Quartz

Source: Wikipedia, the free encyclopedia.
(Redirected from
Quartz crystal
)
This article is about the mineral. For other uses, see Quartz (disambiguation).

Quartz
hexagonal, piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic)
References[6][7][8][9]

Quartz is a hard,

crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is, therefore, classified structurally as a framework silicate mineral and compositionally as an oxide mineral. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.[10]

Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are

chiral
. The transformation from α-quartz to β-quartz takes place abruptly at 573 °C (846 K; 1,063 °F). Since the transformation is accompanied by a significant change in volume, it can easily induce microfracturing of ceramics or rocks passing through this temperature threshold.

There are many different varieties of quartz, several of which are classified as gemstones. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Europe and Asia.

Quartz is the

Mohs scale of hardness, a qualitative scratch
method for determining the hardness of a material to abrasion.

Etymology

The word "quartz" is derived from the German word Quarz,[11] which had the same form in the first half of the 14th century in Middle High German and in East Central German[12] and which came from the Polish dialect term kwardy, which corresponds to the Czech term tvrdý ("hard").[13] Some sources, however, attribute the word's origin to the Saxon word Querkluftertz, meaning cross-vein ore.[14][15]

The

philosophers (including Theophrastus) understood the mineral to be a form of supercooled ice.[16] Today, the term rock crystal is sometimes used as an alternative name for transparent coarsely crystalline quartz.[17][18]

Early studies

Roman naturalist Pliny the Elder believed quartz to be water ice, permanently frozen after great lengths of time.[19] 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. This idea persisted until at least the 17th century. He also knew of the ability of quartz to split light into a spectrum.[20]

In the 17th century, Nicolas Steno's study of quartz paved the way for modern crystallography. He discovered that regardless of a quartz crystal's size or shape, its long prism faces always joined at a perfect 60° angle.[21]

Crystal habit and structure

Crystal structure of α-quartz (red balls are oxygen, grey are silicon)
Crystal structure of β-quartz
A chiral pair of α-quartz

Quartz belongs to the

hexagonal crystal system above 573 °C (846 K; 1,063 °F). The ideal crystal shape is a six-sided prism terminating with six-sided pyramid-like rhombohedrons at each end. In nature, quartz crystals are often twinned (with twin right-handed and left-handed quartz crystals), 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.[22][23]

Well-formed crystals typically form as a druse (a layer of crystals lining a void), of which quartz geodes are particularly fine examples.[24] The crystals are attached at one end to the enclosing rock, and only one termination pyramid is present. However, doubly terminated crystals do occur where they develop freely without attachment, for instance, within gypsum.[25]

α-quartz crystallizes in the trigonal crystal system, space group P3121 or P3221 (space group 152 or 154 resp.) depending on the chirality. Above 573 °C (846 K; 1,063 °F), α-quartz in P3121 becomes the more symmetric hexagonal P6422 (space group 181), and α-quartz in P3221 goes to space group P6222 (no. 180).[26]

These space groups 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 a change in the way they are linked.[22][27] However, there is a significant change in volume during this transition,[clarification needed] and this can result in significant microfracturing in ceramics[28] and in rocks of the Earth's crust.[29]

  • Sceptered quartz
    Sceptered quartz
  • Sceptered quartz (as aggregates: "Elestial quartz")
    Sceptered quartz (as aggregates: "Elestial quartz")
  • Prismatic quartz with black hematite
    Prismatic quartz with black hematite
  • Druse quartz
    Druse quartz
  • Granular quartz
    Granular quartz
  • "Herkimer diamond"
    "Herkimer diamond"
  • Twinned quartz
    Twinned quartz
  • Massive quartz
    Massive quartz

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.[30]

