Cristobalite

Cristobalite is a mineral polymorph of silica that is formed at very high-temperatures. It is used in dentistry as a component of alginate impression materials as well as for making models of teeth [6]

It has the same chemical formula as quartz, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group, which also include coesite, tridymite and stishovite. Cristobalite occurs as white octahedra or spherulites in acidic volcanic rocks and in converted diatomaceous deposits in the Monterey Formation of the US state of California and similar areas. Cristobalite is stable only above 1470 °C, but can crystallize and persist metastably at lower temperatures. It is named after Cerro San Cristóbal in Pachuca Municipality, Hidalgo, Mexico.

The persistence of cristobalite outside its thermodynamic stability range occurs because the transition from cristobalite to quartz or tridymite is "reconstructive", requiring the breaking up and reforming of the silica framework. These frameworks are composed of SiO4 tetrahedra in which every oxygen atom is shared with a neighbouring tetrahedron, so that the chemical formula of silica is SiO2. The breaking of these bonds required to convert cristobalite to tridymite and quartz requires considerable activation energy and may not happen on a human time frame. Framework silicates are also known as tectosilicates.

There is more than one form of the cristobalite framework. At high temperatures, the structure is cubic, Fd3m, No. 227, Pearson symbol cF104.[7] A tetragonal form of cristobalite (P41212, No. 92, Pearson symbol tP12[8]) occurs on cooling below about 250 °C at ambient pressure and is related to the cubic form by a static tilting of the silica tetrahedra in the framework. This transition is variously called the low-high or transition. It may be termed "displacive"; i.e., it is not generally possible to prevent the cubic β-form from becoming tetragonal by rapid cooling. Under rare circumstances the cubic form may be preserved if the crystal grain is pinned in a matrix that does not allow for the considerable spontaneous strain that is involved in the transition, which causes a change in shape of the crystal. This transition is highly discontinuous. The exact transition temperature depends on the crystallinity of the cristobalite sample, which itself depends on factors such as how long it has been annealed at a particular temperature.

The cubic β phase consists of dynamically disordered silica tetrahedra. The tetrahedra remain fairly regular and are displaced from their ideal static orientations due to the action of a class of low-frequency phonons called rigid unit modes. It is the "freezing" of one of these rigid unit modes that is the soft mode for the α–β transition.

In the α–β phase transition only one of the three degenerate cubic crystallographic axes retains a fourfold rotational axis in the tetragonal form. The choice of axis is arbitrary, so that various twins can form within the same grain. These different twin orientations coupled with the discontinuous nature of the transition can cause considerable mechanical damage to materials in which cristobalite is present and that pass repeatedly through the transition temperature, such as refractory bricks.

When devitrifying silica, cristobalite is usually the first phase to form, even when well outside its thermodynamic stability range. This is an example of Ostwald's step rule. The dynamically disordered nature of the β-phase is partly responsible for the low enthalpy of fusion of silica.

The micrometre-scale spheres that make up precious opal exhibit some x-ray diffraction patterns that are similar to that of cristobalite, but lack any long-range order so they are not considered true cristobalite. In addition, the presence of structural water in opal makes it doubtful that opal consists of cristobalite.[9][10]

B-cristobal1

An idealized model of β-cristobalite, showing corner-bonded SiO4 tetrahedra. In reality the tetrahedra are constantly tumbling.

B-cristobal2

β-cristobalite viewed along another direction.

A-cristobal

The crumpled framework of α-cristobalite, related to the β-form by static tilting of the tetrahedra.

Α-Cristobalite

Unit cell of α-cristobalite; red spheres are oxygen atoms.

Β-Cristobalite

Unit cell of β-cristobalite; red spheres are oxygen atoms.

Cristobalite
Cristobalite-Fayalite-40048
Cristobalite spheres formed by devitrification from the obsidian matrix (California, USA) 5.9×3.8×3.8 cm
General
CategoryOxide mineral, quartz group
Formula
(repeating unit)
SiO2
Strunz classification4.DA.15
Dana classification75.1.1.1
Crystal systemTetragonal
Crystal classTrapezohedral (422)
Space groupP41212, P43212
Unit cella = 4.9709(1) Å,
c = 6.9278(2) Å;
Z = 4 (α polytype)
Identification
ColorColorless, white
Crystal habitOctahedra or spherulites up to several cm large
Twinningon {111}
FractureConchoidal
TenacityBrittle
Mohs scale hardness6–7
LusterVitreous
StreakWhite
DiaphaneityTransparent
Specific gravity2.32–2.36
Optical propertiesUniaxial (−)
Refractive indexnω = 1.487
nε = 1.484
Birefringence0.003
PleochroismNone
Melting point1713 °C (β)[1]
References[2][3][4][5]

