illite

noun

1. 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.

noun

1. a clay mineral of the mica group, found in shales and mudstones. Crystal structure: monoclinic. Formula: K 1–1.5 Al 4 (Si 6.5–7 Al 1–1.5 O 20)(OH) 4