THE CRYSTALLIZATION OF CELSIAN POLYMORPHS IN SOME ALKALINE-EARTH ALUMINOSILICATE GLASSES

The crystallisation of some glasses based on the diopside-anorthite eutectic with celsian additions was studied. Hexacelsian and diopsidic solid solution were the primary crystalline phases developed in barium-rich glasses. With increasing time and temperature of heat treatment, anorthite tends to appear and the metastable hexacelsian transforms sluggishly and irreversibly into its stable monoclinic modification between 1100 and 1220-degrees-C. Depending on cooling rate, hexacelsian may invert reversibly into another metastable orthorhombic modification at 225-degrees-C. Both the orthorhombic and the monoclinic celsian phases originated essentially from the transformation of the hexacelsian developed initially and not by direct crystallisation from the glass. The crystallisation products under conditions approaching equilibrium were mainly monoclinic celsian, diopside, and anorthite.