From granite to kaolin The formation of kaolin (white clay indispensable for making porcelain and fine earthenware) began several kilometres deep, 300 million years ago. The circulation of hot (above 150°C) and pressurised magmatic fluids altered and fractured the already crystallised granite. The released silica recrystallised in the form of quartz veins. Later, during the Eocene (from 50 to 35 million years BC), as the climate of the Limousin became humid tropical, the deposit surfaced due to erosion. Warm waters penetrated the pegmatite veins and caused kaolinisation of the feldspars, yielding deposits like those of Saint-Yrieix-la-Perche, which made Limoges porcelain famous. From clay to granite If clays are carried deep underground, by burial or tectonic movements, they transform. The sheets composing them pack together, reorganising into more compact structures, and the clay rock becomes slate. When pressure and temperature increase further, the rock melts more. For example, if we take the products of the alteration of a granite: the granitic sand (with its clays and quartz) that we melt and let cool very slowly will become a rock of a granitic nature.