Composition,
Structure and Associated Minerals:
This page is about the five minerals known as amphibole
asbestos, one of six minerals described by the term asbestos. The other one
is
chrysotile or serpentine asbestos, and it is described separately.
Asbestos is a trade term that is applied to minerals that are fibrous,
difficult to melt and are poor conductors of heat so that may be used in
making certain products for protection against fire. They also have
structural strength to bind materials like concrete or fireproof coatings.
Most of them are magnesian minerals. These include the fiberous or
asbestiform varieties of chrysotile, amosite, anthophyllite, actinolite,
tremolite and crocidolite. The last five noted are amphibole minerals.
Asbestos occurs principally in rocks that have been crushed and sheared
under great pressure. The fibers are generally very long, fine, flexible,
and easily separated by the fingers. The ancients also called it "amianthus"
(undefiled), in allusion to the ease with which cloth, woven from it, was
cleaned by throwing it into the fire. The name amianthus is now restricted
to the more silky kinds. The term asbestos, in the strictest sense, is
confined to the fibrous forms of actinolite; but commercially speaking,
asbestos includes also fibrous forms of serpentine, an example being the
Canadian asbestos.
Identification and Diagnostics
The amphiboles are distinguished from other minerals by their
crystallization and their cleavage. Before the blowpipe all the members of
the group fuse to a glass which is colorless, green or black, according to
the quantity of iron present. The fusing temperature for tremolite being
about 1290 and for actinolite about 1150. The amphibole asbestos varieties
rich in iron
(crocidolite)
are attacked by
acids. The crystals of the monoclinic amphiboles are short columnar, while
all the aasbestos forms are long and acicular. |
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Occurrence,
Localities and Origins of Amphibole Asbestos:
The asbestos of mineralogy is a monoclinic
amphibole which develops in seams and slips in normal amphibolitic rocks,
especially where the rocks have been subjected to pressure and movement.
Chemically it is a calcium magnesium metasilicate. According to the series
of analyses given by Merrill the silica varies from 52 to 58 per cent., the
lime from 12 to 16 per cent., the magnesia from 20 to 30 per cent. Other
constituents are alumina, varying from 1 to 6 per cent., and ferrous oxide,
usually from 1 to 6 per cent., though in some cases considerably higher.
Water is always present, the amount generally varying between 2 and 5 per
cent. Although contrary to the views of some authorities, the conclusion can
hardly be avoided that the water is an essential constituent and that the
mineral is really a hydrated form of tremolite or actinolite.
The extinction angle appears, however, to be that characteristic of these
amphiboles, or about 18. No experiments appear to have been made as to the
temperatures at which the water is driven off. The normal varieties of
amphibole also hold a little water, but in far smaller quantities than
asbestos. The fibers are polygonal in outline and run out into needle-like
points; down to a diameter of 0.002 or 0.001 millimeter the fibers retain
their uniform diameter and polygonal outlines. The color of amphibole
asbestos is usually white to greenish white. Only the finer kinds are
utilized, but even these are less valued than the serpentine asbestos. They
are apt to be less flexible and somewhat brittle. Most of the small quantity
of amphibole asbestos that has been mined in the United States was of the
tremolite or actinolite variety, and it often occurs in limestones which
have been partly metamorphosed to amphibolitic rocks. The mineral is classed
as slip-fiber or cross-fiber, according to the position of the fibers in the
veinlets. The radial or divergent structures are designated as mass-fiber.
There are many occurrences, mainly in pre-Cambrian rocks along the
Appalachian Mountain system, from Vermont to Alabama.
Several types of asbestos fibers are recognized: cross-fiber, slip-fiber,
and mass-fiber. The cross-fiber asbestos occurs in veins as much as several
inches wide, and the fibers are about normal to the walls of veins. The
slip-fiber occurs on slipping planes, and the fibers are parallel to the
planes of movement. Mass-fiber is found as masses not occupying veins or
slipping planes, and the threads are arranged haphazard or are radiating.
