Crystallization is the process by which a solid forms, where the atoms or molecules are organized into a structure known as a crystal. Some of the ways by which crystals form are precipitating from a solution, freezing, or more deposition directly from a gas. Attributes of the resulting crystal depend on factors such as temperature, air pressure, in the case of liquid crystals, time of fluid evaporation. Crystallization occurs in two major steps; the first is nucleation, the appearance of a crystalline phase from either a supercooled liquid or a supersaturated solvent. The second step is known as crystal growth, the increase in the size of particles and leads to a crystal state. An important feature of this step is that loose particles form layers at the crystal's surface lodge themselves into open inconsistencies such as pores, etc; the majority of minerals and organic molecules crystallize and the resulting crystals are of good quality, i.e. without visible defects. However, larger biochemical particles, like proteins, are difficult to crystallize.
The ease with which molecules will crystallize depends on the intensity of either atomic forces, intermolecular forces or intramolecular forces. Crystallization is a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. In chemical engineering, crystallization occurs in a crystallizer. Crystallization is therefore related to precipitation, although the result is not amorphous or disordered, but a crystal; the crystallization process consists of two major events and crystal growth which are driven by thermodynamic properties as well as chemical properties. In crystallization Nucleation is the step where the solute molecules or atoms dispersed in the solvent start to gather into clusters, on the microscopic scale, that become stable under the current operating conditions; these stable clusters constitute the nuclei. Therefore, the clusters need to reach a critical size; such critical size is dictated by many different factors.
It is at the stage of nucleation that the atoms or molecules arrange in a defined and periodic manner that defines the crystal structure — note that "crystal structure" is a special term that refers to the relative arrangement of the atoms or molecules, not the macroscopic properties of the crystal, although those are a result of the internal crystal structure. The crystal growth is the subsequent size increase of the nuclei that succeed in achieving the critical cluster size. Crystal growth is a dynamic process occurring in equilibrium where solute molecules or atoms precipitate out of solution, dissolve back into solution. Supersaturation is one of the driving forces of crystallization, as the solubility of a species is an equilibrium process quantified by Ksp. Depending upon the conditions, either nucleation or growth may be predominant over the other, dictating crystal size. Many compounds have the ability to crystallize with some having different crystal structures, a phenomenon called polymorphism.
Each polymorph is in fact a different thermodynamic solid state and crystal polymorphs of the same compound exhibit different physical properties, such as dissolution rate, melting point, etc. For this reason, polymorphism is of major importance in industrial manufacture of crystalline products. Additionally, crystal phases can sometimes be interconverted by varying factors such as temperature. There are many examples of natural process. Geological time scale process examples include: Natural crystal formation. Human time scale process examples include: Snow flakes formation. Crystal formation can be divided into two types, where the first type of crystals are composed of a cation and anion known as a salt, such as sodium acetate; the second type of crystals are composed for example menthol. Crystal formation can be achieved by various methods, such as: cooling, addition of a second solvent to reduce the solubility of the solute, solvent layering, changing the cation or anion, as well as other methods.
The formation of a supersaturated solution does not guarantee crystal formation, a seed crystal or scratching the glass is required to form nucleation sites. A typical laboratory technique for crystal formation is to dissolve the solid in a solution in which it is soluble at high temperatures to obtain supersaturation; the hot mixture is filtered to remove any insoluble impurities. The filtrate is allowed to cool. Crystals that form are filtered and washed with a solvent in which they are not soluble, but is miscible with the mother liquor; the process is repeated to increase the purity in a technique known as recrystallization. For biological molecules in which the solvent channels continue to be present to retain the three dimensional structure intact, microbatch crystallization under oil and vapor diffusion methods have been the common methods. Equipment for the main industrial processes for crystallization. Tank crystallizers. Tank crystallization is an old method still used in some specialized cases.
