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chamosite Silicate mineral (see SILICATES) and member of the chlorite group and particularly of the septichlorite group which also includes the minerals amesite, greenalite, and cronstedite; composition Fe2+ 10Al2[Si3AlO10]2(OH)16; soft but massive; occurs in sedimentary ironstones where it may form by alteration from the original siderite ore, and is found in association with chert and clay.