Quartz

• α - quartz
• Low quartz

Called quartz, quartz, or even low - α quartz, is a mineral having the chemical composition of SiO2 and trigonal symmetry. He is on the earth's surface stable form ( modification) of the silica and after the feldspars the second most common mineral of the Earth's crust. At a temperature of about 573 ° C ( under a pressure of 1 bar) depth of quartz proceeds at high crystal by changing the crystal structure.

With a Mohs hardness of 7 Quartz is one of the hard minerals and serves as a reference to the to 10 ( diamond) reaching scale after Friedrich Mohs. He often provides well-developed crystals of great variety of shapes and colors (see modifications and varieties ), whose crystal surfaces have vitreous luster. Quartz has no cleavage, conchoidal breaks like glass and shows the fracture surfaces of a greasy shine.

In industry, quartz is one of the most important minerals and has equally as a building material as a raw material for the ceramic, glass and cement industries worldwide importance. Quartz pebbles and quartz gebrochenener are raw material for the production of silicon.

In addition, quartz and its color varieties are valued as a gemstone since ancient times (see use ).

Quartz crystals are produced synthetically: cut from them, among other quartz crystals are used as a clock in electronic circuits and quartz watches.

Quartz is occasionally confused with the calcite, but can be easily distinguished by its greater hardness, lower birefringence and the reaction of the calcite with dilute hydrochloric acid therefrom.

• 6.1 colors by foreign ions and radiation
• 6.2 colors by inclusions
• 6.3 Microcrystalline SiO2
• 6.4 Other varieties and trade names

• 7.1 Crystal and growth forms
• 7.2 crystal twins

• 9.1 The raw material
• 9.2 As the material
• 9.3 Use of the piezoelectric properties
• 9.4 As a gemstone

Special Features

Chemical composition

Quartz is a very pure compound and builds other elements only in traces in the crystal lattice. Natural Crystals can 13-15000 ppm ( but usually only a few 100 ppm) Al3 , 9-1400 ppm Na , 3-300 ppm K , as well as lesser amounts of Fe3 , Ti4 , P5 , H and Li included.

The incorporation of these ions is usually done via a coupled substitution (substitution ) of a Si4 ion by a trivalent and monovalent ion such as Al 3 and Na . The impurity ions are incorporated on both the Si positions in the lattice as well as on otherwise empty interstitial sites. The incorporation of iron and aluminum is responsible for the different colors of the quartz varieties, together with the action of ionizing radiation.

Piezoelectricity

Crystal shows a strong piezoelectric effect perpendicular to the prism axis along the a-axis. Under pressure or train a quartz crystal reacts with an electric polarization along the direction of force. Conversely, the application of an electrical direct voltage leads to an expansion or compression of the crystal. When an alternating voltage of suitable frequency is applied, the crystal may be excited into resonance. The resonant frequency is dependent on the geometry ( shape and size ) of the crystal. Due to the regularity and accuracy of these oscillations quartz crystals used in crystal oscillators and clock as a time base for electronic circuits, such as in watches, computers, devices, digital technology and the wireless technology.

Optical activity

By the crystallization of the crystal in an enantiomorphic structure, the plane of oscillation of the light that passes through a low- quartz in the direction of c-axis, is rotated. The providing exact measurements of this rotation is difficult because measurement results due to various confounding factors such as undetected Verzwillingungen of right and left quartz or smallest impurities scatter strongly. In addition, manufacturing tolerances complicate the construction -oriented quartz accurate cuts. Furthermore, the strength of the rotation of the oscillation plane of the light depends on the wavelength of light (for example, sodium D line: 589.3 nm, a green filter for mercury lamp: 546 nm). Thus, the specification of the optical rotatory power in quartz varies depending on the source wavelength and 21 to 28 ° / mm.

