<h2><SPAN name="COMMON_MINERALS_AND_VALUABLE_ORES" id="COMMON_MINERALS_AND_VALUABLE_ORES"></SPAN>COMMON MINERALS AND VALUABLE ORES.</h2>
<p class="ac">II.—QUARTZ AND THE SILICATES.</p>
<p class="ac">THEO. F. BROOKINS, B.S.<br/>
<span class="smaller">Principal Au Sable Academy, N. Y.</span></p>
<p>COMPARATIVELY few persons
associate the gem opal, with its
brilliant internal colored reflections,
with that material forming
so large a part of the soil, sand.
Yet the two are almost identical in
composition. The mineral constituent
of sand and of opal is quartz, though
the latter often contains in addition
some water.</p>
<p>Quartz is composed of the two
elements occurring the most abundantly
in the earth's crust, silicon and
oxygen, both non-metals. As already
indicated, the most common representative
of the mineral substance is the
sand of the soil. The sand grains are
generally so eroded by the atmosphere
and surface waters as to show little of
the true quartz structure. A typical
specimen of quartz, commonly known
as "rock crystal," clear, transparent
and approximately perfect in form, is
not difficult to obtain for study. If
not occurring in the particular locality
it may be obtained from a dealer in
minerals at slight expense. As studied
by means of the rock crystal, quartz is
remarkable for its transparency, its
regular crystal form, and its great degree
of hardness. Its transparency is
such that printing may be read through
the crystal. Its crystalline form
affords an unfailing means to the
mineralogist of recognizing the substance
as quartz. If our specimen be
large and perfect, we note that it is
bounded by planes in such manner
that we have a hexagonal prism terminated
at either end by a hexagonal
pyramid. With convenient apparatus
for measurement, we learn the all-important
fact to the mineralogist, that
the angle between any prism face and
an adjacent pyramid face is 141° 47´.
The mineralogist obtains his accurate
measurements by means of an instrument
known as the goniometer. We
may obtain cruder results by bending
a readily flexible wire over the two
faces, perpendicular to the edge of
their intersection, until it is tight
against either face. Then placing one
arm of the bent wire along the base of
a protractor, the point of flexure at the
center of the base, the number of degrees
between the two arms may be
read, thus giving roughly the angle between
the prism and pyramid faces of
the quartz crystal. The great hardness
of quartz is apparent in that it cannot
be scratched with the point of a
knife and that it will cut glass. Often
clear parts of quartz crystals occur
studding the surface of a rock structure,
in the form known as a crystal aggregate.
One property of quartz rock
(any sandstone or quartzite) we must
not fail to notice is the irregular fracture.
This is recognized in the statement
that quartz has no cleavage.</p>
<p>The study of the rock crystal should
not lead us into the false conclusion
that quartz is commonly transparent.
Instead it occurs in various shades and
colors from smoky white through yellow,
red, purple, and brown to black.
The cause of the abundance of sand on
the soil surface is also liable to misinterpretation.
While sand is naturally
of great abundance, yet its commonness
at the surface of the soil is due
largely to its great resistant powers to
the agencies of weathering.</p>
<p>Quartz has an economic value directly
in glass sand and of course as a
soil constituent. In the latter capacity,
it is taken up by many plants, and is
the silica that studs the saw edges of
the blades of sedges and grasses. The
precious stones, agate, amethyst, and
jasper are varieties of quartz.</p>
<table class="sp2 mc w50" title="QUARTZ AND SILICATES.">
<tbody>
<tr>
<td colspan="3"><span class="ac w100 figcenter">
<SPAN name="i_100.jpg" id="i_100.jpg"> <ANTIMG style="width:100%"
src="images/i_100.jpg" alt="" /></SPAN></span>
</td>
</tr>
<tr>
<td class="xx-smaller ac w30">FROM COL. CHI. ACAD. SCIENCES.<br/>
A. W. MUMFORD, PUBLISHER, CHICAGO.</td>
<td class="x-smaller ac w40">QUARTZ AND SILICATES.<br/>
¾ Life-size.</td>
<td class="xx-smaller ac w30">COPYRIGHT 1900, BY<br/>
NATURE STUDY PUB. CO., CHICAGO.</td>
</tr>
<tr>
<td colspan="3"><span class="smaller">1—Asbestus. 2—Feldspar.
3—Quartz Crystal. 4—Small Garnets in rock. 5—Garnet.
6—Opal. 7—Smoky Quartz.</span></td>
</tr>
</tbody>
</table>
<p>The silicon that is so important a
constituent of quartz, composes with
aluminum a large part of various
minerals comprised under the name
feldspar. This substance is slightly
less hard than quartz and has many
<span class="pagenum"><SPAN name="Page_41" id="Page_41">[Pg 41]</SPAN></span>
variations in color; but, unlike quartz,
shows regular cleavage faces. Feldspar
is always crystalline, but good
crystals are not common. It is very
difficultly soluble, yet readily yields to
the influence of weathering. A feldspathic
rock hence readily crumbles.
During the process of disintegration,
the feldspar may change from a clear,
hard, glassy mineral to a dull, opaque
substance. This product of disintegration
is our common white clay. With
quartz, then, feldspar is of great importance
in the forming of soil.</p>
<p>Allied to the feldspar group of
minerals as regards cleavage, and yet
of far different special characteristics
is the class of substances known as
mica. How many of us ever think of
the so-called isinglass of our stove
doors as a mineral substance? Yet
transparent mica, muscovite, is the
source of that household convenience.
A study of the specimens of mica in
our stove door will provide abundant
ideas of the nature of mica. We have
often noticed how, under the influence
of excessive heat, the isinglass splits
into thin sheets, thus showing the
cleavage of the mineral. These plates
of mica are of especial value in giving
cleavage to rocks which would otherwise
fracture irregularly. The cleavage
of slates and of the common shale
rocks is due to the presence of mica
particles which have, at some period in
the history of the earth's crust, through
the action of heat and pressure, been
arranged along definite planes. Isinglass
represents the transparent variety
of mica. Other varieties are brown
and even black, owing to the presence
of traces of potassium, magnesium,
iron, etc., in varying degree. Some
micas do not easily decay, and so we
frequently see glittering particles
among the fine grains of soil and the
sands of beaches.</p>
<p>The minerals already mentioned,
quartz, feldspar, and mica, are the components
of a large part of our granites.
In the case of the red Scotch granite,
another silicate, hornblende, replaces
the mica. Various silicates of economic
value are asbestos, a variety of hornblende,
and augite, which are silicates
of magnesium and iron or calcium;
and talc, which is a silicate of magnesium
containing water. A great number
of gems are found among the silicates,
including tourmaline, garnet, topaz,
beryl, and chrysolite.</p>
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