Saturated solutions, in tank crystallization, are allowed to cool in open tanks. After a period of time the mother liquo
In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter; the smallest group of particles in the material that constitutes this repeating pattern is the unit cell of the structure. The unit cell reflects the symmetry and structure of the entire crystal, built up by repetitive translation of the unit cell along its principal axes; the translation vectors define the nodes of the Bravais lattice. The lengths of the principal axes, or edges, of the unit cell and the angles between them are the lattice constants called lattice parameters or cell parameters; the symmetry properties of the crystal are described by the concept of space groups. All possible symmetric arrangements of particles in three-dimensional space may be described by the 230 space groups.
The crystal structure and symmetry play a critical role in determining many physical properties, such as cleavage, electronic band structure, optical transparency. Crystal structure is described in terms of the geometry of arrangement of particles in the unit cell; the unit cell is defined as the smallest repeating unit having the full symmetry of the crystal structure. The geometry of the unit cell is defined as a parallelepiped, providing six lattice parameters taken as the lengths of the cell edges and the angles between them; the positions of particles inside the unit cell are described by the fractional coordinates along the cell edges, measured from a reference point. It is only necessary to report the coordinates of a smallest asymmetric subset of particles; this group of particles may be chosen so that it occupies the smallest physical space, which means that not all particles need to be physically located inside the boundaries given by the lattice parameters. All other particles of the unit cell are generated by the symmetry operations that characterize the symmetry of the unit cell.
The collection of symmetry operations of the unit cell is expressed formally as the space group of the crystal structure. Vectors and planes in a crystal lattice are described by the three-value Miller index notation; this syntax uses the indices ℓ, m, n as directional orthogonal parameters, which are separated by 90°. By definition, the syntax denotes a plane that intercepts the three points a1/ℓ, a2/m, a3/n, or some multiple thereof; that is, the Miller indices are proportional to the inverses of the intercepts of the plane with the unit cell. If one or more of the indices is zero, it means. A plane containing a coordinate axis is translated so that it no longer contains that axis before its Miller indices are determined; the Miller indices for a plane are integers with no common factors. Negative indices are indicated with horizontal bars, as in. In an orthogonal coordinate system for a cubic cell, the Miller indices of a plane are the Cartesian components of a vector normal to the plane. Considering only planes intersecting one or more lattice points, the distance d between adjacent lattice planes is related to the reciprocal lattice vector orthogonal to the planes by the formula d = 2 π | g ℓ m n | The crystallographic directions are geometric lines linking nodes of a crystal.
The crystallographic planes are geometric planes linking nodes. Some directions and planes have a higher density of nodes; these high density planes have an influence on the behavior of the crystal as follows: Optical properties: Refractive index is directly related to density. Adsorption and reactivity: Physical adsorption and chemical reactions occur at or near surface atoms or molecules; these phenomena are thus sensitive to the density of nodes. Surface tension: The condensation of a material means that the atoms, ions or molecules are more stable if they are surrounded by other similar species; the surface tension of an interface thus varies according to the density on the surface. Microstructural defects: Pores and crystallites tend to have straight grain boundaries following higher density planes. Cleavage: This occurs preferentially parallel to higher density planes. Plastic deformation: Dislocation glide occurs preferentially parallel to higher density planes; the perturbation carried by the dislocation is along a dense direction.
The shift of one node in a more dense direction requires a lesser distortion of the crystal lattice. Some directions and planes are defined by symmetry of the crystal system. In monoclinic, rhombohedral and trigonal/hexagonal systems there is one unique axis which has higher rotational symmetry than the other two axes; the basal plane is the plane perpendicular to the principal axis in these crystal systems. For triclinic and cubic crystal systems the axis designation is arbitrary and there is no principal axis. For the special case of simple cubic crystals, the lattice vectors are orthogonal and of equal length. So, in this common case, the Miller indices and both denote normals/directions in Cartesian coordinates. For cubic crystals with lattice constant a, the spacing d between adjacent l
Euhedral and anhedral
Euhedral crystals are those that are well-formed, with sharp recognised faces. The opposite is anhedral: a rock with an anhedral texture is composed of mineral grains that have no well-formed crystal faces or cross-section shape in thin section. Anhedral crystal growth occurs in a competitive environment with no free space for the formation of crystal faces. An intermediate texture with some crystal face-formation is termed subhedral. Crystals that grow from cooling liquid magma do not form smooth faces or sharp crystal outlines; as magma cools, the crystals grow and touch each other, preventing crystal faces from forming properly or at all. When snowflakes crystallize, they do not touch each other. Thus, snowflakes form six-sided twinned crystals. In rocks, the presence of euhedral crystals may signify that they formed early in the crystallization of magma or crystallized in a cavity or vug, without hindrance from other crystals. "Euhedral" is derived from the Greek eu meaning well and hedron meaning shape.