Etymology and history

Quartz was a name for the mine and for all crystals in the Middle Ages. Only with Georgius Agricola in the 16th century, the term was limited to rock crystals. The word origin is unclear. In the Slavonic question comes tvurdu for "hard". The Middle High German quartz, quärz or quantity designation Querze is the New Latin quarzum ( silex ) " gravel, rock " related and comes from the older name quaterz or quaderz "bad ore ", which lasted until the 16th century. According to others, accepting the word is contracted from gewärz " outgrowth ". Finally, the word as a contraction of the term Saxon miners ' Querklufterz " said to have originated. The name " quartz " has established itself internationally ( with slight language-specific modifications such as " quartz" in English and French ).

Classification

After the 8th and 9th edition of strunz between systematics of the minerals quartz belongs to the mineral class of oxides with a metal - oxygen ratio of 1:2 due to its chemical composition.

In the 8th edition of the Mineral classification, he is also named for a group of chemically similar or the same minerals, the quartz group, whose other members coesite, cristobalite, melanophlogite, Moganite, opal, tridymite and stishovite are.

The 9th edition of mineral classification by Strunz divided the oxides but fine. Quartz and its related minerals beta - quartz ( existence only known as synthesis) coesite, cristobalite, melanophlogite, Moganite, Seifertit, Opal, stishovite and tridymite are now the subdivision (Chemical compound ) With small cations: attributed to silica - family. That in the scheme also listed with Lechatelierite ( silica glass) but still has a questionable mineral status and is therefore of the International Mineralogical Association (IMA ) is also not recognized as a separate mineral.

The systematics of James Dana arranges the minerals according to their crystal structure. In the crystal silicon is tetrahedrally surrounded by four oxygen atoms. This SiO4 tetrahedra are linked through their corners to a three-dimensional scaffold, and quartz is therefore assigned to the framework silicates in the scheme of Dana.

Education and Locations

Quartz crystallized SiO2 - rich melt during cooling and is the primary component of SiO2 - rich plutonic rocks (quartz rich granitoids, granites, granodiorites, tonalites, quartz syenite, quartz monzonite, quartz diorite), dykes (eg aplites ) and the corresponding volcanics ( rhyolites, Dacite, andesite, quartz trachyte, quartz latite ). The quartz contents of these rocks are one of the main criteria for the classification of igneous rocks using the Streckeisen diagram.

Quartz is found in many metamorphic rocks (eg, shale and gneiss ) and is degraded or formed over countless mineral reactions during metamorphism. For example, marks the quartz- producing reaction of chloritoid and staurolite and quartz aluminosilicate to the boundary between greenschist facies and Amphibolithfazies in metapelites.

Because of its hardness and resistance to weathering quartz is common to find in sedimentary rocks such as sandstones and soils. In the Obernkirchener sand quarries of the so-called Bremer stone is mined for centuries. However, well-trained quartz crystals are preferentially formed in clefts, corridors and as a lining of natural cavities, so-called geodes.

Modifications

Quartz is the most stable form on the surface of the earth ( modification ) of the crystalline silica. Numerous other modifications will occur at higher pressures and temperatures. Some may remain metastable at the earth's surface.

At low temperatures ( 70-200 ° C) crystallized from SiO2 gel is a mixture of quartz and Moganite, a characteristic component of quartzine and Chalcedon.

At temperatures above 573 ° C ( 1013.2 hPa) located quartz converts to high quartz. The phase transition takes place very rapidly, and high quartz never remains metastable even if rapid cooling. Although found in some igneous quartz crystals with the crystal form of high- quartz ( Paramorphose ), structurally it is, however, to quartz.

At higher temperatures, high quartz to tridymite converts only to (from 867 ° C), then in cristobalite ( from 1.470 ° C). Cristobalite melts at 1,727 ° C ( temperatures in each case based on 1013.2 hPa).

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

At high pressure, as prevails in the mantle or occurs with meteorite impacts, particularly dense SiO2 phases form. From 2 GPa coesite forms ( 3.01 g / cm ³), from 7.5 GPa stishovite (4.3 g / cm ³) and from about 78 GPa Seifertit ( 4.12 g / cm ³).

