A soil amendment and medium for root cuttings, vermiculite is a hydrous phyllosilicate mineral and 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
Merriam-Webster Online Dictionary
vermiculite (noun) any of various micaceous minerals that are hydrous silicates resulting usually from expansion of the granules of mica at high temperatures to give a lightweight highly water-absorbent material
Expanded mica, exfoliated vermiculite, hydrated laminar mineral, soil conditioner, horticultural additive
