Asbestos is the generic name for six naturally occurring minerals that have been used in commercial products for their strength, flexibility, low electrical conductivity, and resistance to heat and chemicals. It is composed of silicon, oxygen, hydrogen, and various metals.


Asbestos can be divided into two basic groups, serpentine and amphibole, which differ in their physical characteristics. Serpentine asbestos develops in a layered or tiered form, whereas amphibole asbestos has a chain–like structure.


Identification
Asbestos is generally made up of fiber bundles which easily separate into long, thin fibers. Positive identification of a specific fiber type requires microscopic analysis and examination. All asbestos fibers are hazardous to human health.


Common Asbestos Types

The three most common types of fibers are:


• Chrysotile (white asbestos): A white curly fiber, chrysotile accounts for about 90% of asbestos in products and is a member of the serpentine group. When people talk about 'asbestos', this is usually what they are referring to. It is a magnesium silicate.


• Amosite (brown asbestos): Brown or gray, straight amosite fibers belong in the amphibole group, and contain iron and magnesium. Amosite is also called grunerite asbestos and can be found in iron ore mines. The Environmental Protection Agency is currently very interested in incidents of amosite asbestos among iron ore miners


• Crocidolite (blue asbestos/Riebeckite): A member of the amphibole group, crocidolite takes the form of blue, straight fibers. It is a sodium iron magnesium silicate. Mesothelioma usually results from crocidolite asbestos exposure and occurs 30 to 40 years after first exposure


Tremolite and Other Asbestos Types
The other asbestos types, all in the amphibole group, are anthophyllite, tremolite, and actinolite. Anthophyllite ranges in color from white to gray to brown. It is associated with talc and other minerals, and is a magnesium iron silicate hydroxide.

Tremolite is a calcium, magnesium, iron silicate, which is white to grayish green and can be found in metamorphic rocks. Actinolite has a chemical formula that is similar to tremolite, but actinolite contains more iron in relation to magnesium than does tremolite. It appears as dark green crystals or fibrous aggregates.


Geographic Occurrence
Major asbestos deposits can be found in rock formations in the United States, Canada, South Africa, and the former Soviet Union. In the United States, asbestos occurs in 20 states and has been mined in 17 states (Asbestos Facts, U.S. Geological Survey). Sepentinite, the most common source for chrysotile, is present in the Appalachians, Cascades, Coast Ranges of California and Oregon, and in other mountainous areas.

Currently, chrysotile is the only type of asbestos mined on a large scale. Small amounts of tremolite asbestos are still mined in India. Commercial production of crocidolite and amosite ended about four years ago in South Africa.