The suggested phase diagrams for the system B2O3‐SiO2 as drawn from the findings of previous investigators are presented. These diagrams show the wide controversy regarding this system. The principal difficulties in obtaining reliable equilibrium data are the volatilization of B2O3, the hydration of B2O3‐rich glasses, and the great viscosity of B2OrSiOz melts. In this investigation firings were made in sealed platinum capsules. Where necessary, they were opened and studied under xylene to eliminate hydration. Phases were identified by optical and X‐ray methods. The liquidus was obtained by determination of the temperatures at which the stable phases of SiO2 completely dissolved in the borosilicate melts. The criteria used for equilibrium are discussed.

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By a combination of solid-state sintering and quenching experiments the validity of calcium hexaluminate as a stable phase and the extent of its primary field in the system CaO-Al2O3-SiO2 have been established. The size of the primary field is considerably reduced from that suggested by earlier work. The anorthite-corundum-calcium hexaluminate invariant point has been relocated a t 28.070 CaO, 39.7% A1203, and 32.3%

Phase equilibrium relations in the celsian-silica-alumina portion of the system Ba0-A1203-SiOz were revised on the basis of data obtained from isothermal firing and from the quenching method. These data verify the existence of a celsian-silica compatibility join by reducing the extent of the mullite field shown in previous diagrams and extending the boundaries of the celsian primary field. The system celsian-silica-alumina is divided into two ternary subsystems, one having a eutectic point and the other an invariant reaction point. The eutectic for the subsystem celsian-silica-mullite is located a t BaO 19, Al,0,3 15, and SiO, 66 we% and 1296°r30C. The pertinent invariant point for the subsystem celsian-mullite-alumina, located a t BaQ 26, Al2O3 27, and Si02 47 wt% and 1554"+4OC, is a reaction point.

A compound corresponding in optical properties and in X-ray powder pattern with the natural mineral sapphirine has been synthesized by repeated sintering of the constituent oxides at atmospheric pressure.The most probable formula for this synthetic compound is 4Mg0.-5Al2O3. 2Si02. An unidentified crystalline phase previously noted in certain magnesia-aluminasilica porcelains has been shown to be identical with this synthetic sapphirine. There is reason to believe that sapphirine possesses a small field of primary crystallization on the liquidus surface of the ternary system Mg0-A1203-Si02. Some revision of the earlier work on this system therefore appears to be necessary. Experiments designed to prove the existence of sapphirine as a synthetic mineral, and to determine its thermal behavior and its stability relations with other crystalline phases in the ternary system, have been carried out. The results indicate that synthetic sapphirine melts incongruently at or near 1475 "C. to yield spinel and liquid, and that immediately below that temperature sapphirine is compatible with cordierite, spinel, and mullite, and at still lower temperatures with corundum also. On the basis of these results a revision of the equilibrium diagram for the system Mg0-A1203-Si02 has been made.