Transformed mineral phases during clay ceramic firing

Ceramic clays are one of the most complicated ceramic systems because of the very complex relationship between the behavior of minerals during the ceramic processing and the transformations during heating. A major challenge is to predict the phase transformations in clay ceramics. The aims are to establish reference data of ceramics products that can be formed based on the mineralogical composition of the local raw materials. These data, in turn, can be compared with the archaeological ceramics in order to study their origins.
The mineralogical compositions and transformations during the firing (550 to 1100ºC under oxidizing conditions) of seven clayey materials sampled from the main clay deposits of Northern Morocco were evaluated by X-ray powder diffraction. Two groups of clays were evidenced according to the type of neoformed high-temperature minerals: non-calcareous clays and calcareous clays. For the non-calcareous raw materials, spinel is produced at 950°C. Cristobalite and mullite were formed above 1000ºC from clays that contain illite, kaolinite and chlorite. In clays containing vermiculite and high amount of chlorite, hematite was formed from 950°C. Firing of calcareous clays at temperatures > 950ºC yielded Ca-silicates (diopside, gehlenite and wollastonite), spinel, cristobalite, hematite and feldspars. Mullite may also form in the calcareous clay products, when the carbonates content exceeds 10%.