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