Major varieties of quartz
Type Color and description Transparency
Herkimer diamond Colorless Transparent
Rock crystal Colorless Transparent
Amethyst Purple to violet colored quartz Transparent
Citrine Yellow quartz ranging to reddish-orange or brown (Madera quartz), and occasionally greenish yellow Transparent
Ametrine A mix of amethyst and citrine with hues of purple/violet and yellow or orange/brown Transparent
Rose quartz Pink, may display diasterism Transparent
Chalcedony Fibrous, variously translucent, cryptocrystalline quartz occurring in many varieties.
The term is often used for white, cloudy, or lightly colored material intergrown with moganite.
Otherwise more specific names are used.
Carnelian Reddish orange chalcedony Translucent
Aventurine Quartz with tiny aligned inclusions (usually mica) that shimmer with aventurescence Translucent to opaque
Agate Multi-colored, curved or concentric banded chalcedony (cf. Onyx) Semi-translucent to translucent
Onyx Multi-colored, straight banded chalcedony or chert (cf. Agate) Semi-translucent to opaque
Jasper Opaque cryptocrystalline quartz, typically red to brown but often used for other colors Opaque
Milky quartz White, may display diasterism Translucent to opaque
Smoky quartz Light to dark gray, sometimes with a brownish hue Translucent to opaque
Tiger's eye Fibrous gold, red-brown or bluish colored chalcedony, exhibiting chatoyancy.
Prasiolite Green Transparent
Rutilated quartz Contains acicular (needle-like) inclusions of rutile
Dumortierite quartz Contains large amounts of blue dumortierite crystals Translucent
Prase Green Translucent

Varieties (according to color)

Quartz crystal demonstrating transparency

Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent 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.[31] These color differentiations arise from the presence of impurities which change the molecular orbitals, causing some electronic transitions to take place in the visible spectrum causing colors.

The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the

silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite.[32] Other opaque gemstone varieties of quartz, or mixed rocks including quartz, often including contrasting bands or patterns of color, are agate, carnelian or sard, onyx, heliotrope, and jasper.[22]

Amethyst

Rock crystal
Amethyst
Blue quartz
Dumortierite quartz
Citrine quartz (natural)
Citrine quartz (heat-altered amethyst)
Milky quartz
Rose quartz
Smoky quartz
Prase

Amethyst is a form of quartz that ranges from a bright vivid violet to a dark or dull lavender shade. The world's largest deposits of amethysts can be found in Brazil, Mexico, Uruguay, Russia, France, Namibia, and Morocco. Sometimes amethyst and citrine are found growing in the same crystal. It is then referred to as ametrine. Amethyst derives its color from traces of iron in its structure.[33]

Blue quartz

Blue quartz contains inclusions of fibrous

crocidolite.[34]

Dumortierite quartz

Inclusions of the mineral

grey sometimes also are present. "Dumortierite quartz" (sometimes called "blue quartz") will sometimes feature contrasting light and dark color zones across the material.[35][36] "Blue quartz" is a minor gemstone.[35][37]

Citrine

Citrine is a variety of quartz whose color ranges from pale yellow to brown due to a submicroscopic distribution of colloidal

hardness. Brazil is the leading producer of citrine, with much of its production coming from the state of Rio Grande do Sul. The name is derived from the Latin word citrina which means "yellow" and is also the origin of the word "citron". Sometimes citrine and amethyst can be found together in the same crystal, which is then referred to as ametrine.[39] Citrine has been referred to as the "merchant's stone" or "money stone", due to a superstition that it would bring prosperity.[40]

Citrine was first appreciated as a golden-yellow gemstone in Greece between 300 and 150 BC, during the

Hellenistic Age. Yellow quartz was used prior to that to decorate jewelry and tools but it was not highly sought after.[41]

Milky quartz

Milk quartz or milky quartz is the most common variety of crystalline quartz. The white color is caused by minute

fluid inclusions of gas, liquid, or both, trapped during crystal formation,[42] making it of little value for optical and quality gemstone applications.[43]

Rose quartz

"Rose Quartz" redirects here. For other uses, see Rose Quartz (disambiguation).

