Nacrite Al2Si2O5(OH)4 is a clay mineral that is polymorphous (or polytypic) with kaolinite. It crystallizes in the monoclinic system. X-ray diffraction analysis is required for positive identification. Nacrite was first described in 1807 for an occurrence in Saxony, Germany. The name is from nacre in reference to the mother of pearl luster of nacrite masses.