References

  1. ^ Deer, W. A., R. A. Howie and J. Zussman, An Introduction to the Rock Forming Minerals, Logman, 1966, pp. 340–355 ISBN 0-582-44210-9.
  2. ^ Mineralienatlas.
  3. ^ Cristobalite. Handbook of Mineralogy.
  4. ^ Cristobalite. Mindat.
  5. ^ "Cristobalite Mineral Data". Webmineral.
  6. ^ Anusavice, Kenneth J. (2013). Phillips' science of dental materials. Elsevier/Saunders. ISBN 9781437724189. OCLC 934359978.
  7. ^ Wright A. F., Leadbetter A. J. (1975). "The structures of the b-cristobalite phases of SiO2 and AlPO4". Philosophical Magazine. 31: 1391–1401. doi:10.1080/00318087508228690.
  8. ^ Downs R. T., Palmer D. C. (1994). "The pressure behavior of a cristobalite" (PDF). American Mineralogist. 79: 9–14.
  9. ^ Deane K. Smith (1998). "Opal, cristobalite, and tridymite: Noncrystallinity versus crystallinity, nomenclature of the silica minerals and bibliography". Powder Diffraction. 13 (1): 2–19. doi:10.1017/S0885715600009696.CS1 maint: uses authors parameter (link)
  10. ^ https://www.osha.gov/dsg/topics/silicacrystalline/smithdk/pdf/nomenc.pdf. Archived 2016-03-04 at the Wayback Machine

Further reading

  • American Geological Institute Dictionary of Geological Terms.
  • Durham, D. L., "Monterey Formation: Diagenesis". in: Uranium in the Monterey Formation of California. US Geological Survey Bulletin 1581-A, 1987.
  • Reviews in Mineralogy and Geochemistry, vol. 29., Silica: behavior, geochemistry and physical applications. Mineralogical Society of America, 1994.
  • R. B. Sosman, The Phases of Silica. (Rutgers University Press, 1965)

External links

Auxetics

Auxetics are structures or materials that have a negative Poisson's ratio. When stretched, they become thicker perpendicular to the applied force. This occurs due to their particular internal structure and the way this deforms when the sample is uniaxially loaded. Auxetics can be single molecules, crystals, or a particular structure of macroscopic matter.

Such materials and structures are expected to have mechanical properties such as high energy absorption and fracture resistance. Auxetics may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material, and sponge mops.

The term auxetic derives from the Greek word αὐξητικός (auxetikos) which means "that which tends to increase" and has its root in the word αὔξησις, or auxesis, meaning "increase" (noun). This terminology was coined by Professor Ken Evans of the University of Exeter.

One of the first artificially produced auxetic materials, the RFS structure (diamond-fold structure) , was invented in 1978 by the Berlin researcher K. Pietsch. Although he did not use the term auxetics, he describes for the first time the underlying lever mechanism and its non-linear mechanical reaction is therefore considered the inventor of the auxetic net.

The earliest published example of a material with negative Poisson's constant is due to A. G. Kolpakov in 1985, "Determination of the average characteristics of elastic frameworks"; the next synthetic auxetic material was described in Science in 1987, entitled "Foam structures with a Negative Poisson's Ratio" by R.S. Lakes from the University of Wisconsin Madison. The use of the word auxetic to refer to this property probably began in 1991.Designs of composites with inverted hexagonal periodicity cell (auxetic hexagon), possessing negative Poisson ratios, were published in 1985.Typically, auxetic materials have low density, which is what allows the hinge-like areas of the auxetic microstructures to flex.At the macroscale, auxetic behaviour can be illustrated with an inelastic string wound around an elastic cord. When the ends of the structure are pulled apart, the inelastic string straightens while the elastic cord stretches and winds around it, increasing the structure's effective volume. Auxetic behaviour at the macroscale can also be employed for the development of products with enhanced characteristics such as footwear based on the auxetic rotating triangles structures developed by Grima and Evans.Examples of auxetic materials include:

Auxetic polyurethane foam

α-Cristobalite.

Certain rocks and minerals

Graphene, which can be made auxetic through the introduction of vacancy defects

Living bone tissue (although this is only suspected)

Tendons within their normal range of motion.

Specific variants of polytetrafluorethylene polymers such as Gore-Tex

Paper, several types. If a paper is stretched in an in-plane direction it will expand in its thickness direction due to its network structure.

