[il-ahyt] 
| any of a group of clay minerals, hydrous potassium aluminosilicates, characterized by a three-layer micalike structure and a gray, light green, or yellowish-brown color. |
illite
any of a group of mica-type clay minerals widely distributed in marine shales and related sediments. Illite contains more water and less potassium than true micas, but it has a micalike sheet structure and is poorly crystallized. It may form a chemical series with both muscovite and montmorillonite; it is a weathering product of muscovite and alters to montmorillonite under humid conditions. For chemical formula and detailed structure and physical properties, see clay mineral (table); mica (table)
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