Euhedral crystals have flat faces with sharp angles. The flat faces are oriented in a specific way relative to the underlying atomic arrangement of the crystal: They are planes of low Miller index; this occurs. As a crystal grows, new atoms attach to the rougher and less stable parts of the surface, but less to the flat, stable surfaces. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. Xenomorph Idiomorph Crystal habit Rock microstructure List of rock textures
In geology, druse known as drusy or druzy, refers to a coating of fine crystals on a rock fracture surface, vein or within a vug or geode. Druse occurs worldwide. Garnet, dolomite and a variety of minerals may occur as druse coatings, it is possible to find drusy natural gemstones in any location in which there is a place for water to collect and evaporate on rock. It most appears along river beds and shorelines, it therefore is helpful for collectors, whether commercial or otherwise, to be aware of the minerals common to an area when they look for a particular type of druse. Because of its sparkling appearance, druse is sometimes used in jewelry making. Both the glittering effect of the tiny crystals and the color of the base mineral are factors when selecting druse for this purpose. Druse is used with a variety of natural gemstones including crystalline quartz and many more. Druse can be colored by electroplating, a process similar to rhodium application, giving the stone a fancy look, a brighter appearance than is natural.
Crystal habit Miarolitic cavity
Quartz is a mineral composed of silicon and oxygen atoms 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 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. Since the transformation is accompanied by a significant change in volume, it can induce fracturing of ceramics or rocks passing through this temperature threshold. There are many different varieties of quartz. Since antiquity, varieties of quartz have been the most used minerals in the making of jewelry and hardstone carvings in Eurasia; 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 in East Central German and which came from the Polish dialect term kwardy, which corresponds to the Czech term tvrdý.
The Ancient Greeks referred to quartz as κρύσταλλος derived from the Ancient Greek κρύος meaning "icy cold", because some philosophers believed the mineral to be a form of supercooled ice. Today, the term rock crystal is sometimes used as an alternative name for the purest form of quartz. Quartz belongs to the trigonal crystal system; the ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are twinned, 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 form in a'bed' that has unconstrained growth into a void. However, doubly terminated crystals do occur where they develop without attachment, for instance within gypsum. A quartz geode is such a situation where the void is spherical in shape, lined with a bed of crystals pointing inward. Α-quartz crystallizes in the trigonal crystal system, space group P3121 or P3221 depending on the chirality.
Β-quartz belongs to space group P6222 and P6422, respectively. These space groups are chiral. Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks; the transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without change in the way they are linked. Although many of the varietal names arose from the color of the mineral, current scientific naming schemes refer 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. Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent, has been used for hardstone carvings, such as the Lothair Crystal. Common colored varieties include citrine, rose quartz, smoky quartz, milky quartz, others; these color differentiation's arise from chromophores which have been incorporated into the crystal structure of the mineral.
Polymorphs of quartz include: α-quartz, β-quartz, moganite, cristobalite and stishovite. The most important distinction between types of quartz is that of macrocrystalline and the microcrystalline or cryptocrystalline varieties; the cryptocrystalline varieties are either translucent or opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a cryptocrystalline form of silica consisting of fine intergrowths of both quartz, its monoclinic polymorph moganite. Other opaque gemstone varieties of quartz, or mixed rocks including quartz including contrasting bands or patterns of color, are agate, carnelian or sard, onyx and jasper. Amethyst is a form of quartz that ranges from a dull purple color; the world's largest deposits of amethysts can be found in Brazil, Uruguay, France and Morocco. Sometimes amethyst and citrine are found growing in the same crystal, it is referred to as ametrine. An amethyst is formed. Blue quartz contains inclusions of fibrous crocidolite. Inclusions of the mineral dumortierite within quartz pieces result in silky-appearing splotches with a blue hue, shades giving off purple and/or grey colors additionally being found.
