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{{Infobox mineral
name = Quartz
category = Oxide mineral
boxwidth = 24
boxbgcolor =
image = Quartz, Tibet.jpg
caption = Quartz crystal cluster from Tibet
formula = Silica (silicon dioxide, SiO2)
molweight =
strunz = 04.DA.05
dana = 75.01.03.01
color = Colorless through various colors to black
habit = 6-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massive
system = α-quartz: trigonal trapezohedral class 3 2; β-quartz: hexagonal 622
lattice = hexagonal
twinning = Common Dauphine law, Brazil law and Japan law
cleavage = {0110} Indistinct
fracture = Conchoidal
tenacity = Brittle
mohs = 7 - lower in impure varieties
luster = Vitreous - waxy to dull when massive
refractive = nω = 1.543 - 1.545 nε = 1.552 - 1.554
opticalprop = Uniaxial (+)
birefringence = +0.009 (B-G interval)
pleochroism = None
streak = White
gravity = 2.65; variable 2.59 - 2.63 in impure varieties
density =
melt = 1670 °C (β tridymite) 1713 °C (β cristobalite)
fusibility =
diagnostic =
solubility = 1 ppmmass at 400 °C and 500 lb/in2 to 2600 ppmmass at 500 °C and 1500 lb/in2
diaphaneity = Transparent to nearly opaque
other = Piezoelectric, pyroelectric, may be triboluminescent
references =}}

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.

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.

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 beta quartz (aka high quartz) is stable. Beta quartz is not the same as normal quartz (or alpha quartz), low quartz or just quartz. Beta 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 beta quartz is quick, reversible and accompanied with a slight energy absorption. The conversion in fact is so easily accomplished that a crystal of quartz heated to beta quartz, cooled back down, heated again to beta quartz, etc. and the crystal when all is done, will be the same as when it started.

The reason that the conversion is so easily accomplished is that the difference between quartz and beta quartz is relatively slight. The bonds between the oxygen and silicon atoms are "kinked" or bent in quartz and are not so "kinked" in beta 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 beta quartz. Sometimes these are labeled as beta quartz but are actually examples of pseudomorphic or "falsely shaped" crystals more correctly labeled 'quartz after beta 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.

Varieties (according to color)

Pure quartz, traditionally called rock crystal (sometimes called clear quartz), is colorless and transparent (clear) or translucent. 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. 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.

Citrine

Citrine
Citrine is a variety of quartz whose color ranges from a pale yellow to brown. It is nearly impossible to tell cut citrine from yellow topaz visibly. Citrine has ferric impurities, and is rarely found naturally. Most commercial citrine is in fact artificially heated amethyst or smoky quartz. Brazilmarker is the leading producer of citrine, with much of its production coming from the state of Rio Grande do Sul.

Citrine is one of three traditional birthstones for the month of November.

Rose quartz

Rose quartz is a type of quartz which exhibits a pale pink to rose red hue. The color is usually considered as due to trace amounts of titanium, iron, or manganese, in the massive material. Some rose quartz contains microscopic rutile needles which produces an asterism in transmitted light. Recent X-ray diffraction studies suggest that the color is due to thin microscopic fibers of possibly dumortierite within the massive quartz.

In crystal form (rarely found) it is called pink quartz and its color is thought to be caused by trace amounts of phosphate or aluminium. The color in crystals is apparently photosensitive and subject to fading. The first crystals were found in a pegmatite found near Rumfordmarker, Mainemarker, USA, but most crystals on the market come from Minas Geraismarker, Brazilmarker.

Rose quartz is not popular as a gem - it is generally too clouded by impurities to be suitable for that purpose. Rose quartz is more often carved into figures such as people or hearts. Hearts are commonly found because rose quartz is pink and an affordable mineral.

Amethyst

Amethyst.
Magaliesburg, South Africa


Amethyst is a popular form of quartz that ranges from a bright to dark or dull purple color.

Smoky quartz

Smoky quartz is a gray, translucent version of quartz. It ranges in clarity from almost complete transparency to a brownish-gray crystal that is almost opaque.Some can also be black.

Milky quartz

Milky quartz sample
Milk quartz or milky quartz may be the most common variety of crystalline quartz and can be found almost anywhere. The white color may be caused by minute fluid inclusions of gas and/or liquid trapped during the crystal formation. The cloudiness caused by the inclusions effectively bars its use in most optical and quality gemstone applications.

Varieties (according to microstructure)

Although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties. This does not always hold true.



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 citrineoccurs naturally, the majority is the result of heat-treated amethyst. Carnelianis 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 autoclavevia the hydrothermal process; emeraldsare also synthesized in this fashion. While these are still commonly referred to as quartz, the correct term for this material is silicon dioxide.

