Of the several zirconium minerals known only two, viz., zircon and
baddeleyite are of commercial importance.
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name asbestos came from ancient times from a Greek word meaning
unquenchable in reference to its resistance to fire and heat. For many
centuries, small cloths woven from asbestos were a luxury item, for
handling of hot items in kitchens and foundries.
"Asbestos" refers to a small number of minerals that are
formed of flexible fibers, and have the useful physical property of
being very heat resistant. Because asbestos forms as flexible fibers,
it is woven to make fabrics for heat-resistant and insulating
Chrysotile asbestos, the fibrous variety of the mineral serpentine,
is by far the most important type of asbestos. It forms in metamorphic
rock, that is, rock that has been altered by intense heat and
pressure. Another asbestos mineral is called crocidolite. Crocidolite
is a dark blue variety of the mineral riebeckite. Crocidolite occurs
in metamorphic rock. Only about 4% of the asbestos consumed is
crocidolite. Other, less important asbestos minerals in occasional use
are amosite, anthophyllite asbestos, tremolite asbestos, and
Asbestos has a very high melting point. This, together with the
flexible nature of the fibers, helps to determine its usefulness.
Since the discovery in the mid-Twentieth Century that asbestos can
cause a fatal lung condition, the mining and use of asbestos has
decreased, and has become closely regulated in all countries.
Asbestos fibers have been used in many different applications in our
industrialized society. The most common uses of asbestos have been:
and acoustic insulation
- Asbestos - concrete
- Plastic products (vinyl floor tiles)
- Paper products
- Gasket, packaging
- Roofing felts, papers
the United States, commercial use of asbestos began in the early
1900's and peaked in the period from World War II into the 1970's.
Under the Clean Air Act of 1970 the Environmental Protection Agency
(EPA) began regulating many asbestos-containing materials which, by
EPA definition, are materials with more than 1- percent asbestos.
Generally, asbestos fibers are long, thin, strong, fireproof,
flexible, and resistant to the action of many chemicals. Chrysotile,
the most common asbestos type, is usually white or off white, with
long wavy fibers. In some mines, the fibers are exceedingly long and
flexible, making this chrysotile excellent for weaving into fire and
heat resistant cloth. Shorter chrysotile fibers were more apt to be
used as binders and strengtheners in plastics, cement or insulation.
Amosite is usually brown or tan, with much straighter and fibers than
chrysotile. Amosite was the second most commonly used type of
asbestos, comprising approximately 5%of the asbestos placed in
buildings and factories.
Crocidolite is a fairly rare form of asbestos, unique because of its
very obvious blue color. Crocidolite fibers appear long and straight,
much like Amosite. The remaining three asbestos types were not
extensively used in commercial products in the United States.
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.
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.
Health Hazards of Asbestos
There are three principal diseases which have been shown in
laboratory experiments to be caused by exposure to one or more of the
asbestos minerals. These are:
cancer, which includes cancer of the trachea, bronchus, and lung
a cancer of the pleural and peritoneal membranes which invest the
lung and abdominal cavities, respectively.
is", a diffuse interstitial fibrosis of the lung tissue, often
leading after long exposure, to severe loss of lung function and
ultimately respiratory failure.
history of asbestos as a carcinogen is an interesting saga-- one that
is clearly not complete. Although there seems to be no question that
the residency of large amounts of asbestos fibers within the lung and
pleura for long periods of time can cause lung cancer, asbestosis and
mesothelioma, the exact mechanism of disease production and the
relative potencies of the different asbestos minerals are subjects of
current study and lots of debate. To illustrate the problem, following
the discovery of the laboratory connection between cancer in rats to
exposure of large quantities of asbestos, the next step was to search
for a correlation between human exposure to asbestos and cancer. Such
a study was made of individuals in the small mining community of
Thetford, Quebec in the early 1970's.This community was/is essentially
built around a mine and mill devoted to the production of chrysotile
asbestos. By studying the mortality rate of male workers since the
asbestos mine opened, it was determined that an unusually high
percentage of cancer-caused deaths occurred in this community relative
to the rest of Canada. This study was responsible for the beginning of
the awareness in the public sector that asbestos causes cancer. The
study was, however, scientifically flawed. It failed to report or
consider the fact that nearly all (85-90%) of the blue-collar workers
at Thetford Mines also were heavy smokers. The media and the more
radical environmentalists used this flawed study to promote the ban on
More recent studies of cancer mortality rates among workers in the
asbestos mining and milling professions paint a less-disastrous
picture. Owing to the fact that the potential hazard is lung cancer,
and that most workers exposed to asbestos also have tended to be heavy
smokers, it is not currently possible to statistically prove a
relationship between asbestos exposure and cancer in a general (i.e.,
normal-smoking, non-asbestos profession) population. Several important
indications are, however, available:
miners and millers, exposed to large concentrations of chrysotile
and tremolite-actinolite-anthophyllite dust over long periods of
time (10-30 years) and who also smoke, have suffered a significant
excess mortality due to lung cancer and asbestosis -- but not to
mesothelioma. There is a strong indication that the risk of lung
cancer is compounded in those who smoke and work in the asbestos
industry. Indeed, the combination may lead to increased risk of
cancer that is up to 100 times greater than the average.
