Asbestos, its Chemical and Physical Properties

Second in a series of articles on asbestos: Its history, chemical and physical properties, uses, health hazards and the legal implications of asbestosis & mesothelioma.

By Roberta C. Barbalace

[Oct. 2004]

General description of Asbestos

Asbestos is the name given to a group of fibrous, naturally occurring silicate minerals. They generally exist in nature in metamorphic or altered basic and ultrabasic igneous rocks. While the name "asbestos" goes back to ancient times, the Environmental Protection Agency created a legal definition for the word asbestos by limiting the term to 6 specific fibrous minerals from two distinct groups: chrysotile (from the Serpentine group); and amosite, crocidolite, tremolite, actinolite and anthophyllite (from the Amphibole group). OSHA defines an asbestos fiber as having a length > 5mm and a length:width ratio of 3:1. EPA, on the other hand, defines a particle as a fiber if the ratio is >5:1 length:width when analyzing bulk samples.

Asbestos is resistant to heat and most chemicals (most forms are chemically inert). The fibers do not evaporate into air or dissolve in water. They have no odor or smell and do not migrate through soil. Indeed, even in prehistoric times, it was considered to be the wonder mineral that had limitless uses. In modern times at least 5,000 different products have been manufactured from asbestos.

Chemical and Physical Properties

For all practical purposes most forms of asbestos are inert. That is one quality that makes them so desirable in industry. They are insoluble in water and organic solvents and are nonflammable. While the serpentine chrysotile is soluble in acid, the amphiboles amosite, crocidolite and anthophyllite are resistant to acids. Interestingly enough, most asbestos minerals have non-asbestos counterparts with identical chemical compositions. In the case of tremolite, anthophyllite and actinolite, the word "asbestos" is added after the mineral name to distinguish it from the non-asbestos form. Chrysotile, crocidolite and amosite do not require "asbestos" to be added because the non-asbestos forms have different names.

Fibrous forms and non-fibrous forms of a mineral may occur in the same deposits. That would make sense when one considers how asbestos is formed. It would seem very likely that temperature and pressure might be sufficient to metamorphose completely some but not all of the igneous rock into the asbestos form.

Physical characteristics of Asbestos

Asbestos has a host of physical properties that make it almost a superstar in the world of industrial chemistry. Its tensile strength surpasses that of steel. It has tremendous thermal stability, thermal and electrical resistance and is non-flammable. It can be subdivided into fine fibers that are strong enough and flexible enough to be spun into material that is a flame retardant, chemically inert thermal and electrical insulator. Note that asbestos binds with better insulating materials to create the ultimate construction materials.

Asbestos fibers have no detectible odor or taste. They are all solids that do not move through soil and are insoluble in water. Its color will vary according to type, and metallic composition. Crocidolite, which has iron and sodium as its only metallic elements, is the most colorful, adorned in a range of colors including shades of lavender, blue and green. In general, asbestos-containing iron may display a green color ranging from a hint of green to solid green depending upon the amount of iron present.

Tremolite contains no iron, but is part of a continuous mineral series with actinolite, in which iron and magnesium can freely substitute with each other. As a result, some specimens of tremolite may show a hint of pale green. Chrysotile and tremolite, which in pure form contain no iron, tend to be white, together with actinolite and anthophyllite are grouped together as "white asbestos" and classified as UN2590 (under the United Nations chemical ID numbering system). Amosite and crocidolite are classified as UN2212. Amosite and crocidolite have been used extensively for commercial use, and are considered to be extremely hazardous. Chrysotile is more flexible and has been considered to be less hazardous than either amosite or crocidolite. Until now, anthophyllite, actinolite and tremolite have been lumped with the "lesser evil" chrysotile under the UN Identification numbering system. Their occurrence in industry has been less extensive. Tremolite has been used in laboratories for filtering chemicals. Actinolite is used for industrial asbestos. There is not much reported use of anthophyllite. All three of these amphiboles also have non-asbestos forms associated with them in nature.

Unique structure

Asbestos can be subdivided into two major classifications of minerals: amphiboles and serpentines. All but one form, chrysotile, are amphiboles. Chrysotile is a serpentine. Both amphibole asbestos and serpentine asbestos are fibrous, but they have very different forms.

The amphiboles are double-chain silicates also called inosilicates. The basic structural unit is (Si4O11)-6 with side groups that are responsible for the overall amphibole structure. Amphiboles are distinguished from one another by the amount and positioning of metal atoms including: sodium, calcium, manganese, magnesium, iron(II), iron(III) and aluminum. There is a complete solid solution between Na and Ca end members and between Mg and Fe end members.

