Quartz

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For other uses, see Quartz (disambiguation).
Quartz
Photo of quartz crystal group
General
Category Mineral
Chemical formula (or Composition) Silica (silicon dioxide, SiO2)
Identification
Color Clear (if no impurities); also see Varieties
Crystal habit 6-sided prism ending in 6-sided pyramid (typical)
Crystal system Hexagonal
Cleavage None
Fracture Conchoidal
Mohs Scale hardness 7 - lower in impure varieties
Luster Vitreous/glossy
Refractive index 1.544-1.553 - DR +0.009 (B-G interval)
Pleochroism None
Streak White
Specific gravity 2.65 constant; variable in impure varieties
Melting point 1650 (±75) °C
Boiling point 2230 °C
Solubility H2O insoluble
Major varieties
Chalcedony Any cryptocrystalline quartz, although generally only used for white or lightly coloured material. Otherwise more specific names are used.
Agate Banded Chalcedony, translucent
Onyx Agate where the bands are straight, parallel and consistent in size.
Jasper Opaque chalcedony, impure
Aventurine Translucent chalcedony with small inclusions (usually mica) that shimmer.
Tiger's eye Fibrous quartz, exhibiting chatoyancy.
Rock Crystal Clear, colourless
Ruby Quartz Crimson glass-like crystal, absorbs vast amounts of solar energy
Amethyst Purple, transparent
Citrine Yellow to reddish orange, greenish yellow
Prasiolite Mint green, transparent
Rose quartz Pink, translucent, may display diasterism
Milk quartz, or snow quartz White, translucent to opaque, may display diasterism
Smoky quartz Brown, transparent
Morion Dark-brown, opaque
Carnelian Reddish orange chalcedony, translucent

Quartz is one of the most common minerals in the Earth's continental crust. It is made of crystallized silica (silicon dioxide, SiO2). The typical outward shape of the crystal ("crystal habit") is a six-sided prism that ends in six-sided pyramids, although this shape may be masked by the crystal's size and natural distortions in the mineral. Quartz can occur in different colored varieties, such as amethyst.

Notable characteristics

Quartz has a hexagonal crystal structure made of trigonal crystallized silica (silicon dioxide, SiO2), with a hardness (resistance to scratching) of 7 on the Mohs scale of 1 to 10. Its density is 2.65 g/cm³. The typical shape of the crystal ("crystal habit") is a six-sided prism that ends in six-sided pyramids. The crystals, however, are often twinned, distorted, or so massive that only part of the shape is apparent from a mined specimen. Additionally, the crystals may take the form of a bed, particularly for varieties such as amethyst, in which the crystals grow up from a matrix and only one termination pyramid appears. A quartz geode consists of a hollow rock (usually with a roughly spherical shape) containing a core lined with a bed of crystals.

History

The name "quartz" comes from the German "Quarz," which is of Slavic origin (Czech miners called it křem). Some sources, however, trace the name to the Saxon word "Querkluftertz," meaning cross-vein ore.

Roman naturalist Pliny the Elder (CE 23–79) believed quartz to be permanently frozen ice. In support of this idea, he said that quartz is found near glaciers in the Alps and that large quartz crystals were fashioned into spheres to cool the hands. He was aware of the ability of quartz to split light into a spectrum.

The study of quartz by Nicolas Steno (1638–1686) 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-degree angle.

Charles B. Sawyer invented the commercial quartz crystal manufacturing process in Cleveland, Ohio. This process initiated the transition from mined to manufactured quartz.

Varieties

Quartz is one of the world's most common crustal minerals and goes by a bewildering array of names. Some types of quartz have a "macrocrystalline" structure, in which individual crystals are visible to the unaided eye, while other types of quartz have a "microcrystalline" or "cryptocrystalline" structure, in which aggregates of crystals are visible only under high magnification. Chalcedony is a generic term for cryptocrystalline quartz. The cryptocrystalline varieties are translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline.

Although many of the varietal names historically arose from the mineral's color, current scientific naming schemes refer primarily to the mineral's microstructure. 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.

Not all varieties of quartz are naturally occurring. Prasiolite, an olive colored material, is produced by heat treatment; natural prasiolite has also been obeserved 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.

Milk quartz.

Given that natural quartz is often twinned, much of the quartz used in industry is synthetically produced. Large, flawless, and untwinned crystals are produced in an autoclave via the hydrothermal process. (Emeralds are also synthesized in this manner.)

Quartz occurs in hydrothermal veins and pegmatites. Well-formed crystals may reach several meters in length and weigh hundreds of kilograms. These veins may bear precious metals such as gold or silver, and form the quartz ores sought through mining. Erosion of pegmatites may reveal expansive pockets of crystals, known as "cathedrals."

Quartz is a common constituent of granite, sandstone, limestone, and many other igneous, sedimentary, and metamorphic rocks.

Tridymite and cristobalite are high-temperature polymorphs of silica (SiO2), occurring in silica-rich volcanic rocks. Lechatelierite is an amorphous silica glass, which is formed by lightning strikes in quartz sand.


Piezoelectricity

A quartz crystal has a property known as piezoelectricity—which means that when mechanical stress is put on the crystal, or when the crystal is made to vibrate, it produces electricity. One of the earliest uses for this property of quartz was in making the pickup for a phonograph.

Today, one of the most ubiquitous piezoelectric uses of quartz is for "crystal oscillators," which are often simply called "quartzes." A quartz crystal oscillator is an electronic circuit in which the mechanical resonance of a vibrating quartz crystal creates an electrical signal with a precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters. The crystal used therein is sometimes called a "timing crystal".

The piezoelectric principle is also used in the quartz crystal microbalance, which can make highly accurate measurements of tiny changes in mass.

See also

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Wikisource has original text related to this article:
EB1911:Quartz
  • Fused quartz
  • List of minerals
  • Shocked quartz

References
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