Occurrence

Quartz is an essential constituent of graniteand other felsicigneous rocks. It is very common in sedimentary rockssuch as sandstoneand shaleand is also present in variable amounts as an accessory mineral in most carbonate rocks. It is also a common constituent of schist, gneiss, quartziteand other metamorphic rocks. Because of its resistance to weatheringit is very common in stream sediments and in residual soils.

Quartz occurs in hydrothermalveinsas gangue along with oreminerals. Large crystals of quartz are found in pegmatites. Well-formed crystals may reach several meters in length and weighhundreds of kilograms.

Naturally occurring quartz crystals of extremely high purity, necessary for the crucibles and other equipment used for growing silicon wafers in the semiconductorindustry, are expensive and rare. A major mining location for high purity quartz is the Spruce Pine Gem Mine in Spruce Pine, North Carolinamarker.

Related silica minerals

Tridymiteand cristobaliteare high-temperature polymorphof SiO2that occur in high-silica volcanicrocks. Coesiteis 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. Stishoviteis a yet denser and higher-pressure polymorph of quartz found in some meteorite impact sites. Lechatelieriteis an amorphoussilica glassSiO2which is formed by lightningstrikes in quartz sand.

History

The word "quartz" comes from the German, which is of Slavic origin (Czech miners called it křemen). Other sources attribute the word's origin to the Saxonword Querkluftertz, meaning cross-vein ore.

Quartz is the most common material identified as the mystical substance mabanin Australian Aboriginal mythology. It is found regularly in passage tomb cemeteries in Europe in a burial context, such as Newgrangemarker or Carrowmoremarker in the Republic of Irelandmarker.The Irishword for quartz is grian cloch, which means 'stone of the sun'.

While jadehas been since earliest times the most prized semi-precious stone for carving in East Asiaand Pre-ColumbianAmerica, in Europe and the Middle East the different varieties of quartz were the most commonly used for the various types of jewelryand hardstone carving, including engraved gemsand 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.

Roman naturalist Pliny the Elderbelieved quartz to be water ice, permanently frozen after great lengths of time. (The word "crystal" comes from the Greek word for purity.) He supported this idea by saying that quartz is found near glaciers in the Alps, but not on volcanic mountains, and that large quartz crystals were fashioned into spheres to cool the hands. He also knew of the ability of quartz to split light into a spectrum. This idea persisted until at least the 1600s.

In the 17th century, Nicolas Steno's study of quartz paved the way for modern crystallography. He discovered that no matter how distorted a quartz crystal, the long prism faces always made a perfect 60° angle.

Charles B.Sawyer invented the commercial quartz crystal manufacturing process in Clevelandmarker, Ohiomarker, United Statesmarker.This initiated the transition from mined and cut quartz for electrical appliances to manufactured quartz.

Quartz's piezoelectricproperties were discovered by Jacquesand Pierre Curiein 1880. The quartz oscillatoror resonator was first developed by Walter Guyton Cadyin 1921. George Washington Pierce designed and patented quartz crystal oscillators in 1923. Warren Marrison created the first quartz oscillator clock based on the work of Cady and Pierce in 1927.

Piezoelectricity

Quartz crystals have piezoelectricproperties; they develop an electric potential upon the application of mechanical stress. An early use of this property of quartz crystals was in phonographpickups. One of the most common piezoelectric uses of quartz today is as a crystal oscillator. The quartz clockis 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 microbalanceand in thin-film thickness monitors.

See also



Notes

  1. Deer, W. A., R. A. Howie and J. Zussman, An Introduction to the Rock Forming Minerals, Logman, 1966, pp. 340-355 ISBN 0-582-44210-9
  2. Handbook of Mineralogy. Quartz
  3. Mindat. Quartz
  4. Webmineral. Quartz
  5. Mindat. Rose Quartz
  6. Colored Varieties of Quartz, Caltech
  7. Milky quartz at Mineral Galleries
  8. German Loan Words in English
  9. Mineral Atlas, Queensland University of Technology


References

External links



Major Varieties of Quartz
Chalcedony Cryptocrystalline quartz and moganite mixture. The term is generally only used for white or lightly colored material. Otherwise more specific names are used.
Agate Multi-colored, banded chalcedony, semi-translucent to translucent
Onyx Agate where the bands are straight, parallel and consistent in size.
Jasper Opaque cryptocrystalline quartz, typically red to brown
Aventurine Translucent chalcedony with small inclusions (usually mica) that shimmer.
Tiger's Eye Fibrous gold to red-brown colored quartz, exhibiting chatoyancy.
Rock crystal Clear, colorless
Amethyst Purple, transparent
Citrine Yellow to reddish orange to brown, greenish yellow
Prasiolite Mint green, transparent
Rose quartz Pink, translucent, may display diasterism
Rutilated quartz Contains acicular (needles) inclusions of rutile
Milk quartz White, translucent to opaque, may display diasterism
Smoky quartz Brown to gray, opaque
Carnelian Reddish orange chalcedony, translucent

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