Quartz

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.

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 mineral defining the value of 7 on the Mohs scale of hardness, a qualitative scratch method for determining the hardness of a material to abrasion.

Quartz

General

Category Tectosilicates, quartz group

Formula SiO2

IMA symbol Qz

Strunz classification 4.DA.05 (oxides)

Dana classification 75.01.03.01 (tectosilicates)

Crystal system α-quartz: trigonal

β-quartz: hexagonal

Crystal class α-quartz: trapezohedral (class 3 2)

β-quartz: trapezohedral (class 6 2 2)

Space group α-quartz: P3221 (no. 154)

β-quartz: P6222 (no. 180) or P6422 (no. 181)

Unit cell a = 4.9133 Å, c = 5.4053 Å; Z = 3

Identification

Formula mass 60.083 g•mol−1

Color Colorless, pink, orange, white, green, yellow, blue, purple, dark brown, or black

Crystal habit 6-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massive

Twinning Common Dauphine law, Brazil law, and Japan law

Cleavage {0110} Indistinct

Fracture Conchoidal

Tenacity Brittle

Mohs scale hardness 7 – lower in impure varieties (defining mineral)

Luster Vitreous – waxy to dull when massive

Streak White

Diaphaneity Transparent to nearly opaque

Specific gravity 2.65; variable 2.59–2.63 in impure varieties

Optical properties Uniaxial (+)

Refractive index nω = 1.543–1.545

nε = 1.552–1.554

Birefringence +0.009 (B-G interval)

Pleochroism None

Melting point 1670 °C (β tridymite); 1713 °C (β cristobalite)

Solubility Insoluble at STP; 1 ppmmass at 400 °C and 500 lb/in2 to 2600 ppmmass at 500 °C and 1500 lb/in2

Other characteristics Lattice: hexagonal, piezoelectric, may be triboluminescent, chiral (hence optically active if not racemic)

Etymology

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

The Ancient Greeks referred to quartz as κρύσταλλος (krustallos) derived from the Ancient Greek κρύος (kruos) meaning "icy cold", because some philosophers (including Theophrastus) understood the mineral to be a form of supercooled ice. Today, the term rock crystal is sometimes used as an alternative name for transparent coarsely crystalline quartz.

Classification

Roman naturalist Pliny the Elder believed quartz to be water ice, permanently frozen after great lengths of time. 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.

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, thus discovering the law of constancy of interfacial angles.

According to the 8th and 9th editions of Strunz's classification of minerals, quartz belongs to the mineral class of oxides due to its chemical composition, with a metal-oxygen ratio of 1:2. In the outdated 8th edition of the mineral systematics, he also gave his name to a group of chemically similar or identical minerals, the "quartz-tridymite-cristobalite group" with the system number IV/D.01a, whose other members were cristobalite, high cristobalite, high quartz, high tridymite and tridymite, as well as melanophlogite in the appendix.

In the Lapis mineral catalog by Stefan Weiß, last revised and updated in 2018, which, out of consideration for private collectors and institutional collections, still follows this old form of Karl Hugo Strunz 's system, the mineral was given the system and mineral number IV/D.01-010. In the " Lapis system, " quartz forms the "quartz series" with the system number IV/D.01 together with bosoite, chibaite, coesite, cristobalite, lechatelierite, melanophlogite, moganite, opal, seifertite, stishovite, and tridymite. 

However, the 9th edition of Strunz's mineral classification, last updated by the International Mineralogical Association (IMA) in 2009, subdivides the oxides more finely. Quartz and its related minerals β-quartz (previously known only as a synthesis), coesite, cristobalite, melanophlogite, moganite, seifertite, opal, stishovite, and tridymite are now assigned to the subdivision "With small cations: Silica family" and largely form individual mineral groups there. Lechatelierite (silica glass), which is also included in the classification, still has a questionable mineral status and is therefore not recognized by the IMA as an independent mineral.

