THERMOCHEMICAL STUDIES OF SILICATE, ALUMINOSILICATE, AND OROSILICATE GLASSES.

High temperature calorimetry has been used to determine enthalpies of mixing and of vitrification and fusion in silicate, aluminosilicate, and borosilicate systems. The systems Na//2O-Al//2O//3-SiO//2 and Na//2O-B//2O//3-SiO//2 show contrasting behavior. In the former, the join SiO//2-NaAlO//2 shows large energetic stabilization near Al/(Al plus Si) equals 0. 5, whereas in the latter, much less pronounced stabilization is seen along the join SiO//2-NaBO//2. More B//2O//3-rich compositions, e. g. , the join SiO//2-Na//0//. //3//1B//1//. //2//3O//2 (B/Na equals 4) show substantial positive heats of mixing, presumably related to immiscibility. Thermodynamic data for the complete Na//2O-B//2O//3-SiO//2 ternary system are being developed. Borosilicate glasses are a leading candidate for immobilization of high-level nuclear waste; thus their thermodynamic characterization is important to predicting the long-term stability of such depositories.