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Occurrence,
Localities and Origins of Chrysotile Asbestos:
This page is about Chrysotile, one of six
minerals described by the term asbestos. The other five, which are known as
the amphibole asbestos minerals, are 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.
Chrysotile asbestos is green or yellowish-green and is easily reduced to a
white fluffy state. The fiber is short, but of very uniform diameter and
great divisibility and flexibility; the decomposing effect of hydrochloric
acid also distinguishes it from amphibole asbestos. In composition it is
practically identical with the purer kinds of serpentine. This variety is
found as veinlets, rarely over 2 inches thick, in serpentine or peridotite,
and has almost always a cross-fiber that is, the silky fibers lie
perpendicularly to the plane of the veinlet. The pure yellowish-green
serpentine which occurs in contact metamorphic limestone and which is an
alteration product of diopside sometimes contains chrysotile of
exceptionally high grade. A deposit of such material is now worked in
Arizona, northeast of Globe.
Chrysotile veinlets may be found in almost any serpentine body, but they are
rarely so abundant and large as to be of economic importance. The views
regarding their origin differ. Dresser shows that serpentinization in the
Canadian deposits proceeded along irregular cracks in the peridotite, and
the chrysotile veinlets are found in the center of the serpentinized bands.
These veinlets were interpreted by Pratt and Merrill as fillings of
contraction cracks, but other authors are probably correct in considering
them the result of a recrystallization of the serpentine, proceeding inward
from the cracks. Taber believes that all cross-fiber veins are formed by a
process of lateral secretion, the growing veins pushing aside the enclosing
walls. Since, however, the material in the veins is derived from the
serpentine itself it is not apparent why there is any need of increase of
volume.
Several classes 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.
Most of the cross-fiber asbestos, which is the highest grade, is chrysotile.
Old commercial names for various asbestos varieties include Mountain Cork,
Mountain Leather, Mountain Wood. These are varieties of asbestos described
based on 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. The fibers of chrysotile are generally shorter than those
of the
amphibole asbestos, rarely exceeding 2 inches in length, but they
have greater strength. Since the amphibole deposits of asbestos can be
mined more easily, it is 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 with
Canadian raw material. 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. Chrysotile has been mined in the Province of Quebec, Canada, just
north of the Vermont line; Vermont; New York; New Jersey; Idaho, Wyoming,
California, and from the area around the Grand Canyon in Arizona, etc. The
most productive deposits of chrysotile asbestos in North America are in
Canada, in a belt of igneous rocks that extends southwestward into northern
Vermont. The Canadian deposits are centered at Asbestos Hill near Thetford,
in the eastern townships of Quebec. The asbestos occurs as cross-fiber
veins, closely spaced in a serpentine that has probably been formed by
alteration from peridotite. These rocks are in places accompanied by
somewhat later gabbro and granite and all of them are intrusive into
Ordovician sediments. The veinlets, which are from a fraction of an inch to
several inches thick, are believed to be alteration products of the igneous
rocks in which they are found. The chrysotile is mined in open pits, one of
which, for instance, is 700 feet long, 200 feet wide, and 165 feet in
greatest depth. A small percentage is obtained by hand cobbing, but the
larger part, up to 60 percent of the crude material quarried is crushed and
screened, and the fibers are separated by air currents. The extraction of
fiber of the milled rock is from 6 to 10 per cent.
Veins of chrysotile asbestos in serpentine were worked in Orleans and
Lamoille counties, Vermont. The Vermont deposits, which were discovered in
1900, occupied a rather limited area in a large serpentine belt. Two types
of chrysotile are found here, one forming branching veinlets similar in
character and quality to the Canadian fiber, the other, of inferior quality,
occurring as short fibers on slickenside fault surfaces. Near Casper,
Wyoming, veins of cross-fiber crysotile asbestos occur in serpentine, which
also is probably an alteration product of peridotite. Arizona was the
principal producer of spinning asbestos in the United States. The deposits
consist of chrysotile of the cross-fiber type, and the veins occur in cherty
limestone, probably of Algonkian age. They yield a product of spinning
grade. In the Sierra Anchar, north of Roosevelt, in Gila County, the
deposits occur in limestone near diabase sills. Mount Baker is in general
capped by thick horizontal sandstones, beneath which extends the
asbestos-bearing limestone, intruded and split by the great sill of diabase.
The upper surface of the diabase is irregular. In some places the diabase
cuts up through the limestone and completely envelops large fragments of it.
At such places the limestone may be fissured and asbestos is likely to be
most abundantly developed.
At Ash Creek in Arizona the relations of the rocks are essentially the same.
The asbestos veins occur near the diabase, for the most part in the lower
portion of the limestone. Their position suggests that the serpentine and
asbestos are the result of hydrothermal metamorphic action of the intruding
diabase upon the limestone. The asbestos is generally found in greatest
abundance in the fissured limestone, where the heated waters that
accompanied the intruding diabase magma may have penetrated the limestone
and converted it into serpentine. The asbestos is said to be most abundant
near the surface of the ground, possibly owing to the leaching of the
limestone, which is dissolved and removed more readily than asbestos.
Chrysotile asbestos is found 28 miles south of Lander, Wyo. It is in part of
spinning grade and occurs in cross-fiber veins along the contact of a dike
of pyroxenite or its alteration product, serpentine, and gneissoid rocks or
schists. The dike has a width of 400 feet. The serpentine includes fragments
or lenses of micaceous schist, and asbestos has been formed locally about
these inclusions, as well as on both sides of the serpentine dike.
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 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 chrysotile asbestos naturally stimulated
prospecting, and deposits of promise were found in Vermont, Wyoming,
California, Montana, and Arizona.
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