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).

When rapidly heated to about 300° C (570° F), vermiculite can expand to 20 times its original thickness; its name, from the Latin word meaning “to breed worms,” alludes to this property. In its natural state the mineral has little commercial use, but exfoliated vermiculite is extremely light (specific gravity as low as 0.09) and is used in lightweight concrete or plaster, for thermal and acoustic insulation, or as a packing medium, a soil conditioner, a starting medium for seeds, and a filler or extender in paper, paint, or plastics.