Vermiculite is a hydrous phyllosilicate mineral. It undergoes significant expansion when heated. Exfoliation occurs when the mineral is heated sufficiently, and the effect is routinely produced in commercial furnaces. Vermiculite is formed by weathering or hydrothermal alteration of biotite or phlogopite. Large commercial vermiculite mines currently exist in Russia, South Africa, China, and Brazil. Vermiculite was first described in 1899 for an occurrence in Millbury, Massachusetts. Its name is from Latin vermiculare, to breed worms, for the manner in which it exfoliates when heated.
It typically occurs as an alteration product at the contact between felsic and mafic or ultramafic rocks such as pyroxenites and dunites. It also occurs in carbonatites and metamorphosed magnesium rich limestone. Associated mineral phases include: corundum, apatite, serpentine and talc. It occurs interlayered with chlorite, biotite and phlogopite.