James Dana 's classification classifies minerals according to their crystal structure. In quartz, silicon is surrounded by four oxygen atoms in a tetrahedral arrangement. These SiO 4 tetrahedra are linked by their vertices to form a three-dimensional framework. Therefore, in Dana's classification, quartz is classified as a framework silicate, specifically in the subdivision " Framework silicates: tetrahedral Si lattice, SiO 2 with -coordinated Si."

Chemistry

Quartz is a very pure compound and incorporates only trace amounts of other elements into its crystal lattice. Natural quartz can contain between 13 and 15,000 ppm (but usually only a few hundred ppm) of Al 3+, between 9 and 1400 ppm of Na +, between 3 and 300 ppm of K +, and smaller amounts of Fe 3+, Ti 4+, P 5+, H +, and Li +.

The incorporation of these ions usually occurs via a coupled replacement (substitution) of a Si 4+ ion by a trivalent and a monovalent ion, such as Al 3+ and Na +. The foreign ions are incorporated both at the Si positions in the lattice and at otherwise empty interstitial sites. In addition to the inclusion of foreign minerals, the incorporation of metal ions, sometimes in conjunction with ionizing radiation, is responsible for the different colors of quartz varieties.

Crystal structure

Quartz belongs to the trigonal crystal system at room temperature, and to the hexagonal crystal system above 573 °C (846 K; 1,063 °F). The former is called α-quartz; the latter is β-quartz. 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.

Well-formed crystals typically form as a druse (a layer of crystals lining a void), of which quartz geodes are particularly fine examples. 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.

Deep quartz is trigonal-trapezohedral (crystal class 32) and crystallizes in the enantiomorphic space groups P 3 1 21 (No. 152) and P 3 2 21 (No. 154). The dimensions of the unit cell are a 1 = a 2 = 4.9124 Å and c = 5.4039 Å. One unit cell contains three formula units of SiO 2. Silicon (Si) and oxygen (O) occupy crystallographically distinct atomic positions: 

Si: x = 0.4701; y = 0; z = 1/3

O: x = 0.4139; y = 0.2674; z = 0.2144

Each oxygen ion is surrounded by two silicon ions at a distance of 1.6054 Å and 1.6109 Å, and six oxygen ions at a distance of approximately 2.62 Å. The Si-O bonds have a large covalent component, which is the reason for the great hardness of quartz. The Si-O-Si bond angle is 143.61°. Accordingly, each silicon ion is surrounded tetrahedrally by four oxygen ions, two at a distance of 1.6054 Å and two at a distance of 1.6109 Å.

SiO 2 framework: The SiO 4 tetrahedra are linked to each other via the tetrahedral vertices, each tetrahedron to four neighboring tetrahedra. Along the c-axis, they are linked to form pairs of spiral chains. These SiO 4 tetrahedral helix pairs, which are not connected to each other, form six-sided, open channels along the c-axis.

α-Quartz crystals of the two enantiomorphic space groups differ in the direction of rotation of the tetrahedral screws. Left-handed α-quartz crystallizes in space group P 3 1 21 (No. 152), and the tetrahedral screws wind clockwise around the c-axis, facing the observer when viewed from above. Similarly, the tetrahedral screws of right-handed α-quartz (space group P 3 2 21 (No. 154)) wind counterclockwise, facing the observer. The spiral tetrahedral chains are linked to six neighboring tetrahedral spirals in such a way that each SiO 4 tetrahedron belongs to two neighboring tetrahedral chains and borders two of the six-sided channels.

Quartz is stable only at low temperatures in the trigonal α-quartz phase. At 573 °C, a phase transformation into the hexagonal β-quartz phase occurs. The transition from the β-quartz phase to α-quartz can be easily visualized by tilting robust tetrahedra around the <100> axis. The tilt direction determines the orientation of the α-quartz.

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).

