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Quartz

2007 Schools Wikipedia Selection. Related subjects: Mineralogy

                                   Quartz
                        Photo of quartz crystal group
                                   General
   Category Mineral
   Chemical formula (or Composition) Silica (silicon dioxide, SiO[2])
                               Identification
   Colour Clear (if no impurities); also see Varieties
   Crystal habit 6-sided prism ending in 6-sided pyramid (typical)
   Crystal system Hexagonal
   Cleavage Yes
   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 H[2]O 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 belongs to the hexagonal crystal system, and is made up of
   silica (SiO[2]) tetrahedra. Quartz has a hardness of 7 on the Mohs
   scale. Density is 2.65 g/cm³. The typical shape is a six-sided prism
   that ends in six-sided pyramids, although these are often twinned,
   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.

Varieties

   Quartz is one of the most common minerals of the continental crust 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; 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 colour.
   Milk quartz
   Enlarge
   Milk quartz

   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. These veins may bear precious metals such as gold or silver,
   and form the quartz ores sought in 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 SiO[2]
   which occur in high silica volcanic rocks. Lechatelierite is an
   amorphous silica glass SiO[2] which is formed by lightning strikes in
   quartz sand.

History

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

   Quartz is the most common material identified as the mystical substance
   maban in Australian Aboriginal mythology.

   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 the 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.

   Charles Sawyer invented the commercial quartz crystal manufacturing
   process in Cleveland, OH. This initiated the transition from mined and
   cut quartz for electrical appliances to manufactured quartz.

   The quartz oscillator or resonator was first developed by Walter Guyton
   Cady in 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 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" or "crystals". The same principle is also used for
   very accurate measurements of very small mass changes by means of the
   quartz crystal microbalance.

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