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.

Origin of illite

First recorded in 1937; Ill(inois)2 + -ite1
Related formsil·lit·ic [ih-lit-ik] /ɪˈlɪt ɪk/, adjective Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for illite


  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

Word Origin for illite

C20: named after Illinois, where it was first found
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012