A clay-size, micaceous mineral; a common component of soil, sediments, sedimentary rocks, and hydrothermal deposits. Generally, the term refers to any relatively nonexpanding clay that possesses a c dimension of approximately 1.0 nanometer. Specifically, illite is considered to possess a smaller layer charge and potassium (K) content than muscovite, its potassium (K) content generally being about 0.8 equivalent per half-unit cell as opposed to a content of −1.0 equivalent for muscovite. It is characterized by the ideal formula Al₂(Si_3.2Al_0.8)O₁₀(OH)₂K_0.8.