The phase relations were established for the system silica‐sodium tetraborate. The silica liquidus curve exhibits strong upward concavity but shows no flattening. The tridymite‐cristobalite transformation occurred at 1410°± 10°C. A lowering of the α to β quartz inversion temperature of about 9°C was detected. Primary crystal habits are discussed. The region of metastable subliquidus immiscibility is also discussed and the nature of initial crystallization of the silica‐rich liqu