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a mineral, hydrated iron aluminosilicate belonging to the hydromica group. It is characterized by variable and complex chemical composition; its main components are silica (49-56 percent), ferrous and ferric oxides (up to 21 percent), aluminum oxide (up to 18 percent), potassium oxide (up to 10 percent), magnesium oxide (up to 7 percent), and water (up to 13 percent). It crystallizes in the monoclinic system.
The color of glauconite is green (from dark green, almost black, to olive green). Its hardness on the mineralogical scale is 2-3, and its density is 2,200-2,300 kg/m3. Its capacity for absorbing water, as well as for ion exchange, is considerable. It is found among sedimentary rocks as round grains of cryp-tocrystalline structure, less frequently as small crystals of roughly hexagonal shape. It is formed at the bottom of the sea, where it settles as a gel. Cases of the formation of glauconite through the substitution of grains of biotite and other minerals in marine ooze are known. Glauconite is widespread in sedimentary rocks of various ages, mainly Mesozoic and Cenozoic.
Because of its capacity for cation exchange, glauconite is used to soften water and, since it contains a considerable amount of potassium oxide, as a soil fertilizer. It is also used for making green paint.