The global thermal behavior of some common ceramics is represented in the ternary diagram SiO2-Al2O3-K2O. in a zone where no experimental data are available. In this study, we experimented the thermal transformations of compounds belonging to two vertical sections between mullite and the eutectic point at 985 degreesC or the peritectic at 1140 degreesC. In both sections, results indicate the particular shape of liquidus curves, which show accentuated slopes at the vicinity of eutectic and peritectic points. Data from liquidus curves were used for the purpose of simulation, using a thermodynamic approach involving a subregular model for the excess Gibbs energy of mixing. The particular sigmoid shape of the liquidus curve and the concave downward shapes of Gibbs energy of mixing with two minimums, strongly suggest the existence of a large subliquidus immiscibility zone in the mullite-rich region. Experiments validated the existence of two horizontal lines at 985 and 1140 degreesC, below which mullite, orthoclase, and silica crystallize. (C) 2004 Elsevier Ltd. All rights reserved.