Mineralogy and firing characteristics of a clay from the valley of Ourika (Morocco)

Raw material and processed samples of a Triassic clay deposit, located at the valley of Ourika (Morocco), were qualitatively and quantitatively investigated by means of X-ray diffraction (XRD), thermal analysis (DTA-TG), infrared spectroscopy, gravimetry and analytical methods (complexometry and atomic absorption spectroscopy). The results showed that the raw material consists of quartz (21 wt.%), poorly crystallized goethite and hematite (6 wt.%), micaceous mineral (39 wt.%) and kaolinite (27 wt.%) together with mixed layer phyllosilicates. The clay suitability for traditional ceramics was discussed in view of its granulometric distribution and the structural changes of the raw material samples have been studied as a function of the firing temperature. The linear shrinkage, the density, the open porosity and the bending strength were measured. The crystallized phases were identified and evaluated by XRD. It was shown that up to about 900 °C, the changes of the measured properties were mainly linked to the compaction of the clay breakdown products. The marked changes observed for T>900 °C were principally related to a viscous flow-type sintering. In addition, it was established that the extensive presence of muscovite and the clay aluminium deficiency were responsible for the development of sanidine (KAlSi3O8), which was the single crystalline phase identified in this study. The occurrence of this K-feldspar was accompanied by a decrease of the amount of quartz due to its partial dissolution in the fused phase.