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(also seybertite, xanthophyllite), a micaceous mineral, a lamellar silicate of the “brittle mica” group; its chemical composition is Ca(Mg, Al)3-2 [Al2Si2O10](OH)2. Clintonite forms brittle, foliated monoclinic crystals, as well as plumose, sometimes radiating, aggregates. It is colorless; some varieties vary in color from yellow to green, owing to an admixture of FeO (for example, valuevite). It has a hardness of 4—6 (hardness is greater along the lateral faces of the crystals than on the cleavage surfaces) on the mineralogical scale and a density of 3,000–3,100 kg/m3. Clintonite usually occurs in contact-metasomatic formations in metamorphosed dolomites and magnesian metamorphic schists with garnet, spinel, magnesioferrite, perovskite, talc, chlorite, and other minerals.