Several types of origami folds like the Diamond-Folding-Structure (RFS), the herringbone-fold-structure (FFS) or the miura fold, and other periodic patterns derived from it.

Tailored structures designed to exhibit special designed Poisson's ratios.

Chain organic molecules. Recent researches revealed that organic crystals like n-paraffins and similar to them may demonstrate an auxetic behavior.

Processed needle-punched nonwoven fabrics. Due to the network structure of such fabrics, a processing protocol using heat and pressure can convert ordinary (not auxetic) nonwovens into auxetic ones.

Cork has an almost zero Poisson's ratio. This makes it a good material for sealing wine bottles.

Cadmium cyanide

Cadmium cyanide is an inorganic compound with the formula Cd(CN)2. It is a white crystalline compound that is used in electroplating. It is very toxic, along with other cadmium and cyanide compounds.

Cementation (geology)

Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals forms

"bridges" between original sediment grains, thereby binding them together. In this way sand becomes "sandstone", and gravel becomes "conglomerate" or "breccia". Cementation occurs as part of the diagenesis or lithification of sediments. Cementation occurs primarily below the water table regardless of sedimentary grain sizes present. Large volumes of pore water must pass through sediment pores for new mineral cements to crystallize and so millions of years are generally required to complete the cementation process. Common mineral cements include calcite, quartz or silica phases like cristobalite, iron oxides, and clay minerals, but other mineral cements also occur.

Cementation is continuous in the groundwater zone, so much so that the term "zone of cementation" is sometimes used interchangeably. Cementation occurs in fissures or other openings of existing rocks and is a dynamic process more or less in equilibrium with a dissolution or dissolving process.

Cement found on the sea floor is commonly aragonite and can take different textural forms. These textural forms include pendant cement, meniscus cement, isopachous cement, needle cement, botryoidal cement, blocky cement, syntaxial rim cement, and coarse mosaic cement. The environment in which each of the cements is found depends on the pore space available. Cements that are found in phreatic zones include: isopachous, blocky, and syntaxial rim cements. As for calcite cementation, which occurs in meteoric realms (freshwater sources), the cement is produced by the dissolution of less stable aragonite and high-Mg calcite. (Boggs, 2011)

Classifying rocks while using the Folk classification depends on the matrix, which is either sparry (prominently composed of cement) or micritic (prominently composed of mud).

Djerfisherite

Djerfisherite is an alkali copper–iron sulfide mineral and a member of the djerfisherite group. It has the chemical formula K6Na(Fe2+,Cu,Ni)25S26Cl.

Its type locality is the Kota-Kota meteorite (Marimba meteorite), Malawi. It was first described in 1966 and named after professor Daniel Jerome Fisher (1896–1988), University of Chicago. It has been reported from meteorites, copper-nickel hydrothermal deposits, skarn, pegmatite, kimberlites and alkalic intrusive complexes. Associated minerals include kamacite, troilite, schreibersite, clinoenstatite, tridymite, cristobalite, daubreelite, graphite, roedderite, alabandite, talnakhite,

pentlandite, chalcopyrite, magnetite, valleriite, sphalerite and platinum minerals.

Imogolite

Imogolite is an aluminium silicate clay mineral with the chemical formula Al2SiO3(OH)4. It occurs in soils formed from volcanic ash and was first described in 1962 for an occurrence in Uemura, Kumamoto prefecture, Kyushu Region, Japan. Its name originates from the Japanese word imogo, which refers to the brownish yellow soil derived from volcanic ash. It occurs together with allophane, quartz, cristobalite, gibbsite, vermiculite and limonite.Imogolite consists of a network of nanotubes with an outer diameter of ca. 2 nm and an inner diameter of ca. 1 nm. The tube walls are formed by continuous Al(OH)3 (gibbsite) sheets and orthosilicate anions (O3SiOH groups). Owing to its tubular structure, natural availability, and low toxicity, imogolite has potential applications in polymer composites, fuel gas storage, absorbents, and as a catalyst support in chemical catalysis.

Obsidian

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) causes a high viscosity which, upon rapid cooling, forms a natural glass from the lava. The inhibition of atomic diffusion through this highly viscous lava explains the lack of crystal growth. Obsidian is hard, brittle, and amorphous; it therefore fractures with very sharp edges. In the past it was used to manufacture cutting and piercing tools and it has been used experimentally as surgical scalpel blades.

