Geological Museum KIGAM

(7) Asbestos

Asbestos is a set of six naturally occurring silicate minerals, which all have in common their eponymous asbestiform habit: long (roughly 1:20 aspect ratio), thin fibrous crystals, with each visible fiber composed of millions of microscopic "fibrils" that can be released by abrasion and other processes. They are commonly known by their colors, as blue asbestos, brown asbestos, white asbestos, and green asbestos.

Six mineral types are defined by the United States Environmental Protection Agency (EPA) as "asbestos" including those belonging to the serpentine class and those belonging to the amphibole class. All six asbestos mineral types are known to be human carcinogens. The visible fibers are themselves each composed of millions of microscopic "fibrils" that can be released by abrasion and other processes.

Serpentine class fibers are curly. Chrysotile is the only member of the serpentine class. It has been used more than any other type and accounts for about 90% of the asbestos market. Chrysotile appears under the microscope as a white fiber. It is more flexible than amphibole types of asbestos, and can be spun and woven into fabric. Chrysotile is obtained from serpentinite rocks, in which occurring as veins, fibers or masses. It is formed by metamorphism or metasomatism of ultramafic igneous rocks.

Amphibole class fibers are needle-like. Amosite, crocidolite, tremolite, anthophyllite and actinolite are members of the amphibole class. Amosite, often referred to as brown asbestos, is a trade name for the amphiboles belonging to the cummingtonite-grunerite solid solution series. Crocidolite, is the fibrous form of the amphibole riebeckite, found primarily in southern Africa, but also in Australia and Bolivia. Crocidolite is seen under a microscope as a blue fiber. Other asbestos minerals, such as tremolite, actinolite and anthophyllite asbestos, are less commonly used industrially but can still be found in a variety of construction materials and insulation materials.

All types of asbestos fibers are known to cause serious health hazards in humans. Prolonged inhalation of asbestos fibers can cause serious and fatal illnesses including lung cancer, mesothelioma, and asbestosis (a type of pneumoconiosis). The International Agency for Research on Cancer (IARC) has classified asbestos as being carcinogenic to humans.