Vermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it's routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by hydration of certain basaltic minerals. Large commercial vermiculite mines currently exist in South Africa, China, Brazil and several other countries. Structure Vermiculite is a 2:1 clay, meaning it has 2 tetrahedral sheets for every one octahedral sheet. It is a limited expansion… (More on Vermiculite)