Listing description
Quartz is the second most abundant mineral
in the Earth's
continental crust, after feldspar.
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.
Detailed descrption
There are many
different varieties of quartz, several of which are semi-precious gemstones. Especially in Europe and the Middle
East, varieties of quartz have been since antiquity the most commonly used
minerals in the making of jewelry and hardstone carvings.
The word
"quartz" is derived from the German word "quarz", which was
imported from Middle
High German,
"twarc", which originated in Slavic (cf. Czech tvrdy ("hard"),
Polish twardy ("hard"), Russian твёрдый ("hard")), from Old
Bulgarian (Church Slavonic) тврьдъ ("firm"), from Proto-Slavic
*tvьrdъ.[6]
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Crystal habit
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. A quartz geode is such a situation where the void is
approximately spherical in shape, lined with a bed of crystals pointing inward.
At surface
temperatures and pressures, quartz is the most stable form of silicon dioxide.
Quartz will remain stable up to 573 °C at 1 kilobar of pressure. As the pressure
increases the temperature at which quartz will lose stability also increases.
Above
1300 °C and at a pressure of approximately 35 kilobars, only β-quartz is
stable. The latter is not the same as normal quartz (or α-quartz), low quartz
or just quartz. β-quartz has higher symmetry, is less dense and has a slightly
lower specific gravity. The conversion, from one solid substance to another
solid substance, of quartz to β-quartz is quick, reversible and accompanied
with a slight energy absorption. The conversion is so easily accomplished that
when a crystal of quartz is heated to β-quartz, cooled back down, heated again
to β-quartz, etc., the quartz will be the same as when it started.
The reason that
the conversion is so easily accomplished is that the difference between quartz
and β-quartz is relatively slight. The bonds between the oxygen and silicon
atoms are "kinked" or bent in quartz and are not so
"kinked" in β-quartz. At the higher temperatures the atoms move away
from each other just enough to allow the bonds to unkink or straighten and
produce the higher symmetry. As the temperature is lowered, the atoms close in
on each other and the bonds must kink in order to be stable and this lowers the
symmetry back down again.
Although all
quartz at temperatures lower than 573 °C is low quartz, there are a few
examples of crystals that obviously started out as β-quartz. Sometimes these
are labeled as β-quartz but are actually examples of pseudomorphic or
"falsely shaped" crystals more correctly labeled 'quartz after
β-quartz'. These crystals are of higher symmetry than low quartz although low
quartz can form similar crystals to them. They are composed of hexagonal
dipyramids which are a pair of opposing six sided pyramids and the crystals
lack prism faces. Quartz's typical termination is composed of two sets of three
rhombic faces that can look like a six sided pyramid.
(Microscopic) crystal structure
α-quartz
crystallizes in the trigonal crystal system, space group P3121
and P3221 respectively. β-quartz belongs to the hexagonal
system, space group P6221 and P6421,
respectively.[7] These spacegroups are truly chiral
(they each belong to the 11 enantiomorphous pairs). Both α-quartz and β-quartz
are examples of chiral crystal structures composed of achiral building blocks
(SiO4 tetrahedra in the present case). The transformation between α-
and β-quartz only involves a comparatively minor rotation of the tetrahedra
with respect to one another, without change in the way they are linked.
Varieties (according to color)
Pure quartz,
traditionally called rock crystal (sometimes called clear quartz),
is colorless and transparent (clear) or translucent, and has often been used for hardstone carvings, such as the Lothair Crystal. Common colored varieties include
citrine, rose quartz, amethyst, smoky quartz, milky quartz, and
others. Quartz goes by an array of different names. The most important
distinction between types of quartz is that of macrocrystalline
(individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals
visible only under high magnification). The cryptocrystalline varieties are
either translucent or mostly opaque, while the transparent varieties tend to be
macrocrystalline. Chalcedony is a cryptocrystalline form of silica
consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite.[8] Other opaque gemstone varieties of
quartz, or mixed rocks including quartz, often including contrasting bands or
patterns of color, are agate, sard, onyx, carnelian, heliotrope, and jasper.
Synthetic and artificial treatments
Not all varieties
of quartz are naturally occurring. Prasiolite, an olive colored material, is
produced by heat treatment; natural prasiolite has also been observed in Lower
Silesia in Poland. Although citrine occurs naturally, the majority is the
result of heat-treated amethyst. Carnelian is widely heat-treated to deepen its
color.
Due to natural
quartz being so often twinned, much of the quartz used in industry
is synthesized. Large, flawless and untwinned crystals are produced in an autoclave via the hydrothermal
process; emeralds are also synthesized in this fashion.
While these are still commonly referred to as quartz, the correct term for this
material is silicon
dioxide[citation needed].
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 weathering 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 hundreds of kilograms.
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 Gem Mine in Spruce
Pine, North Carolina,
United
States.[13]
Related silica minerals
Tridymite and cristobalite are high-temperature polymorphs of SiO2 that occur in high-silica volcanic rocks. Coesite is a denser polymorph of quartz found
in some meteorite impact sites and in metamorphic rocks formed at pressures
greater than those typical of the Earth's crust. Stishovite is a yet denser and higher-pressure
polymorph of quartz found in some meteorite impact sites. Lechatelierite is an amorphous silica glass SiO2 which is formed by lightning strikes in quartz sand.
History
The word
"quartz" comes from the German
Quarz (help·info),[14] which is of Slavic origin (Czech
miners called it křemen). Other sources attribute the word's origin to
the Saxon word Querkluftertz, meaning cross-vein
ore.[15]
Quartz is the
most common material identified as the mystical substance maban in Australian Aboriginal mythology. It is found regularly in passage
tomb cemeteries in Europe in a burial context, such as Newgrange or Carrowmore in the Republic
of Ireland.
The Irish word for quartz is grian cloch,
which means 'stone of the sun'. Quartz was also used in Prehistoric
Ireland, as well as many
other countries, for stone tools; both vein quartz and rock crystal
were knapped as part of the lithic technology of the prehistoric peoples.
While jade has been since earliest times the most prized
semi-precious stone for carving in East Asia and Pre-Columbian America, in Europe and the Middle
East the different varieties of quartz were the most commonly used for the
various types of jewelry and hardstone carving, including engraved gems and cameo gems, rock crystal vases, and extravagant vessels. The
tradition continued to produce objects that were very highly valued until the
mid-19th century, when it largely fell from fashion except in jewelry. Cameo
technique exploits the bands of color in onyx and other varieties.
Piezoelectricity
Quartz crystals
have piezoelectric properties; they develop an electric
potential upon the application of mechanical stress. An early use of this property of
quartz crystals was in phonograph pickups. One of the most common
piezoelectric uses of quartz today is as a crystal
oscillator.
The quartz
clock is a familiar
device using the mineral. The resonant frequency of a quartz crystal oscillator
is changed by mechanically loading it, and this principle is used for very
accurate measurements of very small mass changes in the quartz
crystal microbalance
and in thin-film
thickness monitors.
PRICE
$9.9/KG OR $4.5/IB
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