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. However, there is a significant change in volume during this transition, and this can result in significant microfracturing in ceramics during firing, in ornamental stone after a fire and in rocks of the Earth's crust exposed to high temperatures, thereby damaging materials containing quartz and degrading their physical and mechanical properties.

Morphology

Well-formed crystals are common, and their shape can vary considerably depending on the growth conditions. The adjacent figure illustrates the typical prismatic crystal form of left-hand quartz and how this form is composed of the basic structures of the trigonal-trapezohedral class (class 32). The numbers in parentheses in the text and in the figure are the Miller indices. They are used in crystallography to designate crystal faces. Indices of crystal faces are placed in round brackets, indices of a group of faces forming a basic structure are placed in curly brackets, and indices of directions (crystal axes) are placed in square brackets.

The crystal form is dominated by a hexagonal prism in position I ({10 1 0}). The prism faces are parallel to the crystallographic c-axis. The prism is bounded at the ends by the positive and negative rhombohedrons ({10 1 1} and {01 1 1}), with the positive principal rhombohedron having larger faces.

Subordinate, i.e. smaller, various trigonal trapezohedra, mostly {51 6 1}, and trigonal bipyramids, mostly {11 2 1}, occur. Of these polyhedra, there are two enantiomorphic (left and right), but otherwise identical forms in crystal class 32. In an untwinned quartz crystal, either only right or only left trapezohedra and bipyramids occur, in left-hand quartz (space group P 3 1 21 (No. 152)) left-hand forms and in right-hand quartz (space group P 3 2 21 (No. 154)) right-hand forms. Right-hand and left-hand quartz can be differentiated based on the arrangement of the trapezohedral and bipyramid faces. In left-handed quartz these occur to the left of the main rhombohedral faces {10 1 1} and in right-handed quartz to the right of the main rhombohedral faces. 

Crystal and growth forms

Specific names have been established for striking growth forms of quartz:

Ticino habitus: quartz whose crystal form is dominated by large, very steep rhombohedral faces.

Skeletal quartz: During rapid crystal growth in supersaturated solutions, growth occurs primarily along the crystal edges and corners. Frame-like, prominent edges form around deeper crystal faces (frame quartz). Sometimes, these deeper crystal faces grow back from the protruding edges, forming thin quartz disks over a cavity (window quartz).

Cap quartz: Quartz crystals in which parts at the end of the crystal can be removed like a cap.

Cube quartz: Quartz whose crystal shape is dominated by the rhombohedral faces {10 1 1}. The angle between these faces in quartz is 85.5°, giving these crystals a cubic habit.

Scepter quartz: When a second, younger generation grows along the main axis of a quartz crystal, so-called scepter quartz crystals are formed. These "daughters" are usually clearer than the parent crystal. If subsequent crystal growth occurs only at one end of the crystal, the characteristic scepter-shaped crystal form is formed.

Thread quartz: Thread quartz forms when a fracture occurs during crystal growth, tearing the crystal apart. As the fracture opens, the crystal grows back together from both sides of the fracture. The fracture itself remains visible as a thin "thread" in the crystal. It appears on the ground and polished surface as grinding marks and a cluster of fine holes in lines (streaks).

Friedland quartz: quartz crystals with surface striations on the faces of the six-sided prism (10 1 0) transverse to the crystallographic c-axis or to the prism.

Phantom quartz: If crystal growth occurs in several phases, the different growth stages are visible in clear crystals through inclusion-rich zones.

Other names are used for certain intergrowths of several crystals:

Sprout quartz or artichoke quartz: Quartz that has formed many individual daughter crystals due to lattice defects, thus forming artichoke-shaped aggregates.

Twisted quartz (Gwindel): Parallel intergrowth of several plate-like crystals along a prism surface, whereby the crystallographic main axes of the individual crystals do not lie in one plane, but are twisted against each other.

