Thermal transformations in ceramic products at high temperatures were studied for admixture of Cretaceous and Mio-Pliocene clays, either carbonatic or with plastic behaviour, used for Lab-tiles and factory biscuits with amounts (25%) of quartz-sand added as temper. Clay admixtures contained illites, smectites series, interstratified I/S species and kaolinite. Firing steps of pastes in the factory took place in a tunneled gas kiln at 1080°C; whereas, tiles used for laboratory experiments were fired in an electric kiln (6001050°C). SEM and XRD patterns help identify minerals disseminated in a glassy matrix as well as idiomorphic crystals formed in pored ceramic tiles, including feldspars (anorthite, sanidine), wollastonite, diopside and melilite (gehlenite). In the fired products, minerals form preferably in pores and crystal genesis is mainly controlled by melt flow. Wollastonite, diopside and gehlenite appear at 800°C. Gehlenite amounts remain nearly constant or may diminish slightly above 1000°C. Enough calcite amounts favor formation of anorthite, gehlenite and wollastonite; whereas gehlenite doesn't appear necessarily as a precursor for anorthite. It is worthy of noting that gehlenite persists at 1100°C and coexists with anorthite. The later crystallizes above 850°C and its amounts increase abruptly above 1000°C. Following mineral crystallization at elevated temperature, the melt precursor can be either completely consumed or remains as relicts in biscuits.