Much of the mass-fiber asbestos is anthophyllite. Old commercial names for
various asbestos varieties include Mountain Cork, Mountain Leather, Mountain
Wood. These are varieties of asbestos described by how they vary in
compactness and in the matting of their fibers.
As a rule, much waste rock is mined with asbestos deposits. In both forms
of asbestos the fibers are easily separated, but the amphibole variety often
contains gritty impurities which are more difficult to remove. The ore is
crushed and the fiber is separated from the waste, usually by means of an
air blast but sometimes by washing with water. It is then sized into various
categories for use in industrial applications from weaving that requires
longer fibers down to the smallest fibers which were used in asbestos paper
manufacture. Since the amphibole deposits of asbestos can be mined
more easily, they are normally cheaper than the chrysotile variety, which,
nevertheless, was in greater demand because more constant in character and
it was suited to a greater variety of uses.
The commercial value of the asbestos depends almost wholly on its property
of being spun, and therefore good asbestos yields long silky fibers when
rubbed between the fingers. Although crocidolite is slightly more easily
fusible but is more resistant than chrysotile to acids and sea water. All
considered, the heat-resisting properties of the various mineral varieties
of asbestos are roughly about the same. Because of the fibrous structure of
asbestos, it is flexibility, incombustible, and because it is a nonconductor
of heat and electricity, asbestos became an exceedingly important economic
product. It has been woven into paper and boards that were used to cover
steam pipes, and to increase electric insulations, and is manufactured into
shingles. The spinning and weaving of fire-proof cloth form was long an
important part of the asbestos industry carried on in the United States.
Asbestos was used in fireproof paints, boiler covering, for packing in fire
safes, and for other purposes where non-conductivity of heat is required. It
was also used in fireproofing, in the manufacture of automobile tires, in
making paints, and as a substitute for rubber in packing steam pipes.
Asbestos was used for making fireproof theater curtains, ropes, fire
resistant clothing, etc. When felted it is a good nonconductor of heat and
electricity and it found many uses as an insulator. The lower grades have
been mixed with cement and manufactured into fireproof shingles (transite).
These were cheap and for some purposes are superior to tile and slate.
Asbestos plaster was used in theaters to deaden noise. Fire proof boards
were made of asbestos and cement.
Because of the health dangers of asbestos, demand has greatly decreased in
recent decades, though it is still used extensively in some third world
countries. In Idaho, 14 miles southeast of Kamiah, anthophyllite rock is
found intruded in mica schist. This rock is quarried and shipped to Spokane,
Wash. Amphibole varieties of asbestos are found at a number of localities in
the crystalline belt of the Appalachians, but at present Sail Mountain in
White County, Georgia, is the only producer, although promising deposits are
known in Polk County, North Carolina, and Bedford County, Virginia, and are
worked occasionally. Most of the low grade amphibole asbestos produced in
the United States is mined in Georgia. At Sail Mountain, Ga., according to
Diller, anthophyllite asbestos occurs in pockets, veins and lenticular
masses in gneiss, which is believed to be an altered igneous rock. Near
Bedford, Va., there are deposits of mass-fiber presumably derived from basic
rocks.
Large deposits of asbestos are known in Russia, in Italy, and Zimbabwe,
Africa. Until about 1895 the small quantity of asbestos used in the United
States came from Italy. After that date the development of the asbestos
industry in Canada was been extremely rapid, and the Canadian mines long
supplied much of the worlds demand for asbestos. In the early 1900s, the
Russian chrysotile deposits from the Ural Mountains and the amphibole
asbestos deposits in southern Rhodesia as well as the crocidolite asbestos
from Griqualand West, Cape Colony, all became important as asbestos usage
increased. In the same time frame, the great increase in asbestos use, as
well as the limited supply of asbestos naturally stimulated prospecting,
and deposits of promise were found in Vermont, Wyoming, California, Montana,
and Arizona.
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