Crystal twins

The two chiral forms of quartz, right-hand quartz and left-hand quartz, sometimes occur intergrown in an oriented manner.

Brazilian twin: Brazilian twin is the oriented intergrowth of the two enantiomorphic forms of deep quartz, right- and left-hand quartz, parallel to the prism face (11 2 0). Brazilian twins are often finely lamellar and typical of amethyst. Brazilian twin lamellae are concentrated in the {101} rhombohedral sectors. The incorporation of traces of iron into the quartz structure appears to play an important role in the formation of the finely lamellar Brazilian twins of amethysts. Corresponding to the concentration of twin lamellae in the {101} rhombohedral sectors, amethysts exhibit a higher iron concentration in these sectors. This sector zoning is visible in the rare variety ametrine (bicolored quartz crystals). The slightly iron-poor sectors are violet, and the slightly iron-rich zones are yellow.

Dauphinée twin (also Swiss or Alpine twin law): Dauphinée twin is the interpenetration of two deep quartz crystals with the same sense of rotation, so that the faces of the positive rhombohedra {h0 h l} of one crystal coincide with the faces of the negative rhombohedra {0h h l} of the other crystal. The twin axis is either or. The pyroelectric and piezoelectric effects of the two crystals cancel each other out. Dauphinée twins are therefore unsuitable for most technical applications.

Japanese twin: Twinning of deep quartz in the dipyramid II configuration (11 2 2). The prism axes of the twinned crystals intersect at an angle of 84° 33', giving the twins a characteristic heart-shaped form.

Liebisch twin

Esterel twin: twinning after (10 1 0)

Sardinia twin: twinning after (10 1 2)

Belodwa Beacon Twin: Twinning after (30 3 2)

Cornish twin: twinning after (20 2 1)

Wheal-Coats twin: twinning after (21 3 1)

Pierre-Levee twin: twinning after (21 3 3)

Piezoelectricity

Quartz crystals have piezoelectric properties; they develop an electric potential upon the application of mechanical stress. Quartz's piezoelectric properties were discovered by Jacques and Pierre Curie in 1880. Quartz exhibits a strong piezoelectric effect perpendicular to the prism axis along the a-axes. A quartz crystal reacts to pressure or tension with an electrical polarization along the direction of the force. Conversely, the application of a direct current leads to an extension or compression of the crystal. If an alternating current of suitable frequency is applied, the crystal can be excited to resonant oscillations. The resonant frequency depends on the geometry (shape and size) of the crystal. Due to the regularity and precision of these oscillations, quartz crystals are used in quartz oscillators as a time base and clock generator for electronic circuits, for example in clocks, computers, digital technology devices and radio technology.

Optical activity

Due to the crystallization of quartz in an enantiomorphic structure, the vibrational plane of the light passing through deep quartz in the direction of the c-axis is rotated. Providing exact measurements of this rotation is difficult because of the strong scatter caused by various interfering factors such as undetected twinning of right- and left-hand quartz or the smallest impurities. Furthermore, manufacturing tolerances make it difficult to produce precisely oriented quartz sections. Furthermore, the strength of the rotation of the vibrational plane of the light depends on the wavelength of the light (example: sodium D-line: 589.3 nm, green filter for mercury vapor lamps: 546 nm). Thus, the specified optical rotatory power for quartz varies between 21 and 28 °/mm depending on the source and wavelength. On the other hand, processed quartz in the form of quartz plates is ideal for testing polarimeters.

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.

The most important microstructure difference between types of quartz is that of macrocrystalline quartz (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 macrocrystalline varieties tend to be more transparent. Chalcedony is a cryptocrystalline form of silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite. Agate is a variety of chalcedony that is fibrous and distinctly banded with either concentric or horizontal bands. While most agates are translucent, onyx is a variety of agate that is more opaque, featuring monochromatic bands that are typically black and white. Carnelian or sard is a red-orange, translucent variety of chalcedony. Jasper is an opaque chert or impure chalcedony.

