Mineralogy and firing transformations of Permo–Triassic clays used in the manufacturing of ceramic tile bodies

This study is focussed on the behaviour of Permo–Triassic ceramic clays from the area around Castellon (Spain) having a large ceramic industry. The aim of this work is to establish the mineralogical composition of Permo–Triassic raw materials and fired ceramic bodies. Particular emphasis is placed on the compositional variations along the lithostratigraphic section. Bulk rock XRD analysis shows a homogeneous composition dominated by quartz, phyllosilicates and hematite. Minor components (< 5%) include potassium feldspar, dolomite and calcite. The clay minerals assemblage is formed by illite ± kaolin ± chlorite ± I/S ± C/S. Test samples have been prepared by extrusion and firing in the range of 900–1150 °C. Analysis of the fired samples was carried out by X-ray diffraction. The results from the study of mineralogical transformations show the persistence of illite up to at least 900 °C. From the destruction of illite, an intermediate phase between spinel and hercynite originates. Quartz and hematites are present in all samples. These clays are an excellent raw material for the formulation of low porosity ceramic stoneware.