Varieties

Pure quartz is completely transparent and colorless, and, if he develops well -formed crystals, known as rock crystal. Crystals are usually due to microscopic inclusions of liquids and gases milky ( milky quartz ) and appear to be growing in rock gray. Under the name rhinestone transparent to milky Roll pieces of rock crystal are also known, which mainly originate from the Alpine region and are found in the Rhine gravel.

The incorporation of coloring ions ( Fe3 or generally Fe2 ), inclusion of colored minerals or exposure to ionizing radiation can be differently colored crystals. The color and the cause of the following varieties are distinguished:

Colors by foreign ions and radiation

• Amethyst: violet coloration by the interaction of the stored iron ions and gamma-ray irradiation
• Ametrine: rare quartz variety, the sectors with Amethyst and Citrinfärbung in a crystal shows
• Citrine: yellow to orange brown colored crystals (also artificially produced by burning)
• Prasiolite (Green Quartz ): leek green and translucent quartz, which occurs rarely natural and artificially created by burning amethyst or yellowish crystals for
• Colored by natural or artificial gamma-ray gray-brown ( smoked color ) to black ( Morion ): Smoky Quartz ( Morion )
• Nickel Quartz: rough green quartz, which is colored by nickel.

Colors by inclusions

• Blue Quartz (sapphire crystal ): blue, opaque unit with embedded crocidolite fibers or Dumortierite. Depending on the type of containment Blue Quartz is also called crocidolite precise quartz or quartz - Dumortierite or Dumortieritquarz.
• Eisenkiesel: maroon colored by Hämatiteinschlüsse quartz
• Strawberry quartz, a variety and commercial name for an irregular pink through reddish brown to red-colored quartz Hämatiteinschlüsse. He is mostly transparent and color stronger than the rose quartz.
• Milky Quartz: milchigtrüber by fluid inclusions of quartz
• Prasem ( emerald quartz): leek green, opaque unit that gets its color by inclusions of actinolite.
• Rose Quartz: by Dumortieriteinschlüsse dull, pink colored quartz, sometimes with asterism by intercalation of fine rutile needles

Microcrystalline SiO2

Under microcrystalline quartz is meant massive aggregates of very fine crystalline quartz with crystal sizes in the micrometer range. Here we distinguish three forms:

• Chalcedony: microcrystalline, fibrous quartz, fibrous grown along a prism surface [ 11-20 ] ( "length - fast").
• Micro quartz: microcrystalline granular quartz with no visible preferred growth direction
• Quartzine: microcrystalline, fibrous quartz, fibrous grown along the basal plane (0001 ) of the hexagonal prism ( "length - slow").

Amethystquarz is an opaque, banded intergrowth of Amethyst and Milky Quartz.

All forms of microcrystalline silica have a high density of lattice defects and Verzwillingungen.

Chert, flint ( flint) and their innumerable colored by contents colored minerals varieties are intergrowths of microcrystalline quartz with Moganite in a regular loose, granular structure. This is strictly speaking not minerals and Mineralvarietäten, but rocks. Of the countless, often ill-defined names for varieties of this silicate rocks only a few are listed below:

• Agate, Onyx: microcrystalline fibrous crystals with parallelfaserigem ( parabolic ) or nodular structure
• Jasper, carnelian ( carnelian, carnelian ), moss agate, heliotrope, sardonyx, Snow Quartz: by impurities colored cherts

Other varieties and trade names

• Aventurine quartz, Hawk's Eye, Tiger's Eye, Cat's Eye Quartz: Quartz with inclusions platy or fibrous minerals such as Fuchsite, rutile, asbestos

The often to be found in the commercial Aqua Aura is not a variety, but mostly rock crystal ( quartz or another ), the (mainly gold) was vapor deposited metal. Result is a transparent, blue-colored crystal, some with multi-colored shimmer.