Rose quartz is a type of quartz that 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 material. Some rose quartz contains microscopic rutile needles that produce asterism in transmitted light. Recent X-ray diffraction studies suggest that the color is due to thin microscopic fibers of possibly dumortierite within the quartz.[44]

Additionally, there is a rare type of pink quartz (also frequently called crystalline rose quartz) with color that 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 Rumford, Maine, US, and in Minas Gerais, Brazil.[45] The crystals found are more transparent and euhedral, due to the impurities of phosphate and aluminium that formed crystalline rose quartz, unlike the iron and microscopic dumortierite fibers that formed rose quartz.[46]

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. The translucency results from natural irradiation acting on minute traces of aluminum in the crystal structure.[47]

Prase

Prase is a green variety of quartz.[48] The green color is caused by inclusions of amphibole.[49]

Prasiolite

Not to be confused with
Praseolite
.

Prasiolite, also known as vermarine, is a variety of quartz that is green in color.[50] The green is caused by iron ions.[49] It is a rare mineral in nature and is typically found with amethyst; most "prasiolite" is not natural – it has been artificially produced by heating of amethyst. Since 1950[citation needed], almost all natural prasiolite has come from a small Brazilian mine, but it is also seen in Lower Silesia in Poland. Naturally occurring prasiolite is also found in the Thunder Bay area of Canada.[50]

Piezoelectricity

Quartz crystals have

Jacques and Pierre Curie in 1880.[52][53]

Occurrence

Quartz vein in sandstone, North Carolina

Quartz is a defining constituent of granite and other felsic igneous rocks. It is very common in sedimentary rocks such as sandstone and shale. It is a common constituent of schist, gneiss, quartzite and other metamorphic rocks.[22] Quartz has the lowest potential for weathering in the Goldich dissolution series and consequently it is very common as a residual mineral in stream sediments and residual soils. Generally a high presence of quartz suggests a "mature" rock, since it indicates the rock has been heavily reworked and quartz was the primary mineral that endured heavy weathering.[54]

While the majority of quartz crystallizes from molten magma, quartz also chemically precipitates from hot hydrothermal veins as gangue, sometimes with ore minerals like gold, silver and copper. Large crystals of quartz are found in magmatic pegmatites.[22] Well-formed crystals may reach several meters in length and weigh hundreds of kilograms.[55]

The largest documented single crystal of quartz was found near

Goiaz, Brazil; it measured approximately 6.1 m × 1.5 m × 1.5 m and weighed 39,916 kilograms.[56]

Mining

Quartz is extracted from open pit mines. Miners occasionally use explosives to expose deep pockets of quartz. More frequently, bulldozers and backhoes are used to remove soil and clay and expose quartz veins, which are then worked using hand tools. Care must be taken to avoid sudden temperature changes that may damage the crystals.[57][58]

Related silica minerals

See also:
Silica minerals
Pressure-temperature diagram showing the stability ranges for the two forms of quartz and some other forms of silica[59]

polymorphs of SiO2 that occur in high-silica volcanic rocks. Coesite is a denser polymorph of SiO2 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 SiO2 found in some meteorite impact sites.[60] Moganite is a monoclinic polymorph. Lechatelierite is an amorphous silica glass SiO2 which is formed by lightning strikes in quartz sand.[61]

Safety

As quartz is a form of silica, it is a possible cause for concern in various workplaces. Cutting, grinding, chipping, sanding, drilling, and polishing natural and manufactured stone products can release hazardous levels of very small, crystalline silica dust particles into the air that workers breathe.[62] Crystalline silica of respirable size is a recognized human carcinogen and may lead to other diseases of the lungs such as silicosis and pulmonary fibrosis.[63][64]

Synthetic and artificial treatments

A long, thin quartz crystal
A synthetic quartz crystal grown by the hydrothermal method, about 19 centimetres (7.5 in) long and weighing about 127 grams (4.5 oz)

Not all varieties of quartz are naturally occurring. Some clear quartz crystals can be treated using heat or gamma-irradiation to induce color where it would not otherwise have occurred naturally. Susceptibility to such treatments depends on the location from which the quartz was mined.[65]