Major varieties of quartz

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

Varieties (according to color)

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. 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.

Pure quartz is completely transparent and colorless and, when it develops well-formed crystals, is called rock crystal (formerly from the Latin Cristallus). Quartz is usually milky-cloudy due to microscopic inclusions of liquids and gases (milky quartz) and appears gray when embedded in the rock. Transparent to milky-cloudy rolled pieces of rock crystal are also known under the name Rhine pebbles. These come primarily from the Alpine region and are found in the Rhine gravel. 

Due to the incorporation of coloring ions (generally Fe 3+ or Fe 2+), inclusion of colored minerals, or exposure to ionizing radiation, quartz can exhibit different colors. Based on the color and its cause, the following varieties are distinguished:

Color variations due to foreign ions or irradiation:

Ametrine from Bolivia

Amethyst: violet color due to the interaction of embedded iron ions and irradiation with gamma rays

Ametrine: rare quartz variety that shows sectors of amethyst and citrine coloration on one crystal

Citrine: yellow to orange-brown colored quartz (also artificially produced by firing)

Prasiolite (green quartz): leek-green and transparent quartz, which rarely occurs naturally and is also produced artificially by burning amethyst or yellowish quartz

Smoky quartz (Morion): colored by natural or artificial gamma rays, grey-brown (smoky) to black (Morion)

Nickel quartz: coarse green quartz colored by nickel.

Color variations due to inclusions

Blue quartz (sapphire quartz): a blue, opaque aggregate with embedded crocidolite fibers or dumortierite. Depending on the type of inclusion, blue quartz is also more precisely referred to as crocidolite quartz or dumortierite quartz. 

Iron silica: reddish-brown colored quartz due to hematite inclusions

Strawberry quartz is a variety and trade name for a quartz irregularly colored pink to red by reddish-brown hematite inclusions. It is usually more transparent and more intense in color than rose quartz.

Milky quartz: milky-cloudy quartz due to liquid inclusions

Prasem (emerald quartz): leek-green, opaque aggregate that gets its color from inclusions of actinolite.

Rose quartz: cloudy, pink colored quartz due to dumortierite inclusions, occasionally with asterism due to the inclusion of finest rutile needles

Microcrystalline SiO 2

Microcrystalline quartz refers to massive aggregates of very fine-crystalline quartz with crystal sizes in the micrometer range. Three forms are distinguished:

Chalcedony: microcrystalline, fibrous quartz, grown fibrously along a prism surface (“length-fast”).

Agate, Onyx: microcrystalline fibrous quartz with parallel-fibered (parabolic) or spherulitic structure

Jasper, Carnelian (Carnelian, Sarder), Moss Agate, Heliotrope, Sardonyx, Snow Quartz

Microquartz: microcrystalline, granular quartz with no recognizable preferred growth direction

Quartzine: microcrystalline, fibrous quartz, grown fibrously along the base surface (0001) of the hexagonal prism (“length-slow”).

Amethyst quartz is an opaque, banded intergrowth of amethyst and milky quartz.

All forms of microcrystalline quartz exhibit a high density of lattice defects and twinning.

Chert and flint are intergrowths of microcrystalline quartz with moganite in a random, granular structure. Strictly speaking, these are not minerals or mineral varieties, but rocks that are also collectively referred to as chert. This sometimes also includes chalcedony and its forms, as well as amorphous SiO 2 (opal).

Other varieties and trade names

Aventurine quartz, hawk's eye, tiger's eye, cat's eye quartz: quartz with inclusions of plate-like or fibrous minerals such as fuchsite, rutile, asbestos

The Aqua Aura often found in stores is not a variety, but rather rock crystal (or another quartz) that has been vapor-coated with metal (predominantly gold). The result is a transparent, blue-colored crystal, sometimes with a multicolored shimmer.

