From a geological point of view, clays are soft, fine-grained, and residual sedimentary rocks
resulting from the weathering of feldspars (e.g., orthoclases and plagioclases) and ferromagnesian silicates (e.g., micas, amphiboles) contained in igneous and metamorphic rocks.
Hence clays are always made of various hydrated aluminosilicates, mainly kaolinite but also
illite and montmorillonite, all exhibiting the typical structure of sheet silicates (i.e., phyllosilicates).