Opal

Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt. Opal is the national gemstone of Australia.There are two broad classes of opal: precious and common. Precious opal displays play-of-color (iridescence), common opal does not. Play-of-color is defined as "a pseudochromatic optical effect resulting in flashes of colored light from certain minerals, as they are turned in white light." The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent or opaque and the background color may be white, black or nearly any color of the visual spectrum. Black opal is considered to be the rarest, whereas white, gray and green are the most common.

Polymorphism (materials science)

In materials science, polymorphism is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements. The complete morphology of a material is described by polymorphism and other variables such as crystal habit, amorphous fraction or crystallographic defects. Polymorphism is relevant to the fields of pharmaceuticals, agrochemicals, pigments, dyestuffs, foods, and explosives.

When polymorphism exists as a result of a difference in crystal packing, it is called packing polymorphism. Polymorphism can also result from the existence of different conformers of the same molecule in conformational polymorphism. In pseudopolymorphism the different crystal types are the result of hydration or solvation. This is more correctly referred to as solvomorphism as different solvates have different chemical formulae. An example of an organic polymorph is glycine, which is able to form monoclinic and hexagonal crystals. Silica is known to form many polymorphs, the most important of which are; α-quartz, β-quartz, tridymite, cristobalite, moganite, coesite, and stishovite. A classical example is the pair of minerals, calcite and aragonite, both forms of calcium carbonate.

An analogous phenomenon for amorphous materials is polyamorphism, when a substance can take on several different amorphous modifications.

Pyrometamorphism

Pyrometamorphism is a type of metamorphism in which rocks are changed by heat coming from the fossil fuel fire. The rocks produced by pyrometamorphism include buchite, clinker and paralava, formed due to thermal changes of sedimentary rocks. Both natural and anthropogenic examples of sites with active pyrometamorphism are known. Natural pyrometamorphic rocks are known, e.g., from the Hatrurim Formation. Xenoliths of sedimentary rocks trapped in volcanic lava may undergo pyrometamorphic transformation. Anthropogenic pyrometamorphic rocks are found in burning coal-mining dumps. A great number of minerals, sometimes very rare, are found within these rocks. Of the silicate minerals, the typical ones are especially cordierite, indialite, fayalite, mullite, tridymite and cristobalite (both may be classified as oxide minerals, too), and sekaninaite. Oxide minerals include corundum, hematite, hercynite, magnesioferrite, and magnetite. Some unique minerals typical for meteorites, like oldhamite, are also found in pyrometamorphic rocks.

Pyroxferroite

Pyroxferroite (Fe2+,Ca)SiO3 is a single chain inosilicate. It is mostly composed of iron, silicon and oxygen, with smaller fractions of calcium and several other metals. Together with armalcolite and tranquillityite, it is one of the three minerals which were discovered on the Moon. It was then found in Lunar and Martian meteorites as well as a mineral in the Earth's crust. Pyroxferroite can also be produced by annealing synthetic clinopyroxene at high pressures and temperatures. The mineral is metastable and gradually decomposes at ambient conditions, but this process can take billions of years.

Quartz

Quartz is a hard, crystalline mineral composed of silicon and oxygen atoms. 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 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). Since the transformation is accompanied by a significant change in volume, it can easily induce fracturing of ceramics or rocks passing through this temperature threshold.

There are many different varieties of quartz, several of which are semi-precious gemstones. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Eurasia.

Quartz inversion

The room-temperature form of quartz, α-quartz, undergoes a reversible change in crystal structure at 573 °C to form β-quartz. This phenomenon is called an inversion, and for the α to β quartz inversion is accompanied by a linear expansion of 0.45%. This inversion can lead to cracking of ceramic ware if cooling occurs too quickly through the inversion temperature. This is called dunting, and the resultant faults as dunts. To avoid such thermal shock faults, cooling rates not exceeding 50 °C/hour have been recommended.At 870 °C quartz ceases to be stable but, in the absence of fluxes, does not alter until a much higher temperature is reached, when, depending on the temperature and nature of the fluxes present, it is converted into the polymorphs of cristobalite and / or tridymite. These polymorphs also experience temperature-induced inversions. The inversion of cristobalite at 220 °C can be advantageous to achieve the cristobalite squeeze. This puts the glazes into compression and so helps prevent crazing.The size of the silica particles influences inversions, conversions and other properties of the ceramic body. The presence of other ceramic raw materials can influence the thermal behaviour of quartz, including:

Talc promotes the conversion of quartz to cristobalite, and if sufficient alumina is available the formation of cordierite.

Nepheline syenite increases the dissolution of silica.

Petalite promotes the formation of cristobalite.

Alumina can react with silica to form mullite.

