Mineralogy of the Grès de Thiviers (northern Aquitaine, France)

This paper reports the mineralogical characterisation of the Grès de Thiviers (GT), a unique natural pigment used in the industry of traditional tile ceramics. The only occurrence of this raw material, composed of quartz and goethite, is a small region in the SW of France were it is exploited and exported to the European and Far East markets. GT is mainly used as a bulk red pigment for white stoneware and its stability in firing is a direct consequence of the unique nature. In fact, despite the nature of the raw materials composing the ceramic mixture and their behaviour at high temperature, GT invariably yields a final red colour to the fired ceramic body. The stability stems from the fact that goethite (yellow colour) which later transforms into hematite (red colour) is the cement of quartz grains. Thus, in firing, because of the protective shield formed by the inert quartz grains, the newly-formed hematite crystals do not get in contact with other reacting phases such as the alkaline glass and retain the red colour (Sempio and Gualtieri,2001). The mineralogy of the GT deposits was described in the light of the peculiar genesis and chemical variability. In fact, GT occurrence is closely related to the karstic system permeating the late Cretaceous limestone in this region. According to the different formation environment it was possible to describe the four typical GT textures: laminar, oolitic, dendritic, and homeoblastic. GT samples contain amorphous substance with composition [SiO2+Fe2O 3 or FeO(OH)] which is the colloidal precursor of the crystalline phases quartz and goethite.