The serpentine group of minerals has the formula Mg3Si2O5(OH)4 . Serpentine structure is a bending sheet. There are only three known serpentines. Two are massive and fine grained (not asbestos form). Chrysotile is the only one in which the sheets are continuous and bend to form continuous tubes, which give the mineral the fibrous habit related to asbestos. Chrysotile is very flexible and less likely to be "friable" than the amphiboles. Friability of asbestos is generally defined as the ability to easily be turned into a dust with finger pressure. It is this friability that releases asbestos fibers into the atmosphere and results in health problems.

Asbestos in nature and in synthetic products

There is a tendency for people to think that all chemicals that are hazardous to health were created in a laboratory. Asbestos is a naturally occurring mineral that is found throughout the world. It has been estimated to exist in two thirds of the rocks in the earth's crust. It is frequently found with its non-asbestos counterpart (a mineral with the exact same chemical formula, that has not been altered by extreme pressure and heat). Asbestos fibers are released into the environment through erosion and carried by the wind. Humans inhale an average of 10,000 to 15,000 asbestos fibers a day. Occasional low-level exposure to asbestos appears to present no health issues. Continuous exposure to elevated levels on the other hand, can be deadly.

Asbestos has been used in over 3,000 products in home and industry. The automotive industry has used insulation in brake shoes, valve packing materials and clutch facings. The building industry is responsible for the vast majority of asbestos use and can be found almost anywhere in buildings. A partial list of these building include: pipe insulation, asphalt floor tile, vinyl sheet flooring, acoustical and decorative plaster, cement products (wallboard, siding, pipes, panels, facings, extruded products, ducts, sheets, shingles), heating and electrical ducts, textured paints, roofing shingles and felt, wallboard blown in insulation, spray applied insulation, vinyl wall coverings, and all sorts of sealants. Friable asbestos use was discontinued in the late 70s, but it can still be found in older buildings.

Is asbestos gone yet?

Because of its association with diseases like mesothelioma and asbestosis, new uses of asbestos have been banned in the USA by the Environmental Protection Agency. In October 1991, a U.S. federal court overturned an EPA regulation that banned most uses of asbestos by 1997. Only products that were not being manufactured, imported or processed on July 1989 remain subject to the prohibition of asbestos. While exposure to asbestos has been lowered as a result of abatement programs, it has recently reared its ugly head in materials that were never thought to contain asbestos. Vermiculite mined from Zonolite Mountain in Libby, Montana was found to contain the asbestos types tremolite and actinolite as well as four other mineral types (winchite, ricterite, fero-edenite and magnesioarfvedsonite) that are not on the EPA list of asbestos minerals, but none the less have all of the properties of asbestos. Evidence supports the belief that these "non-asbestos forms" have resulted in asbestoses and mesothelioma among mine workers in Libby Montana. In addition, friable asbestos present in asbestos mined at Libby, Montana under the name Zonolite may be a health hazards for nursery workers, installers of popped vermiculite insulation and individuals living or working in one of the millions of buildings that contain Zonolite insulation. Some industrial grade talc has also been found to contain asbestos. Cosmetic grade talc, on the other hand, appears to be asbestos free.

In answer to the question "Is asbestos gone?", no, it is not gone, nor will it ever be gone. Unlike PCBs, DDT and dioxin asbestos is part of the natural environment. The answer is not to be totally rid of it, but to maintain a working and living environment in which airborne asbestos is in a concentration low enough that it will not result in impaired health or death from diseases like asbestosis, mesothelioma, etc. When asbestos is found to be in a state in which it is non- friable and unlikely to become friable, the present trend is to leave it alone. Sometimes removal of non-friable asbestos from buildings can increase rather than decrease the chances for exposure to friable asbestos. If it is found to be friable (crumbling and powdery, or of a type that by its very nature is likely to become airborne), removal is frequently the only answer. This process must be left to asbestos abatement specialists. Most importantly, asbestos management must also be controlled at the source. How is it entering industrial and consumer products? At one time the only pathway considered to be important in introducing asbestos to our homes and businesses was via products in which it was an intentional ingredient. Suddenly it has been found to be an incidental impurity (as in vermiculite and talc). In addition, mineral types previously thought to be unassociated with the asbestos minerals have been found to have asbestos-like properties. Testing for asbestos at the source is necessary to limit exposure to workers and the general public. It will require government, industry and the general public working together to face the new challenges of asbestos contamination.

Related Articles


  • Agency for Toxic Substances and Disease Registry. Anne Olin, Editor. Asbestos, Chemical Specific Health Consultation Tremolite Asbestos and Other Related Types of Asbestos. 2001.
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