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The vermiculite ores from these mines are different forms of hydrated phlogopite or biotite mica which has the remarkable ability of being able to expand to many times its original volume when heated---a property known as exfoliation. Vermiculite mines are surface operations where ore is separated from other minerals, and then screened or classified into several basic particle sizes.
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.
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. The bulk density of crude vermiculite or vermiculite concentrate is in the range of 640-1120 kg/m3 and exfoliated or expanded vermiculite is in the range of 64-160 kg/m3.