A general term for a group of three-layer (14Aa), micalike clays (K,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀((OH)₂,H₂O) ; widely distributed in argillaceous sediments and derived soils; intermediate in composition and structure between muscovite and montmorillonite; contains less potassium and more water than muscovite, but more potassium than kaolinite or montmorillonite; potassium is generally replaced by calcium and/or magnesium; named from studies by Grimm of shales and clays in Illinois. Compare: pholidoide; phyllite.