Found this on the internut at The Garden Geek...

Up until 1990, about 70 to 80% of the vermiculite used in the U.S. was mined in Libby, Montana. The processed vermiculite from Libby was often sold under the trade name "Zonolite". W.R. Grace was the company that owned the mine from 1963 - 1990. A problem specific to the Libby mine was that the vermiculite deposit was also associated with tremolite asbestos, a rare naturally-occurring mineral. Health problems associated with asbestos have been known for many years; however, it is uncertain just how long the health problems have been known at the Libby plant. Due to the high amount of asbestosis, lung cancer, and mesotheloioma related deaths among workers and family members associated with the Libby plant, the mine was closed down in 1990. Because ten years have passed since the closing of the mine, it is unlikely that any of the vermiculite used today comes from the Libby, Montana mine.

Vermiculite is currently mined in South Carolina and Virginia as well in South Africa, Brazil, China, Egypt, India, Japan and Russia. The largest mine today is located in South Africa. No asbestos related disorders have been reported in any of the major vermiculite mines except for the Libby, Montana mine. Testing done by EPA and the Ontario Research Foundation found that no asbestos could be detected in the vermiculite from the South Africa mine. Based on these reports, it can be reasonably assumed that vermiculite used today does not contain significant amounts of asbestos.

When using vermiculite, as well as any other material that inherently contains dust sized particles, it is recommended that dust control measures be adopted and/or personal protective equipment (dust masks) be used to protect against dust inhalation. Keeping the vermiculite moist will greatly reduce dust problems.

If it is known that Libby vermiculite was used in a garden bed, health risks can be reduced by simply covering the area with sod or mulch to ensure that dust from the area is kept to a minium. As a last resort the area can be excavated by professionals and sent off to a landfill.

For further information on vermiculite and health related issues please refer to the following web sites:

http://www.vermiculite.org
http://www.epa.gov/region01/qa.html
http://www.mcn.net/~vermiculite/overview.htm