Perlite is a naturally occurring siliceous rock. The presence of two to six percent combined water in the crude perlite rock. When quickly heated to above 1600°F (871°C), the crude rock pops in a manner similar to popcorn .
re:Vermiculite is also an expanded mineral like Perlite but with different horticultural uses .......
Vermiculite itself does not contain asbestos; it was just the Libby, Montana mine which was contaminated because of the presence in the mountain of a secondary mineral called diopside.
see ......
http://www.schundler.com/verm-asb.htm
Perlite aerates soil but vermiculite holds water.