A. Characteristics
Once the student has become familiar with some of the characteristics of minerals in general it is time to focus in on some specific examples. Quartz is a good one with which to work as samples of a variety of forms are easily obtainable and have the all-important virtues of both durability and beauty. Quartz is at position 7 on the Mohs scale which means that it is one of the hardest common minerals. It crystallizes from the molten state at rather high temperatures but also can be deposited on the ocean floor at only a few degrees above zero. It forms very beautiful crystals that can be used as gem stones and is the source of crystals used for technical purposes. It is also the major ingredient in sand. Flint is a historically important form of quartz due to its use in the early weapons used by man to hunt wild game.
In 1880 the Curies discovered another peculiar property of quartz while studying the electrical conductivity of crystalline bodies. They discovered that pressure on plates of quartz caused a deflection of the needle on a sensitive electrometer. This is called the piezoelectric effect. It occurs when the crystal is squeezed slightly out of shape and then springs back. This shape change actually affects the crystal at the atomic level causing a movement of ions, with their attendant electric charges. This motion of the electrically charged particles constitutes flow of electrons, or electricity. This particular characteristic is now used to control and stabilize the frequency of a radio transmitter or to regulate watches.
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B. Types of Quartz
Probably the most familiar form of quartz is the clear, colorless form known as
rock crystal
. These crystals can range in size from the most minute to the gigantic; one from Brazil weighed in at 5.5 tons! This clear material has been used far back into history for carved goblets, bowls, and other objects.
Another form of quartz known and valued for its beauty is
amethyst
. This mineral is almost pure SiO2 with only a trace of iron. As the amount of iron increases, so does the intensity of the violet color, so it is believed to be the coloring agent. According to folklore the amethyst gives its wearer great power, increased intelligence, and strength.
Smoky quartz
does not differ from clear quartz in chemical composition. In fact when it is heated to very high temperatures the “smoky” color vanishes, and it looks identical to clear crystalline quartz. The color can be restored by treating the crystal with a beam of x-ray radiation. Scientists believe that the color of smoky quartz is a result of natural radiation in the earth.
Agate
is a common form of quartz which does not have any external evidence of its crystal nature. The extremely tiny crystalline particles are so intergrown that they appear smoothly mixed. Agate is used decoratively and as jewelry, especially in the onyx form.
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A form of quartz that is unique and appears to be most “un-quartz like” is the
opal
. It contains a rather large percentage of water, ranging from four to twenty percent. And a complex internal structure if microscopic silica fitted together in a lattice-pattern results in diffraction of the light hitting it, forming rainbows of brilliant color as the gem is rotated. Opal is a low-pressure and low-temperature mineral and is formed at the earth’s surface by deposition from ground water or by the evaporation of
hydrothermal
, or hot water, springs as they rise to the surface and cool leaving opal mineral behind. Because of the rather large amount of water present in opal, it tends to be relatively soft (5.5 to 6.5) and low in specific gravity. These qualities limit its use as a gem, and it is usually found mounted in pendants and pins where the stone is relatively protected.
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