Vermiculite is, primarily, a hydrated magnesium-aluminum-iron silicate mineral. It also contains smaller quantities of K, Ca, Cr, Mn, and Cl. The "expanded" form, which is what most people see, is used in potting soils as has already been mentioned. It is also used as a loose fill insulation and a packing material. It can absorb large amounts of water and still remain free flowing. Bill