Distribution of Asbestos
Geographical Distribution on Earth
In its natural state, asbestos occurs throughout much of the planet: indeed, it is found in two-thirds of the rocks in the earth's crust. The following discussion concentrates on the places where asbestos had or still have industrial and economical importance.
The greatest percentage of asbestos mined is chrysotile, which is found over most of the world's crust, the amphiboles, including the rest five of asbestos, are distributed much less widely, being confined mainly to relatively small deposits in South Africa, China and Australia.
Corcidolite, or "blue" asbestos has been mined only from Northwest Cape, Transvaal and Paakkila mines in Cape Province South Africa, Western Australia and Bolivia. Only the South African mines are still active.
In addition to its crocidolite mines, clay deposits containing naturally occurring crocidolite are also found in two areas of China where residents using the crocidolite clay extensively in their homes for making stoves, bricks and tiles have high rate of asbestos related health problems.
In Western Australia, the amphiboles occur as major components of the "greenstone belts" where most of the State's major gold and nickel deposits are found and in the banded iron formations of the Hamersley.
It's believed that the process that concentrates most iron and nickel into viable ore deposits alters asbestos into harmless secondary minerals, such as talc, chlorate, clay, iron oxides and hydroxides and various silica species. The small and isolated pockets of more intact occurrences scatter among the altered mineral deposits and form the basis of the blue asbestos mining industry at Wittenoom, which was one of the major world sources of crocidolite. Crocidolite was commercially mined and milled at Wittenoom Gorge from 1943 to 1966.
Qu¨¦bec where the world's largest asbestos mines, where chrysotile, or "white" asbestos is mined. It used to supply the U.S. with 90% of its asbestos demand. Chrysotile is still being mined, primarily in Canada, Africa, and the former Soviet Republic.
Asbestos in the United States
Asbestos has been identified in 20 States (see figure) and mined in 17 States over the past 100 years. It is found in many common rocks. Serpentinite, the most widely occurring host rock for chrysotile, is present throughout the Appalachians, Cascades, Coast Ranges of California and Oregon, and other mountain belts.
In general, chrysotile and amphibole asbestos varieties occur in areas where the original rock, under elevated temperatures and pressures, has been changed by heated fluids (a process referred to as metamorphism). This type of altered rock occurs predominantly along the eastern seaboard from Alabama to Vermont, along the western seaboard from California to Washington, and in the upper Midwest in Minnesota and Michigan. Small occurrences of asbestos are in other areas, such as Arizona, Idaho, and Montana.
Although asbestos can be present in most of the metamorphic rocks described above, the bulk of the rock mass does not contain asbestos. In fact, most commercial asbestos deposits contain less than 6 percent asbestos by volume. Only a few deposits contain 50 percent or more asbestos (such as chrysotile deposits near Coalinga, Calif.).
The NOA Problem
Naturally Occurring Asbestos, or NOA, has been a problem in California, because asbestos fibers occur naturally in the soil associated with ultramafic rock formations in 44 of California's 58 counties. The worst areas include the Coastal Ranges, the
Klamath Mountains, and Sierra foothills, where NOA is commonly exposed near faults.
Type of Occurrence
Most common type of chrysotile deposit grow with the fibers at right angles to the walls of cracks (cross-vein) in massive serpentine formations. In some occurrences, the relative motions (slipping) of blocks in the host rock, during or after fiber growth, lead to veins in which the fibers are inclined or parallel to the vein axes (slip fibers). In still other local conditions, dispersed aggregates of fibers are found with no preferential orientation but with high fiber content, up to 50% of the surrounding rocks. These are called mass-fiber deposits.
The Amphiboles occur as small and isolated pockets, irregular veins in the more metamorphism-altered rock formations.
Relative Abundance of Asbestos
Chrysotile is more abundant than amphiboles. It occurs in two thirds of the Earth¡¯s crust.
As a matter of fact, figures show that between 90% and 95% of all asbestos found in buildings and other commercial products that contain asbestos is of the Chrysotile variety.
The amphiboles are found in limited areas, mainly in parts of Australia, Africa, China and Parts of former Soviet Union.
Asbestos in Building Materials
Asphalt Floor Tile
Ceiling Tiles and Lay-in Panels
Construction Mastics (floor tile, carpet, ceiling tile, etc.)
Ductwork Flexible Fabric Connections
Electrical Panel Partitions
Elevator Brake Shoes
Elevator Equipment Panels
Heating and Electrical Ducts
High Temperature Gaskets
HVAC Duct Insulation
Laboratory Hoods/Table Tops
Packing Materials (forwall/floor penetrations)
Pipe Insulation (corrugated air-cell block)
Taping Compounds (thermal)
Thermal Paper Products
Vinyl Floor Tile
Vinyl Sheet Flooring
Vinyl Wall Coverings