Prasiolite, an olive colored material, is produced by heat treatment;[66] natural prasiolite has also been observed in Lower Silesia in Poland.[67] Although citrine occurs naturally, the majority is the result of heat-treating amethyst or smoky quartz.[66] Carnelian has been heat-treated to deepen its color since prehistoric times.[68]

Because natural quartz is often

autoclave via the hydrothermal process.[69][22][70]

Like other crystals, quartz may be coated with metal vapors to give it an attractive sheen.[71][72]

Uses

Quartz is the most common material identified as the mystical substance

Australian Aboriginal mythology. It is found regularly in passage tomb cemeteries in Europe in a burial context, such as Newgrange or Carrowmore in Ireland. 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.[73]

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.

Efforts to synthesize quartz began in the mid-nineteenth century as scientists attempted to create minerals under laboratory conditions that mimicked the conditions in which the minerals formed in nature: German geologist Karl Emil von Schafhäutl (1803–1890) was the first person to synthesize quartz when in 1845 he created microscopic quartz crystals in a pressure cooker.[74] However, the quality and size of the crystals that were produced by these early efforts were poor.[75]

Elemental impurity incorporation strongly influences the ability to process and utilize quartz. 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. These high-purity quartz are defined as containing less than 50 ppm of impurity elements.[76] A major mining location for high purity quartz is the Spruce Pine Gem Mine in Spruce Pine, North Carolina, United States.[77] Quartz may also be found in Caldoveiro Peak, in Asturias, Spain.[78]

By the 1930s, the electronics industry had become dependent on quartz crystals. The only source of suitable crystals was Brazil; however, World War II disrupted the supplies from Brazil, so nations attempted to synthesize quartz on a commercial scale. German mineralogist Richard Nacken (1884–1971) achieved some success during the 1930s and 1940s.[79] After the war, many laboratories attempted to grow large quartz crystals. In the United States, the U.S. Army Signal Corps contracted with Bell Laboratories and with the Brush Development Company of Cleveland, Ohio to synthesize crystals following Nacken's lead.[80][81] (Prior to World War II, Brush Development produced piezoelectric crystals for record players.) By 1948, Brush Development had grown crystals that were 1.5 inches (3.8 cm) in diameter, the largest at that time.[82][83] By the 1950s, hydrothermal synthesis techniques were producing synthetic quartz crystals on an industrial scale, and today virtually all the quartz crystal used in the modern electronics industry is synthetic.[70]

An early use of the piezoelectricity of quartz crystals was in phonograph pickups. One of the most common piezoelectric uses of quartz today is as a crystal oscillator. The quartz oscillator or resonator was first developed by Walter Guyton Cady in 1921.[84][85] George Washington Pierce designed and patented quartz crystal oscillators in 1923.[86][87][88] The quartz clock is a familiar device using the mineral. Warren Marrison created the first quartz oscillator clock based on the work of Cady and Pierce in 1927.[89] 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.[90]

  • Rock crystal jug with cut festoon decoration by Milan workshop from the second half of the 16th century, National Museum in Warsaw. The city of Milan, apart from Prague and Florence, was the main Renaissance centre for crystal cutting.[91]
    Rock crystal jug with cut festoon decoration by
    National Museum in Warsaw. The city of Milan, apart from Prague and Florence, was the main Renaissance centre for crystal cutting.[91]
  • Synthetic quartz crystals produced in the autoclave shown in Western Electric's pilot hydrothermal quartz plant in 1959
    Synthetic quartz crystals produced in the autoclave shown in Western Electric's pilot hydrothermal quartz plant in 1959
  • Fatimid ewer in carved rock crystal (clear quartz) with gold lid, c. 1000
    Fatimid ewer in carved rock crystal (clear quartz) with gold lid, c. 1000

Almost all the industrial demand for quartz crystal (used primarily in electronics) is met with synthetic quartz produced by the hydrothermal process. However, synthetic crystals are less prized for use as gemstones.

child labor.[93]

See also

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