Re: Vermiculite

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.