Quartz
Crystals
Quartz is an abundant mineral
in the Earth's continental crust. It is
made up of a continuous framework of
SiO4 silicon–oxygen tetrahedra, with
each oxygen being shared between two
tetrahedra, giving an overall formula
SiO2. There are many different varieties
of quartz, several of which are
semi-precious gemstones. Throughout the
world, varieties of quartz have been,
since antiquity, the most commonly used
minerals in the making of jewelry and
hardstone carvings.
Quartz belongs to the trigonal
crystal system. The ideal crystal shape
is a six-sided prism terminating with
six-sided pyramids at each end. In
nature quartz crystals are often
twinned, distorted, or so intergrown
with adjacent crystals of quartz or
other minerals as to only show part of
this shape, or to lack obvious crystal
faces altogether and appear massive.
Well-formed crystals typically form in a
'bed' that has unconstrained growth into
a void, but because the crystals must be
attached at the other end to a matrix,
only one termination pyramid is present.
There are exceptions as doubly
terminated crystals do occur. An
occurrence in Herkimer County, New York
is noted for these Herkimer diamonds
with terminations at both ends. A quartz
geode is such a situation where the void
is approximately spherical in shape,
lined with a bed of crystals pointing
inward.
Occurrence
Quartz is an essential
constituent of granite and other felsic
igneous rocks. It is very common in
sedimentary rocks such as sandstone and
shale and is also present in variable
amounts as an accessory mineral in most
carbonate rocks. It is also a common
constituent of schist, gneiss, quartzite
and other metamorphic rocks. Because of
its resistance to erosion it is very
common in stream sediments and in
residual soils. Quartz, therefore,
occupies the lowest potential to weather
in the Goldich dissolution series.
Quartz occurs in hydrothermal
veins as gangue along with ore minerals.
Large crystals of quartz are found in
pegmatites. Well-formed crystals may
reach several meters in length and weigh
as much as 1,400 pounds (640 kg).
Naturally occurring quartz
crystals of extremely high purity,
necessary for the crucibles and other
equipment used for growing silicon
wafers in the semiconductor industry,
are expensive and rare. A major mining
location for high-purity quartz is the
Spruce Pine Mining District in Spruce
Pine, North Carolina, United States.
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