“As Attic Insulation from 1920 to 1970”
Vermiculite was sold as Zonolite attic insulation, used as early as 1920, but very popular from the 1950’s to the late 1970’s. The majority of the vermiculite used worldwide was from a mine in Libby, Montana, owned and operated since 1963 by W.R. Grace. The mine was closed in 1990. As well as being rich in vermiculite, this mine had the misfortune of having a deposit of tremolite, a type of asbestos. When the vermiculite was extracted, some tremolite came in with the mix.
Vermiculite is the mineralogical name given to hydrated laminar magnesium-aluminum-iron silicate that resembles mica in appearance. All vermiculite ores contain a range of other minerals that were formed along with the vermiculite in the rock. Vermiculite ores from some sources have been found to contain asbestos minerals but asbestos is not intrinsic to vermiculite and only a few ore bodies have been found to contain more than tiny trace amounts.
Vermiculite mines are surface operations where ore is separated from other minerals, and then screened or classified into several basic particle sizes. Vermiculite is found in various parts of the world. Locations of the predominant commercial mines are in Australia, Brazil, China, Kenya, South Africa, the USA and Zimbabwe.
When subjected to heat, vermiculite has the unusual property of exfoliating or expanding into worm-like pieces (the name vermiculite is derived from the Latin ‘vermiculare’ – to breed worms). This characteristic of exfoliation, the basis for commercial use of the mineral, is the result of the mechanical separation of the layers by the rapid conversion of contained water to steam. During this exfoliation process, asbestos fibers can potentially be released. The increase in bulk volume of commercial grades is 8 to 12 times, but individual flakes may exfoliate as many as 30 times. There is a color change during expansion that is dependent upon the composition of the vermiculite and furnace temperature.
Vermiculite has been used in various industries for over 80 years. It is used in the construction, agricultural, horticultural and industrial markets. Examples of products that contain vermiculite include potting soil mixes, loose fill insulation, and packing material.