Carbonate and silicate phase reactions during ceramic firing

Mineralogical, textural and chemical analyses of clay-rich materials following firing, evidence that initial mineralogical differences between two raw materials (one with carbonates and the other without) influence the textural and mineralogical evolution of the ceramics as T increases from 700 to 1100°C. Mineralogical and textural changes are interpreted considering local marked disequilibria in a system that resembles a small-scale high-T metamorphic process (e.g., contact aureoles in pyrometamorphism). In such conditions, rapid heating induces significant overstepping in mineral reaction, preventing stable phase formation and favoring metastable ones. High-T transformations in non-carbonate materials include microcline structure collapse and/or partial transformation into sanidine; and mullite plus sanidine formation at the expenses of muscovite and/or illite at T ³ 800°C. Mullite forms by muscovite-out topotactic replacement, following the orientation of mica crystals: i.e., former (001)muscovite are ^ to (001)mullite. This reaction is favored by minimization of free energy during phase transition. Partial melting followed by fingered structure development at the carbonate-silicate reaction interface enhanced high-T Ca (and Mg) silicates formation in carbonate-rich materials. Gehlenite, wollastonite, diopside, and anorthite form at carbonate-silicate interfaces by combined mass transport (viscous flow) and reaction-diffusion processes. These results may add to a better understanding of the complex high-T transformations of silicate phases in both natural (e.g., pyrometamorphism) and artificial (e.g., ceramic processing) systems. This information is important to elucidate technological achievements and raw material sources of ancient civilizations and, it can also be used to select appropriate clay composition and firing temperatures for new bricks used in cultural heritage conservation interventions.