching the melts at about 2273–2473 K in the pressure range from 3 to 12 GPa. The enthalpies of the glasses were measured by a differential drop-solution calorimetry method in 2PbO·B₂O₃ solvent, using a Calvet-type microcalorimeter. Densities of the glasses were also measured by the Archimedes method. Relative enthalpies and densities in the K₂Si₄O₉ and Na₂Si₄O₉ glasses increase with increasing the synthesis pressure. Compared with those of the 1 atm-glass, relative enthalpy and density of the K₂Si₄O₉ glass synthesized at 8 GPa are higher by 28.1 kJ/mol and 11.0 %, respectively, and those of Na₂Si₄O₉ glass higher by 33.1 kJ/mol and 25.1 %, respectively. These large enthalpy increases are consistent with pressure-induced structural changes in K₂Si₄O₉ and Na₂Si₄O₉ glasses in which the proportion of five- and six-coordinated Si increases with pressure: total abundance of the high-coordinated Si increases up to about 12 % in all of Si in the Na₂Si₄O₉ glass synthesied at 12 GPa and about 5 % in the K₂Si₄O₉ glass at 5.5 GPa. In contrast to these large enthalpy changes in the alkali tetrasilicates, enthalpy increase in NaAlSi₃O₈ glasses quenched from melt at 1.5–2.5 GPa was only 2–3 kJ/mol, because only a small structural change in the medium range (decrease of mean T-O-T angles) occured in this pressure range.