Brasilit contrast, the commercial name for a greenish- yellow to pale yellow colored by firing quartz. In Safiental (Graubünden, Switzerland ), the world's first finds the jacket quartz were found, the tip of which is sunk a little in the prism.

Morphology

Well -formed crystals are common and their shape may be quite different depending on growth conditions. The figure illustrates the typical prismatic crystal form of quartz links and how this form of the basic bodies of Trigonal trapezoedrischen class ( Class 32 ) is composed. The numbers in brackets in the text and in the figure are the Miller indices. They can be used in crystallography for designating crystal faces. Indices of crystal faces are enclosed in parentheses, indices of an area group that form a body, in braces and indexes of directions (crystal axes) in square brackets.

Dominating the crystal form from hexagonal prism I. position ({ 1010 }). The prism surfaces are parallel to the crystallographic c -axis. The prism is limited to the ends of the positive and negative rhombohedral ({ 1011 } and { 0111 }), wherein the positive Hauptrhomboeder occurs with larger surfaces.

Subordinate, that is, made smaller, experience various trigonal trapezohedron, mostly { 5161 }, and trigonal bipyramids, mostly { 1121 } on. Of these polyhedra exist in the crystal class 32 two enantiomorphic (left and right ), but otherwise identical shapes. At one untwinned quartz crystal occur either right or left only trapezohedron and bipyramids on the links quartz (space group P3121 ) left on the right forms and quartz (space group P3221 ) right forms. Can be distinguished right and left crystals based on the arrangement of the trapezohedron and Bipyramidenflächen. When these links quartz occur on the left of the Hauptrhomboederflächen { 1011 } and the legal right of the quartz Hauptrhomboederflächen.

Crystal and growth forms

For conspicuous growth forms of quartz is its own name have been established:

• Ticino Habit: Crystals, whose crystal structure are dominated by large, steep rhombohedron.
• Skeletal crystal: In a fast crystal growth in supersaturated solutions, the growth takes place particularly along the crystal edges and corners. It formed like a frame highlighted edges to deeper crystal faces ( frame quartz). Sometimes these crystal faces grow deeper by the protruding edges forth again, with thin quartz disks over a cavity form (window Crystals ).
• Cap Quartz: Quartz crystals in which roles such as a cap are removable at the end of the crystal.
• Cube Quartz: crystals, the crystal shape is dominated by the rhombohedron { 1011 }. The angle between these faces is the quartz 85.5 °, which gives the crystals a cube-shaped habit.
• Scepter Quartz: Grows on a quartz crystal in direction along the major axis, a second, younger generation, so-called scepter quartz crystals form. The "daughters" are usually clearer than the parent crystal. If the subsequent crystal growth at only one end of the crystal, the characteristic, scepter -shaped crystal form is emerging.
• Quartz thread: A thread quartz arises when during the crystal growth, a chasm crack occurs and the crystal pulling apart. During the opening of the gap of the crystal from both sides of the crack from growing back together. The crack itself is visible as a thin "thread" in the crystal. It occurs on the ground and polished surface as sanding marks and accumulation of fine holes in lines ( stripes) to days.
• Friedlander quartz: Quartz crystals Flächenstreifung the surfaces of the hexagonal prism ( 1010) transversely to the crystallographic c -axis and the prism.
• Phantom Quartz: If the crystal growth in several phases, the different stages of growth are visible by inclusion- rich zones in clear crystals.

Other names are used for specific adhesions of several crystals:

• Sprout crystals or artichokes Crystals: Crystals that have formed due to lattice defects formed many individual daughter crystals and so artichokes shaped aggregates.
• Spiral Crystals ( Gwindel ): Parallelverwachsung several platy crystals along a prism surface, the main crystallographic axes of the individual crystals are not in a plane but are rotated relative to each other.

Crystal twins

The two chiral forms of quartz, quartz right and left quartz, occur sometimes oriented grown up together.

