Vermiculite is a hydrated basaltic mineral with some unusual properties which have caused it to be popular in industrial manufacturing since the early 1900s. Many consumers probably interact with vermiculite, since it is used as a filler in a wide range of products, and it can also be found in things like insulation, absorbent materials, and soil mixes. Direct contact with vermiculite is relatively rare, since it is used as a constituent rather than a primary ingredient, although people who work with some forms of insulation may handle vermiculite.
There are two interesting things about vermiculite which make the mineral desirable to manufacturers. The first is its physical structure, which takes the form of crystalline layers like mica. The second is the water trapped inside the vermiculite. If the mineral is heated, the water turns to steam, forcing the mineral to expand, and the layers fold out like an accordion, creating strands of very lightweight, porous material. When vermiculite is used, it is usually heated to expand it, in a process known as exfoliation.
The name “vermiculite” is derived from the Latin word for worm, a reference to the wormy threads of material which form when vermiculite is exfoliated. This property was well known to people in the 19th century, although it was treated as a novelty rather than a potentially useful mineral until people realized its potential for things like insulation and concrete mixing. The mineral was named, incidentally, by Thomas Webb in 1924. It is more formally known as hydrated laminar magnesium-aluminum-iron silicate, which is rather a mouthful.
Some people have raised health concerns about vermiculite. The mineral itself is not harmful, but it often contains impurities which are not healthy, such as asbestos. In addition, the exfoliation process can generate silicate fibers which could be dangerous to inhale. Because removing impurities from the mineral is not really feasible, people should handle vermiculite carefully to reduce the potential exposure to health risks. When handling substances like insulation, nose and mouth protection should be worn so that people do not inhale small shards of silicates which could damage their lungs.
One of the largest sources of vermiculite historically was a mine owned by W.R. Grace in Libby, Montana. Although this mine is now closed, vermiculite can be found in other regions of the United States such as Virginia. There are also large mines in South Africa, China, and Australia. Depending on where it is mined, the mineral may have greater or fewer impurities.