Rigid unit modes

Rigid unit modes (RUMs) represent a class of lattice vibrations or phonons that exist in network materials such as quartz, cristobalite or zirconium tungstate. Network materials can be described as three-dimensional networks of polyhedral groups of atoms such as SiO4 tetrahedra or TiO6 octahedra. A RUM is a lattice vibration in which the polyhedra are able to move, by translation and/or rotation, without distorting. RUMs in crystalline materials are the counterparts of floppy modes in glasses, as introduced by Jim Phillips and Mike Thorpe.

Seifertite

Seifertite is a silicate mineral with the formula SiO2 and is one of the densest polymorphs of silica. It has only been found in Martian and lunar meteorites, where it is presumably formed from either tridymite or cristobalite – other polymorphs of quartz – as a result of heating during the atmospheric re-entry and impact to the Earth, at an estimated minimal pressure of 35 GPa. It can also be produced in the laboratory by compressing cristobalite in a diamond anvil cell to pressures above 40 GPa. The mineral is named after Friedrich Seifert (born 1941), the founder of the Bayerisches Geoinstitut at University of Bayreuth, Germany, and is officially recognized by the International Mineralogical Association.Seifertite forms micrometre-sized crystalline lamellae embedded into a glassy SiO2 matrix. The lamellae are rather difficult to analyze, as they vitrify within seconds under laser or electron beams used for standard Raman spectroscopy or electron-beam microanalysis, even at much reduced beam intensities. Nevertheless, it was possible to verify that it is mainly composed of SiO2 with minor inclusions of Na2O (0.40 wt.%) and Al2O3 (1.14 wt.%). X-ray diffraction reveals that the mineral has scrutinyite (α-PbO2) type structure with an orthorhombic symmetry and Pbcn or Pb2n space group. Its lattice constants a = 4.097, b = 5.0462, c = 4.4946, Z = 4 correspond to the density of 4.294 g/cm3, which is among the highest for any forms of silica (for example, density of quartz is 2.65 g/cm3). Only stishovite has a comparable density of about 4.3 g/cm3.

Silicon dioxide

Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula SiO2, most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as synthetic product. Notable examples include fused quartz, fumed silica, silica gel, and aerogels. It is used in structural materials, microelectronics (as an electrical insulator), and as components in the food and pharmaceutical industries.

Inhaling finely divided crystalline silica is toxic and can lead to severe inflammation of the lung tissue, silicosis, bronchitis, lung cancer, and systemic autoimmune diseases, such as lupus and rheumatoid arthritis. Inhalation of amorphous silicon dioxide, in high doses, leads to non-permanent short-term inflammation, where all effects heal.

Silicosis

Silicosis is a form of occupational lung disease caused by inhalation of crystalline silica dust. It is marked by inflammation and scarring in the form of nodular lesions in the upper lobes of the lungs. It is a type of pneumoconiosis. Silicosis (particularly the acute form) is characterized by shortness of breath, cough, fever, and cyanosis (bluish skin). It may often be misdiagnosed as pulmonary edema (fluid in the lungs), pneumonia, or tuberculosis.

Silicosis resulted in 46,000 deaths globally in 2013 down from 55,000 deaths in 1990.The name silicosis (from the Latin silex, or flint) was originally used in 1870 by Achille Visconti (1836–1911), prosector in the Ospedale Maggiore of Milan. The recognition of respiratory problems from breathing in dust dates to ancient Greeks and Romans. Agricola, in the mid-16th century, wrote about lung problems from dust inhalation in miners. In 1713, Bernardino Ramazzini noted asthmatic symptoms and sand-like substances in the lungs of stone cutters. With industrialization, as opposed to hand tools, came increased production of dust. The pneumatic hammer drill was introduced in 1897 and sandblasting was introduced in about 1904, both significantly contributing to the increased prevalence of silicosis.

Smoky quartz

Smoky quartz is a grey, translucent variety of quartz that ranges in clarity from almost complete transparency to an almost-opaque brownish-gray or black crystal. Like other quartz gems, it is a silicon dioxide crystal. The smoky colour results from free silicon formed from the silicon dioxide by natural irradiation.

Tetragonal crystal system

In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base (a by a) and height (c, which is different from a).

Tridymite

Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is SiO2. Tridymite was first described in 1868 and the type location is in Hidalgo, Mexico. The name is from the Greek tridymos for triplet as tridymite commonly occurs as twinned crystal trillings (compound crystals comprising three twinned crystal components).

Crystalline
Cryptocrystalline
Amorphous
Miscellaneous
Notable varieties

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