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clay mineral similar to montmorillonite in structure and, in some cases, composition. Vermiculite is typically formed by the alteration of biotite, and it occurs both as large pseudomorphs replacing biotite and as small particles in soils and ancient sediments. It is also formed at the interface between acidic intrusive rocks and basic rocks such as pyroxenites and dunites. Large deposits occur in South Africa, Australia, Russia, and Brazil. In the United States, it is found in Montana and the Carolinas. For chemical formula and detailed physical properties, see clay mineral (table)