Brasilite, on the other hand, is the trade name for a quartz that has a greenish-yellow to pale yellow color when fired. The Safien Valley (Graubünden, Switzerland) is the world's first location for mantle quartz, whose tip is slightly recessed into the prism.

Modifications

Quartz is the stable form (modification) of crystalline silicon dioxide on the Earth's surface. Numerous other modifications occur at higher pressures and temperatures. Some can remain metastable at the Earth's surface.

At low temperatures (70–200 °C) a mixture of quartz and moganite, a characteristic component of quartzine and chalcedony, crystallizes from SiO 2 gel.

At temperatures above 573 °C (at 1013.2 hPa), quartz transforms into high quartz (quartz jump ). The phase transformation occurs very rapidly, and high quartz never remains metastable, even upon rapid cooling. Although quartz crystals with the crystal form of high quartz (paramorphism) are found in some magmatites, structurally they are quartz.

At higher temperatures, high quartz first transforms into tridymite (from 867 °C), then into cristobalite (from 1470 °C). Cristobalite melts at 1727 °C (temperatures based on 1013.2 hPa).

The transition temperatures depend on the pressure. In general, they increase with increasing pressure.

At high pressures, such as those found in the Earth's mantle or during meteorite impacts, particularly dense SiO 2 phases form. Coesite (3.01 g/cm³) forms at 2 G Pa and above, stishovite (4.3 g/cm³) at 7.5 G Pa, and seifertite (4.12 g/cm³) at approximately 78 G Pa.

Occurrence

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. 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.

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. Well-formed crystals may reach several meters in length and weigh hundreds of kilograms.

The largest documented single crystal of quartz was found near Itapore, Goiaz, Brazil; it measured approximately 6.1 m × 1.5 m × 1.5 m (20 ft × 5 ft × 5 ft) and weighed over 39,900 kg (88,000 lb).

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.

Related silica minerals

Tridymite and cristobalite are high-temperature 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. Moganite is a monoclinic polymorph. Lechatelierite is an amorphous silica glass SiO2 which is formed by lightning strikes in quartz sand.

Uses

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

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. However, the quality and size of the crystals that were produced by these early efforts were poor.

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. A major mining location for high purity quartz is the Spruce Pine Gem Mine in Spruce Pine, North Carolina, United States. Quartz may also be found in Caldoveiro Peak, in Asturias, Spain.

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. 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. (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. 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.

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. George Washington Pierce designed and patented quartz crystal oscillators in 1923. 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. 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.

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. The popularity of crystal healing has increased the demand for natural quartz crystals, which are now often mined in developing countries using primitive mining methods, sometimes involving child labor.

As a raw material

Quartz sand or powder, together with kaolin and feldspar, is an additive for porcelain and a variety of other ceramic materials.

Quartz sand or ground quartz rock is melted to produce glass and quartz glass. Quartz glass is a glass-like solid made from fused (crystalline) quartz or silicon dioxide; the correct name is therefore silica glass. Quartz glass and artificial quartz single crystals (pure rock crystal) are ground into optical prisms and lenses. Quartz glass is also used in standard scales and weights, as a thread for torsion balances, and as an optical fiber.

In addition, quartz gravel and crushed quartz are starting materials for the production of silicon.

As material

Quartz and quartz glass react with only a few chemicals. Hydrofluoric acid is the only acid capable of dissolving quartz, forming silicon tetrafluoride or hexafluorosilicic acid. This property is beneficial for a variety of applications:

Containers for chemicals

In fluidized bed combustion, quartz sand is swirled with the air to improve heat transfer and optimize the combustion process.

Quartz is also used in the form of fireproof stones.

Its high strength, which prevents plant growth, leads to the mineral's use as railway ballast. Quartz is unsuitable for road gravel because it is too hard, binds poorly, and causes rapid tire wear.

Quartz crystal plates made of unweathered quartz are used in electroacoustics.

