kaolinite - soft white kaolin - large display specimen of the primary constituent of kaolin clay

kaolinite  Al2(Si2O5)(OH)4 Kaolinite is the primary constituent of kaolin clay and is one of the kaolin group of minerals. Its physical structure is like a sandwich, with aluminum hydroxide layers, called gibbsite layers, bonded to silicate sheets. These sheets are electrically neutral, so they are bonded by very weak van de Waals forces, easily sliding apart to give kaolinite its usual softness and greasy feel. It has a similar structure to minerals in the serpentine group.   Rocks with a silica-rich composition that contain aluminosilicate minerals weather to clays. This kaolinite was formed by the hydrothermal alteration of Pleistocene lacustrine sandstone and an underlying rhyolite. The sandstone was derived from granitic rocks, which contain abundant feldspar and after hydrothermal alteration or weathering are a common source for kaolinite. Because kaolinite does not absorb water, it does not expand when it comes in contact with water and is the preferred type of clay for the cera