"Dumortierite quartz" will sometimes feature contrasting light and dark color zones across the material. Interest in the certain quality forms of blue quartz as a collectible gemstone arises in India and in the United States. Citrine is a variety of quartz whose color ranges from a pale yellow to brown due to ferric impurities. Natural citrines are rare. However, a heat-treated amethyst will have small lines in the crystal, as opposed to a natural citrine's cloudy or smokey appearance, it is nearly impossible to differentiate between cut citrine and yellow topaz visually, but they differ in hardness. Brazil is the leading producer of citrine, with much
Tibet is a historical region covering much of the Tibetan Plateau in Inner Asia. It is the traditional homeland of the Tibetan people as well as some other ethnic groups such as Monpa, Qiang and Lhoba peoples and is now inhabited by considerable numbers of Han Chinese and Hui people. Tibet is the highest region on Earth, with an average elevation of 5,000 metres; the highest elevation in Tibet is Mount Everest, Earth's highest mountain, rising 8,848 m above sea level. The Tibetan Empire emerged in the 7th century, but with the fall of the empire the region soon divided into a variety of territories; the bulk of western and central Tibet was at least nominally unified under a series of Tibetan governments in Lhasa, Shigatse, or nearby locations. Thus Tibet remained a suzerainty of the Mongol and Chinese rulers in Nanjing and Beijing, with reasonable autonomy given to the Tibetan leaders; the eastern regions of Kham and Amdo maintained a more decentralized indigenous political structure, being divided among a number of small principalities and tribal groups, while often falling more directly under Chinese rule after the Battle of Chamdo.
The current borders of Tibet were established in the 18th century. Following the Xinhai Revolution against the Qing dynasty in 1912, Qing soldiers were disarmed and escorted out of Tibet Area; the region subsequently declared its independence in 1913 without recognition by the subsequent Chinese Republican government. Lhasa took control of the western part of Xikang, China; the region maintained its autonomy until 1951 when, following the Battle of Chamdo, Tibet became incorporated into the People's Republic of China, the previous Tibetan government was abolished in 1959 after a failed uprising. Today, China governs western and central Tibet as the Tibet Autonomous Region while the eastern areas are now ethnic autonomous prefectures within Sichuan and other neighbouring provinces. There are tensions regarding dissident groups that are active in exile. Tibetan activists in Tibet have been arrested or tortured; the economy of Tibet is dominated by subsistence agriculture, though tourism has become a growing industry in recent decades.
The dominant religion in Tibet is Tibetan Buddhism. Tibetan Buddhism is a primary influence on the art and festivals of the region. Tibetan architecture reflects Indian influences. Staple foods in Tibet are roasted barley, yak meat, butter tea; the Tibetan name for their land, Bod བོད་, means "Tibet" or "Tibetan Plateau", although it meant the central region around Lhasa, now known in Tibetan as Ü. The Standard Tibetan pronunciation of Bod, is transcribed Bhö in Tournadre Phonetic Transcription, Bö in the THL Simplified Phonetic Transcription and Poi in Tibetan pinyin; some scholars believe the first written reference to Bod "Tibet" was the ancient Bautai people recorded in the Egyptian Greek works Periplus of the Erythraean Sea and Geographia, itself from the Sanskrit form Bhauṭṭa of the Indian geographical tradition. The modern Standard Chinese exonym for the ethnic Tibetan region is Zangqu, which derives by metonymy from the Tsang region around Shigatse plus the addition of a Chinese suffix, 区 qū, which means "area, region, ward".
