High temperature calorimetric study of mixing, phase separation, and crystallization in silicate glasses

Several glass forming systems have been studied by in situ scanning calorimetry and by solution calorimetry in molten 2PbO·B₂O₃ at 973 K. In systems which show macroscopic glass-glass phase separation on annealing, namely K₂O-La₂O₃-SiO₂ and Li₂O-SiO₂, the enthalpies of solution of optically clear quenched glasses and milky annealed glasses are almost identical. This behavior, coupled with a linear variation of heat of solution with composition along joins which have a large change in overall degree of polymerization, support the contention from spectroscopy that short range clustering dominates even in nominally single phase materials, with exsolution observed as the initially present heterogeneous regions grow in size. Alkali titanosilicate melts show C_p anomalies consistent with breakdown of alkali titanate complexes or changes in Ti coordination. Calorimetry is a sensitive quantitative probe of crystallization as a function of time in both ceramic and geologic systems.