Liquidus surface of MgO-CaO-Al₂O₃-SiO₂ glass-forming systems

Alkaline earth aluminosilicate systems are of widespread technological importance, and it is therefore essential to have accurate phase equilibrium diagrams available. While the phase equilibria for the single alkaline earth CaO-Al₂O₃-SiO₂ and MgO-Al₂O ₃-SiO₂ systems are fairly well established, the mixed MgO-CaO-Al₂O₃-SiO₂ systems have received little attention despite their important role in the high-tech glass industry. Here, the liquidus surface (liquidus temperature T_liq and primary devitrification phase) is determined by x-ray diffraction phase analyses of isothermally reacted samples from powder mixtures. The phase equilibrium diagrams are presented based on measurements on a total of 120 compositions. In the composition range of interest for industrial glasses, T_liq tends to increase with increasing silica content and decreasing alkaline earth-to-alumina ratio. Cristobalite, mullite, anorthite, and cordierite are found to be the dominant devitrification phases for the compositions with high SiO₂, Al₂O₃, CaO, and MgO contents, respectively. Anorthite extends further into the cristobalite and mullite fields for the [CaO]/[MgO] = 3 system compared to the cordierite behavior in systems with [MgO]/[CaO] = 3. For systems with [MgO] = [CaO], the liquidus temperatures are shifted to lower values since the formation of anorthite and cordierite requires some critical concentration of CaO and MgO, respectively.