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(Minerals) a clay mineral of the mica group, found in shales and mudstones. Crystal structure: monoclinic. Formula: K1–1.5Al4(Si6.5–7Al1–1.5O20)(OH)4
[C20: named after Illinois, where it was first found]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
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.
il•lit•ic (ɪˈlɪt ɪk) adj.
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