Quartz is a mineral of wide-spread occurrence which is characterized by
the following properties: (1) it is considerably harder than glass or
steel, (2) it has a high luster, glassy to oily, (3) it breaks with an
irregular or rough glistening fracture, and (4) it crystallizes in
six-sided crystals when it grows unobstructed. Ordinary acids do not
attack quartz, and it is relatively unaffected by chemical weathering in
Missouri. Its composition is silicon dioxide, SiO₂.
Quartz occurs in granite as the lustrous, partially rounded grains which
constitute perhaps 20% of the rock (feldspar makes up most of the more
opaque remainder which breaks with many small flat faces), and is
recognized in the small glistening grains in the porphyry. Hence it is
an important igneous rock-forming mineral.
Sandstone in Missouri is made almost entirely of quartz grains which
have been broken, worn, and more or less rounded during their long
travel history. The sandstone formation quarried and mined near Pacific
and Crystal City, named the St. Peter sandstone formation, is an
outstandingly pure quartz sandstone and therefore usable in the glass
industry. It is obvious that quartz is an important constituent of
sedimentary rocks.
[Illustration: Lustrous, translucent quartz. The irregular fracture
and oily luster are characteristic.]
Further, in the sedimentary dolomite formation near Potosi, fine to
coarse quartz crystals line the surfaces of cavities and pockets in the
stone. This cavity coating of quartz which reflects light brilliantly
from the many small crystal faces is called drusy quartz by the
mineralogist, but is locally and popularly known as “blossom rock.”
Thousands of pounds of “blossom rock” are sold each year for rock
gardens and various ornamental purposes.
In northeastern Missouri quartz crystals line the hollow, more or less
spherical bodies called Geodes, which vary in size from small nuts to
melons, and weather out of the so-called Warsaw formation. Other types
of hollow cavities in many Missouri rocks may contain quartz growing
inward from their walls.
Missouri chert is composed primarily of quartz in microscopically fine
grains; likewise, agate and petrified wood may contain abundant quartz.
Other varieties are rock crystal, rose quartz, amethyst, false topaz,
bloodstone, carnelian, and onyx.
Quartz crystallized in an igneous rock as the hot fluid cooled through
its “freezing” temperature interval, which was probably not below
1000°F. In the cases of the quartz in geodes, the drusy quartz, or that
in cavities within petrified wood, quartz crystals grew from ground
water solutions which must have carried very low concentrations of
silica in solution, and whose temperatures did not depart far from that
of rocks buried at various depths today. Although quartz is a very
common and abundant mineral, our specific knowledge about its transport
and deposition is surprisingly meager.
[Illustration: Quartz crystal cluster. Crystals are six-sided. From
Arkansas.]
Quartz crystals are used in large quantities in radio apparatus where it
is necessary to maintain very close control on the tuning of a circuit.
This use requires quartz of highest quality and crystals above minimum
size, which have never been found in Missouri and probably are not
present. Silica production from this state is in its sandstone, tripoli,
chert chats, and rock garden ornamental stone.