Technically, Vermiculite encompasses a large group of hydrated laminar magnesium-aluminum-iron silicates, which resemble mica. Here are two keys to the unique properties of Vermiculite. The first is its laminar (or layered) crystalline structure, which provides the hinged plates that make the material expand or unfold in a linear manner, like an accordion. The second is the fact that it contains trapped water, which flashes into steam when heated to force the layers open. There are a great many naturally occurring Vermiculite minerals and soils, and their identification often requires sophisticated scientific analysis.
One of the most common forms of Vermiculite is generally known as commercial Vermiculite. This is the form that is mined and processed for various end uses. It is derived from rocks containing large crystals of the minerals biotite and iron-bearing phlogopite. As these rocks are exposed to the weather, they start to decompose, allowing water to enter and react with the various chemicals present. As the decomposition and chemical reactions proceed, Vermiculite is formed.