Vermiculite

n. any of various hydrated silica minerals which expand into worm-like shapes when exposed to heat and are used for heat insulation and as medium for plant growing

Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by hydration of certain basaltic minerals. The former W.R. Grace mine in Libby, Montana was the largest and oldest vermiculite mine in the United States, but other deposits exist in South Carolina and Virginia, and large commercial vermiculite mines currently exist in South Africa, China, Brazil, and a several other countries.
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n. vermiculite, any of various hydrated silica minerals which expand into worm-like shapes when exposed to heat and are used for heat insulation and as medium for plant growing

n. vermiculite (Geology)

(n.)
A group of minerals having, a micaceous structure. They are hydrous silicates, derived generally from the alteration of some kind of mica. So called because the scales, when heated, open out into wormlike forms.