Cation distribution between cristobalite, tridymite, and coexisting glass phase in used silica bricks

The chemical composition of cristobalite, tridymite, glass, and accessory phases of different zones of used silica bricks taken from the roof of a glass tank was studied with a high resolution microprobe. Tridymite and cristobalite contain as impurities TiO2 (≤ 0.36 wt%), Al2O3 (≤ 0.37 wt%), and Na2O (≤ 0.27 wt%). Main constituents of the glass phase coexisting with crystalline silica are: SiO2 (74 to 60 wt%), TiO2 (0.4 to 9 wt%), Al2O3 (1 to 5 wt%), Fe2O3 (0.3 to 3 wt%), CaO (5 to 20 wt%), and Na2O (8 to 17 wt%). Temperature curves within the bricks during operation of the glass tank have been estimated using direct temperature measurement at the hot front of the bricks, and the transition temperatures of cristobalite to tridymite (∼ 1450°C), and of α- to β-wollastonite (∼ 1200°C). Microchemical data and supposed temperatures were correlated with the Nernst distribution law. The applicability of the Nernst law shows that local equilibrium conditions were reached during the use of the bricks; they have been preserved during cooling of bricks. The results of the Nernst law cation distribution imply that structural saturation with Al2O3, TiO2, and Na2O was not reached in the investigated composition range. Al3+ is believed to substitute Si4+ at tetrahedral lattice sites. Al3+ substitution is favoured with decreasing temperature in relation to the Al2O3 content in the glass phase. Al3+ → Si4+ substitution produces charge deficiency, which is compensated by interstitial entry of Na+ into structural channels and voids of tridymite and cristobalite. Ti4+ incorporation into the cristobalite and tridymite structures is favoured at higher temperatures with respect to the TiO2 content of the glass phase. The close reciprocal dependence between Al3+ and Ti4+ in silica may indicate that Ti4+ is tetrahedrally incorporated as well.