Tibetan people and culture, regardless of where they are from, are referred to as Zang although the geographical term Xīzàng is limited to the Tibet Autonomous Region. The term Xīzàng was coined during the Qing dynasty in the reign of the Jiaqing Emperor through the addition of a prefix meaning "west" to Zang; the best-known medieval Chinese name for Tibet is Tubo. This name first appears in Chinese characters as 土番 in the 7th century and as 吐蕃 in the 10th-century. In the Middle Chinese spoken during that period, as reconstructed by William H. Baxter, 土番 was pronounced thux-phjon and 吐蕃 was pronounced thux-pjon. Other pre-modern Chinese names for Tibet include Wusiguo, Wusizang and Tanggute. American Tibetologist Elliot Sperling has argued in favor of a recent tendency by some authors writing in Chinese to revive the term Tubote for modern use in place of Xizang, on the grounds that Tubote more includes the entire Tibetan plateau rather than the Tibet Autonomous Region; the English word Tibet or Thibet dates back to the 18th century.
Historical linguists agree that "Tibet" names in European languages are loanwords from Semitic Ṭībat orTūbātt, itself deriving from Turkic Töbäd, literally: "The Heights". Linguists classify the Tibetan language as a Tibeto-Burman language of the Sino-Tibetan language family although the boundaries between'Tibetan' and certain other Himalayan languages can be unclear. According to
A crystal or crystalline solid is a solid material whose constituents are arranged in a ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations; the scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification; the word crystal derives from the Ancient Greek word κρύσταλλος, meaning both "ice" and "rock crystal", from κρύος, "icy cold, frost". Examples of large crystals include snowflakes and table salt. Most inorganic solids are not crystals but polycrystals, i.e. many microscopic crystals fused together into a single solid. Examples of polycrystals include most metals, rocks and ice. A third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever.
Examples of amorphous solids include glass and many plastics. Despite the name, lead crystal, crystal glass, related products are not crystals, but rather types of glass, i.e. amorphous solids. Crystals are used in pseudoscientific practices such as crystal therapy, along with gemstones, are sometimes associated with spellwork in Wiccan beliefs and related religious movements; the scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called the crystal structure. A crystal is a solid where the atoms form a periodic arrangement.. Not all solids are crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a polycrystalline structure. In the final block of ice, each of the small crystals is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does not have a periodic arrangement of atoms, because the periodic pattern is broken at the grain boundaries.
Most macroscopic inorganic solids are polycrystalline, including all metals, ice, etc. Solids that are neither crystalline nor polycrystalline, such as glass, are called amorphous solids called glassy, vitreous, or noncrystalline; these have no periodic order microscopically. There are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the latent heat of fusion, but forming a crystal does. A crystal structure is characterized by its unit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement; the unit cells are stacked in three-dimensional space to form the crystal. The symmetry of a crystal is constrained by the requirement that the unit cells stack with no gaps. There are 219 possible crystal symmetries, called crystallographic space groups; these are grouped into 7 crystal systems, such as hexagonal crystal system. Crystals are recognized by their shape, consisting of flat faces with sharp angles.
These shape characteristics are not necessary for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is present and easy to see. Euhedral crystals are those with well-formed flat faces. Anhedral crystals do not because the crystal is one grain in a polycrystalline solid; the flat faces of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: they are planes of low Miller index. This occurs; as a crystal grows, new atoms attach to the rougher and less stable parts of the surface, but less to the flat, stable surfaces. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. One of the oldest techniques in the science of crystallography consists of measuring the three-dimensional orientations of the faces of a crystal, using them to infer the underlying crystal symmetry.
A crystal's habit is its visible external shape. This is determined by the crystal structure, the specific crystal chemistry and bonding, the conditions under which the crystal formed. By volume and weight, the largest concentrations of crystals in the Earth are part of its solid bedrock. Crystals found in rocks range in size from a fraction of a millimetre to several centimetres across, although exceptionally large crystals are found; as of 1999, the world's largest known occurring crystal is a crystal of beryl from Malakialina, Madagascar, 18 m long and 3.5 m in diameter, weighing 380,000 kg. Some crystals have formed by magmatic and metamorphic processes, giving origin to large masses of crystalline rock; the vast majority of igneous rocks are formed from molten magma and the degree of crystallization depends on the conditions under which they solidified. Such rocks as granite, which have cooled slowly and under great pressures, have crystallized.