Vermiculite
Vermiculite, any of a number of minerals formed by the combination of certain micas with water. It is most commonly formed from the kind of mica called biotite. Vermiculite is rotproof and fireproof, and in fine particles can absorb up to 500 times its weight in water.
When mined, vermiculite is in the form of flakes made up of numerous thin layers. When vermiculite is exposed briefly to high temperatures, steam produced from the water in the mineral spreads the layers, causing the flakes to expand to as much as 20 times their original size.
Vermiculite is used mainly in its expanded form, primarily in construction and agriculture. Some is used as a filler in paint, plastics, and rubber.
In construction, vermiculite is used in place of sand to make lightweight, fireproof concretes and plasters that provide excellent thermal and acoustical insulation. Vermiculite concrete cannot support heavy weight, but its properties make it useful for roof decks, partitions, prefabricated panels, and firewalls. Vermiculite plaster is used primarily for fireproofing. In the form of pellets, vermiculite is used for insulation in walls and attics.
In agriculture, vermiculite serves as a mulch and as a soil conditioner. It holds water, prevents the soil from crusting, and insulates the soil from heat and cold. Vermiculite is also used to add bulk to some fertilizers and insecticides.