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Quartz

From Academic Kids

For other uses of this word, see Quartz (disambiguation).
Quartz
Photo of quartz
General
CategoryMineral
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 Trigonal
Cleavage None
Fracture Conchoidal
Mohs Scale hardness 7 - lower in impure varieties
Luster Vitreous
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
Solubility
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
Amethyst Purple, transparent
Citrine Yellow to reddish orange, greenish yellow
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 the second most abundant mineral in the Earth's crust. It has a hexagonal crystal structure made of trigonal crystallized silica (silicon dioxide, SiO2), with a hardness of 7 on the Mohs scale. Density is 2.6 g/cm³. The typical shape is a six-sided prism that ends in six-sided pyramids, although these are often distorted, or so massive that only part of the shape is apparent from a mined specimen. Additionally a bed is a common form, particularly for varieties such as amethyst, where the crystals grow up from a matrix and thus only one termination pyramid is present. A quartz geode consists of a hollow rock (usually with an approximately spherical shape) with a core lined with a bed of crystals.

Contents

Varieties

Quartz is one of the world's most common crustal minerals and goes by a bewildering 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). Chalcedony is a generic term for cryptocrystalline quartz. The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline.

Although many of the varietal names historically arose from the colour of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Colour 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 coloured material, is produced by heat treatment. Although citrine occurs naturally, the majority is the result of heat-treated amethyst. Carnelian is widely heat-treated to deepen its colour.

Missing image
QuartzUSGOV.jpg

Because natural quartz is so often twinned, much 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.

Quartz occurs in hydrothermal veins and pegmatites. Well-formed crystals may reach several metres in length and weigh hundreds of kilograms. 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.

Some quartz crystal structures are piezoelectric and are used as oscillators in electronic devices such as quartz clocks and radios.

A amorphous (glass) SiO2, called lechatelierite, is caused by lightning strikes in sand, distinct from typical window glass that is impure.

History

The name "quartz" comes from the German "Quarz", which is of Slavic origin.

Roman naturalist Pliny the Elder believed quartz to be permanently frozen ice. He supported this idea by saying that quartz is found near glaciers in the Alps and that large quartz crystals were fashioned into spheres to cool the hands. He also knew of ability of quartz to split light into a spectrum.

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 degree angle.

Piezoelectricity

Quartz is also a type of piezoelectric crystal that creates electricity through a process called piezoelectricity when mechanical stress is put upon it. One of the earliest uses for a quartz crystal was a phonograph pickup. Today, one of the most ubiquitous piezoelectric uses of quartz is as a crystal oscillator -- in fact these oscillators are often simply called "quartzes".

See also

References and external links

de:Quarz et:Kvarts es:Cuarzo eo:Kvarco fr:Quartz it:Quarzo he:קוורץ nl:Kwarts ja:石英 pl:Kwarc pt:Quartzo ro:Cuarţ ru:Кварц zh:石英

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