Posted: 25/02/2016 at 20:45
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.
Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration of obsidian.
So Vermiculite is a shaped like flat plates and Perlite is little balls.
Both hold water, but in a different way. Perlite soaks it up, but Vermiculite holds it between the plates.
Both are chemically inert so they have no effect on the pH of the compost.