Quartz sand serves as an abrasive and filler, as well as for arc quenching in fuses. It is also used in the production of water glass and silicate paints. Mixed with polymers, it also serves as a material for creating hard surfaces for floors and countertops.

Use of piezoelectric properties

Artificially grown quartz single crystals are used for piezoelectric applications, among other things. The piezoelectric properties of quartz are exploited in quartz oscillators, which, when suitably excited by an electrical voltage, oscillate mechanically at a fixed frequency. This made it possible to build highly accurate quartz clocks. Today, quartz oscillators are found as clock generators in virtually all electronic devices. Quartz is also suitable for pressure measurements, in high-frequency technology, and as an acousto-optical Q-switch in lasers.

The two chiral forms of quartz, right-hand quartz and left-hand quartz, exhibit opposing piezoelectric effects. In such twins, the piezoelectric effects cancel each other out in the entire crystal, making them unsuitable for piezoelectric applications and less commonly used than synthetic quartz. For technical applications, the twins are often cut parallel to the (O1-1) plane (AT cut) or (O23) plane (BT cut), since the piezoelectric effect perpendicular to these planes is almost independent of temperature.

As a gemstone

Quartz varieties such as agate, violet amethyst, lemon-yellow citrine, blood-red jasper or black-and-white striped onyx are used in the jewelry industry to make gemstones because of their great hardness and the mineral's ability to be cut and polished.

Quartz and fossilization

If silica-rich groundwater penetrates the tissue of dead woody plants, these plants can fossilize through the crystallization of microcrystalline quartz (Si(OH) 4 → SiO 2 + 2 H 2 O). Although the woody tissue is replaced by quartz, the original cell structure is often preserved. Paleobotanists can use this information to draw conclusions about the plant's former growth conditions. Fossilized Araucaria cones from Patagonia are also known.

Silicification also occurs in animals. In this case, an exoskeleton or shell formerly made of calcium carbonate (CaCO 3) is often replaced by microcrystalline quartz. Known examples include silicified corals from the Miocene of Florida  and the Triassic of British Columbia and Alaska, opalescent snails, mussels, and vertebrate remains from the Lower Cretaceous of the Lightning Ridge in Australia, and silicified snails from the Deccan Traps (Upper Cretaceous) in India. If the interior of these snails' shells was not completely filled with sediment after the extensive decomposition of the soft parts, agate-like druses may also have formed within them.

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. 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.

However, hardly any dust is generated during the cutting of gemstones because the grinding process is always sufficiently cooled and absorbed with an excess of water, emulsion, petroleum, or a special grinding oil. Dry grinding would also damage or destroy most gemstones.

Synthetic and artificial treatments

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.

Prasiolite, an olive colored 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-treating amethyst or smoky quartz. Carnelian has been heat-treated to deepen its color since prehistoric times.

Because natural quartz is often twinned, synthetic quartz is produced for use in industry. Large, flawless, single crystals are synthesized in an autoclave via the hydrothermal process.

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

Esotericism

Already in ancient times, but also in the Middle Ages, various gemstones were considered to be "outflows of the stars" and according to this superstition, gemstones with alleged magical properties were assigned to the known planets, and later also to the zodiac signs and months. For example, quartz in the rock crystal variety represents the zodiac sign Leo, and in the quartz-cat's eye variety, it represents Capricorn. Depending on the source, quartz or rock crystal can also be assigned to the zodiac signs Taurus, Gemini or Sagittarius. As the "month stone", rock crystal represents April and as a "planetary stone" according to Richardson and Huett (1989), along with tiger's eye, it represents Saturn and, alongside several other minerals, Neptune. 

In modern Western esotericism, pure quartz (rock crystal) is considered a healing stone that is said to protect against harmful rays, relieve headaches and various inflammations, cleanse the liver and kidneys, and strengthen blood circulation (varicose veins). However, there is no scientific evidence for the listed effectiveness.

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