• Brazilian twins: As a Brazilian, twin is called the oriented intergrowth of the two enantiomorphic forms of the low- quartz, right and left quartz parallel to the prism surface ( 1120). Brazilian twins are often feinlamellar and typical of amethyst. There you will find Brazilians twin lamellae concentrated in the { 101 } - Rhomboedersektoren. The incorporation of traces of iron in the quartz structure seems to play an important role in the formation of fine lamellar Brazilian twins of amethysts. According to the concentration of the twin lamellae in the { 101 } - Rhomboedersektoren show amethysts a higher iron concentration in these sectors. In the rare variety Ametrine (two colored quartz crystals) is this Sektorzonierung visible. The somewhat poorer iron sectors are purple and some iron -rich zones in yellow.
• Dauphinee - twin (also Swiss or alpine twin law ): As Dauphinee - twin is defined as the interpenetration of two low- quartz crystals with the same direction, so that the faces of the positive rhombohedron { h0hl } of a crystal individual with the surfaces of the negative rhombohedron { 0hhl } of the other individual crystal coincide. The twin axis is either or. The pyro - and piezoelectric effects of the two crystal individuals stand out here on each other. Dauphinee Twins are therefore unsuitable for most technical applications.
• Japanese twin: twinning of deep quartz after Dipyramid II position ( 1122). The prism axes of the twinned crystals intersect here at an angle of 84 ° 33 ', which gives the twins a characteristic heart-shaped form.

• Liebisch twin
• Esterel twin: twinning by (1010 )
• Sardinia - twin: twinning by ( 1012)
• Belodwa beacon twin: twinning by ( 3032 )
• Cornish - twin: twinning by (2021)
• Wheal Coats - twin: twinning by (2131)
• Pierre- Levee - twin: twinning by ( 2133 )

Crystal structure

Low quartz is trigonal trapezoedrisch ( crystal class 32) and crystallizes in the enantiomorphic space groups P 1 and P 2 31 32 2 1 The dimensions of the unit cell are a1 = a2 = 4.9124 Å and c = 5.4039 Å. A unit cell contains three formula units of SiO2. 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 about 2.62 Å. The Si -O bonds have a large covalent component, what is the cause for the great hardness of quartz. Si -O -Si bond angle is 143.61 °. Accordingly, each Siliciumion is tetrahedrally surrounded by four oxygen ions, two at a distance of 1.6054 Å and two at a distance of 1.6109 Å.

SiO2 framework: The SiO4 tetrahedra are linked to each other via the tetrahedron corners, each tetrahedron with four neighboring tetrahedra. In the direction of c-axis, they are linked in pairs by helical chains. This SiO4 tetrahedra helix pairs that are not interconnected, forming 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 tetrahedra screws. Links Händischer α - quartz crystallizes in the space group P3121 and the tetrahedron screws wind in a clockwise direction around the c- axis toward the viewer when viewed from the top to the c- axis. Accordingly, the tetrahedron screws of the right-handed α - quartz (space group P3221 ) wind counterclockwise towards the viewer. The helical chains of tetrahedra are six neighboring tetrahedra spirals linked so that each SiO4 tetrahedron belongs to two adjacent chains of tetrahedra and is adjacent to two of the six-sided channels.

Quartz is only stable at low temperature in the α - trigonal crystal phase. At 573 ° C a phase transformation takes place in the hexagonal β - quartz phase. The higher the symmetry of the β - crystal, among other things leads to a loss of the piezoelectric properties. The transition from the β - quartz phase to α - quartz can be simplified slightly by tilting robust tetrahedra around the <100 > axis illustrate. The tilt direction determines the orientation of the α - quartz.