miners and millers, exposed to large concentrations of crocidolite
and amosite dust over moderate periods of time (5-10 years) and have
suffered a significant excess mortality due to lung cancer,
asbestosis and to mesothelioma. The amphibole fibers used
commercially (amosite, crocidolite) are extremely hazardous. Because
of their chemical structure and straight, needlelike fibers,
amphiboles are very dusty, as well as highly biopersistent. Once in
the human body, they can remain indefinitely in the lung tissue, and
may cause cancer and mesothelioma.
the most common serpentine fiber, is considerably less hazardous
than ampibole varieties. Silky in texture, with curly fibers,
serpentine asbestos is unlikely to remain suspended in the air.
Thus, less of it is inhaled, and it does not stay in the lungs very
long. The human immune system can eliminate these fibers fairly
quickly. The Health & Safety Executive (HSE) of Great Britain
recently concluded that, like asbestosis, the appearance of lung
cancer linked to chrysotile is a threshold phenomenon, meaning that
there is an exposure level below which the health risk, if any, is
so low as to be undetectable. Moreover, the HSE confirms that very
few cases of mesothelioma are attributable to chrysotile, despite
extensive exposure of thousands of workers in the past.
one example, McDonald and others (1980) found that for men exposed to
chrysotile dust averaging 20 fibers/cm3, the total mortality was less
than expected in a population of workers not exposed to asbestos.
Exposures to 20 fibers/cm3 are about an order of magnitude greater
than those currently experienced in asbestos mines and mills; thus,
chrysotile miners working a lifetime under the present dust levels
should not be expected to suffer any measurable excess cancer. It
turns out that about 95% of the asbestos in the current US market is
chrysotile. Given these data, has our recent concern about asbestos in
schools and homes really been justified?
At the present time it seems that many of the benefits obtainable
from asbestos may be retained with minimal health risk through
utilization of the common chrysotile form of asbestos, provided that
dust emissions are controlled. It appears that instead of treating all
asbestos minerals as equally potent carcinogens, each mineral should
be examined on its own merits with regard to its usefulness to society
and its potential to cause disease.
Although asbestos fibre has been known to man since antiquity, it was
not until the end of the nineteenth century that it came into
industrial and commercial use. Its excellent insulating properties,
durability and fire retardance have resulted in its use in a variety
of industrial applications.
Asbestos-cement pipe has been used in water distribution systems
around the world for more than 70 years. Pressure pipe is used
primarily for the distribution of potable water, sewer force mains and
industrial effluent process piping. Non-pressure pipe is used for
sanitary and storm drainage systems, casings for electric cables or
Asbestos-cement construction products include flat and corrugated
sheets used for commercial and residential buildings, roofing shingles
Asbestos fibres are combined with a resin or woven with metallic
fibre to produce temperature resistant and durable friction linings.
They can also be mixed with various additives, then molded to produce
disk and drum brake linings and clutch facings.
There are close to 3000 known product applications for asbestos,
though today its use is limited primarily to high density products in
which the asbestos fibre is locked into a cementitious or resinous
matrix. Modern-day products include asbestos-cement (pipe, roofing
tiles and sheet), friction materials, gaskets, specialty papers and
additives, with asbestos-cement accounting for approximately 85% of
chrysotile fibre consumption.
1995 World Production (metric tonnes)
|Commonwealth of Independent
The majority of the 500,000 metric tons of chrysotile asbestos
produced annually in Québec is exported. Canada uses some 6,000
tons of chrysotile asbestos every year. Friction products, composite
materials and asbestos textiles are the main domestic market outlets.
Unlike most countries, Canada no longer manufactures chrysotile-cement
products. In the residential sector, the North American market has
turned to wood because of the availability of this raw material. On
the infrastructure side, which includes networks of water mains made
of asbestos-cement pipe, the work is for all intents and purposes
complete. Though no longer manufactured in Québec, 2,000 to
3,000 tons of asbestos cement are imported and used on construction
sites each year.
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