\-lә-▐ nīt\ n (1867) Al2O3·2SiO2.2H2O. A finely divided crystalline form of hydrated aluminum silicate that occurs as monoclinic crystals with a basal cleavage, resulting chiefly from the alteration of feldspars under conditions of hydrothermal or pneumatolytic metamorphism. It is an important clay mineral. Density, 2.58 g/cm3 (21.5 lb/gal); refractive index, 156; O.A., 32 to 55; Mohs hardness, 2.5. (Hibbard MJ (2001) Mineralogy. McGraw-Hill, New York; Solomon DH, Hawthorne DG (1991) Chemistry of pigments and fillers. Krieger, New York).