α - quartz structure: pair of helical chains along the c - axis, a chain highlighted in blue; Look at the a- c plane

α - quartz structure: pair of helical chains along the c - axis, a chain highlighted in blue; View along the c axis (3- fold axis )

α - quartz structure: Linking spiral chains, a chain highlighted in blue; View along the c axis (3- fold axis )

Use

The raw material

• Quartz sand or powder, together with kaolin and feldspar, an aggregate of porcelain and a variety of other ceramic materials.
• Quartz sand or ground quartz rock is melted to glass and quartz glass production. Quartz glass is, a melted from ( crystalline ) quartz or silica, glassy frozen solid; the correct term is, therefore, silica glass. Quartz glass and artificial quartz crystals ( Pure rock crystal) are ground to optical prisms and lenses. Quartz glass is used in standard scales and standard weights and as a thread for torsion and as an optical waveguide.
• Furthermore, quartz gravel and crushed quartz starting material for the production of silicon.

As the material

Quartz and silica react with only a few chemicals, hydrofluoric acid is the only acid that can dissolve quartz; this is silicon tetrafluoride or hexafluorosilicic form. This property is beneficial for a variety of applications:

• Vessels for chemicals
• In fluidized bed combustion quartz sand is fluidized with air to enhance heat transfer and to optimize the combustion process.
• In addition, quartz is used in the form of refractory bricks.
• Its high strength, which prevents plant growth, leads to the use of the mineral as railway ballast body. Quartz is unsuitable as road gravel because it is too hard, binds poorly and causes rapid wear of the tires.
• Quartz crystal plates from unweathered quartz used in electro-acoustics.
• Silica sand is used as an abrasive and filler, and for arc quenching in fuses.
• Quartz sand mixed with polymers to create hard surfaces for floors and countertops.

Use of the piezoelectric properties

Artificially drawn quartz crystals are among others used for piezoelectric applications. The piezoelectric properties of the crystal can be exploited in quartz crystals, which vibrate like a tuning fork when excited by an electrical voltage at a fixed frequency mechanical. The construction of very accurate continuous quartz watches was possible. Today you will find in almost all electronic devices quartz crystals as the clock source. In addition, quartz is also suitable for pressure measurements in the high-frequency technology as well as acousto-optic Q-switches in lasers.

The two chiral forms of quartz, quartz right and left quartz, to show an opposite piezoelectric effect. In such twins, therefore, the piezoelectric effects in the overall crystal pick up, which makes them unusable for piezoelectric applications and are used less frequently over synthetic quartz. For technical applications, the twins are level ( AT-cut ) or ( 023 ) plane (BT section) cut parallel to the common ( 01-1 ) because the piezoelectric effect is perpendicular to these planes is nearly independent of temperature.

As a gemstone

Quartz varieties such as agate, amethyst, the violet, the lemon-yellow citrine, the blood-red jasper or black and white striped onyx are processed into semi-precious stones because of the high hardness and good cutting and polishing of the mineral in the jewelry industry.

Quartz and fossilization

Penetrates silica- rich groundwater into the tissue of dead, woody plants, which these are brought to crystallization of quartz ( Si ( OH) 4 → SiO2 2 H2O ) the original cell structure can fossilize, the woody tissue is indeed replaced by crystalline quartz, but often receive remains. Palaeobotanist can for example draw conclusions about the former growth conditions of the plant from today.

Precautions

The degradation of quartz, it is partly to substantial deposit of dust inhaled over a long time, lead to the dreaded among miners silicosis. However, it comes in grinding the gemstones never to dust formation, since the grinding operation is always sufficiently cooled with water, emulsion, petroleum or a special grinding oil. A dry sanding would damage or destroy the most precious stones.

Esoteric

In the esoteric pure quartz ( rock crystal ) is considered a healing stone, the protection from harmful rays, relieve headaches and various inflammations, liver and kidney cleanse and to strengthen the blood circulation ( varicose veins ). Quartz is associated with the zodiac sign of Leo, the planet Saturn and Neptune and the month of April. The different varieties such as yellow citrine or amethyst purple properties are also mainly attributed to that can be derived from the mythology of their color, for example yellow for power and purple for spirituality. In biodynamic agriculture, the preparation horn silica is employed. Scientific evidence for the effectiveness, however, are not available.