Thermal analysis of the Na2O-rich concentration region of the quasi-binary system Na2O–SiO2

The phase diagram of the Na2O-rich concentration region of the binary system Na2O–SiO2 was derived from STA measurements. The eutectic was established at about 1134 ± 9 K between 78 and 80 mole per cent of sodium oxide. The XRD measurements showed the existence of a crystal structure which was unknown so far. Its composition is most likely Na10SiO7 which was also deducted from the shape of the liquidus line obtained for all samples measured in the high Na2O concentration range of the binary system Na2O–SiO2. A second eutectic is placed between 85 and 87 mole per cent of sodium oxide at 1118 ± 15 K. The structure change of Na2O (β → α) should take place at 1243 K but was not observed in any sample in the Na2O-rich concentration region of the binary system Na2O–SiO2.