Misplaced Pages

#650349

84-512: Three polytypes of chrysotile are known. These are very difficult to distinguish in hand specimens, and polarized light microscopy must normally be used. Some older publications refer to chrysotile as a group of minerals—the three polytypes listed below, and sometimes pecoraite as well—but the 2006 recommendations of the International Mineralogical Association prefer to treat it as a single mineral with

168-436: A co-crystal of caffeine and maleic acid (2:1) is dissolved in chloroform and when the solvent is allowed to evaporate slowly. Whereas form I has monoclinic space group P 2 1 / c , the new form has space group Pc . Both polymorphs consist of sheets of molecules connected through hydrogen bonding of the carboxylic acid groups: in form I, the sheets alternate with respect of the net dipole moment , while in form II,

252-426: A polymorph as “a crystalline phase of a given compound resulting from the possibility of at least two different arrangements of the molecules of that compound in the solid state.” These defining facts imply that polymorphism involves changes in physical properties but cannot include chemical change. Some early definitions do not make this distinction. Eliminating chemical change from those changes permissible during

336-407: A polymorphic transition is "A reversible transition of a solid crystalline phase at a certain temperature and pressure (the inversion point) to another phase of the same chemical composition with a different crystal structure." Additionally, Walter McCrone described the phases in polymorphic matter as "different in crystal structure but identical in the liquid or vapor states." McCrone also defines

420-418: A ball of crystal much overcome the energetic barrier to the r > r c r i t {\displaystyle r>r_{crit}} part of the energy landscape. Chest The thorax ( pl. : thoraces or thoraxes ) or chest is a part of the anatomy of mammals and other tetrapod animals located between the neck and the abdomen . In insects , crustaceans , and

504-419: A certain variation in its naturally occurring forms. Clinochrysotile is the most common of the three forms, found notably at Val-des-Sources, Quebec , Canada . Its two measurable refractive indices tend to be lower than those of the other two forms. The orthorhombic paratypes may be distinguished by the fact that, for orthochrysotile, the higher of the two observable refractive indices is measured parallel to

588-412: A few kJ/mol in lattice energy. Approximately 50% of known polymorph pairs differ by less than 2 kJ/mol and stability differences of more than 10 kJ/mol are rare. Polymorph stability may change upon temperature or pressure. Importantly, structural and thermodynamic stability are different. Thermodynamic stability may be studied using experimental or computational methods. Polymorphism is affected by

672-427: A hexagonal (alpha) form and a rhombohedral (beta) form. Polymorphism in binary metal oxides has attracted much attention because these materials are of significant economic value. One set of famous examples have the composition SiO 2 , which form many polymorphs. Important ones include: α-quartz , β-quartz , tridymite , cristobalite , moganite , coesite , and stishovite . A classical example of polymorphism

756-575: A human carcinogen by the International Agency for Research on Cancer (IARC) and by the U.S. Department of Health and Human Services . These state that "Asbestos exposure is associated with parenchymal asbestosis , asbestos-related pleural abnormalities, peritoneal mesothelioma , and lung cancer , and it may be associated with cancer at some extra- thoracic sites". In other scientific publications, epidemiologists have published peer-reviewed scientific papers establishing that chrysotile

840-624: A key role in stabilising the crystal lattice of both the molecules. Efficient crystal packing of amoxapine seems to be contributing towards its monomorphic behaviour as compared to the comparatively less efficient packing of loxapine molecules in both polymorphs. The combination of experimental and computational approaches has provided a deeper understanding of the factors influencing the solid-state structure and diversity in these compounds. Hirshfeld surfaces using Crystal Explorer represent another way of exploring packing modes and intermolecular interactions in molecular crystals. The influence of changes in

924-418: A more or less pigmented disc, the areola . The apex of a normal heart is in the fifth left intercostal space, three and a half inches from the mid-line. Different types of diseases or conditions that affect the chest include pleurisy , flail chest , atelectasis , and the most common condition, chest pain. These conditions can be hereditary or caused by birth defects or trauma . Any condition that lowers

SECTION 10

#1732779533651

1008-501: A polymorphic transition delineates polymorphism. For example, isomerization can often lead to polymorphic transitions. However, tautomerism (dynamic isomerization) leads to chemical change, not polymorphism. As well, allotropy of elements and polymorphism have been linked historically. However, allotropes of an element are not always polymorphs. A common example is the allotropes of carbon , which include graphite, diamond, and londsdaleite. While all three forms are allotropes, graphite

1092-714: A single polymorph. In a classic patent dispute, the GlaxoSmithKline defended its patent for the Type II polymorph of the active ingredient in Zantac against competitors while that of the Type I polymorph had already expired. Polymorphism in drugs can also have direct medical implications since dissolution rates depend on the polymorph. Polymorphic purity of drug samples can be checked using techniques such as powder X-ray diffraction, IR/Raman spectroscopy, and utilizing

1176-431: A symptom of myocardial infarctions ('heart attack'). If this condition is present in the body, discomfort will be felt in the chest that is similar to a heavy weight placed on the body. Sweating, shortness of breath, lightheadedness , and irregular heartbeat may also be experienced. If a heart attack occurs, the bulk of the damage is caused during the first six hours, so getting the proper treatment as quickly as possible

1260-654: A white tetragonal form. When cooled below 13.2 degrees, alpha-tin forms which is gray in color and has a cubic diamond form. A classic example of a nonmetal that exhibits polymorphism is carbon. Carbon has many allotropes, including graphite, diamond, and londsdaleite. However, these are not all polymorphs of each other. Graphite is not a polymorph of diamond and londsdaleite, since it is chemically distinct, having sp hybridized bonding. Diamond, and londsdaleite are chemically identical, both having sp hybridized bonding, and they differ only in their crystal structures, making them polymorphs. Additionally, graphite has two polymorphs,

1344-422: Is Mg 3 ( Si 2 O 5 )( OH ) 4 , although some of the magnesium ions may be replaced by iron or other cations . Substitution of the hydroxide ions for fluoride , oxide or chloride is also known, but rarer. A related, but much rarer, mineral is pecoraite , in which all the magnesium cations of chrysotile are substituted by nickel cations. Chrysotile is resistant to even strong bases (asbestos

1428-462: Is chest pain . The word thorax comes from the Greek θώραξ thṓrax " breastplate , cuirass , corslet " via Latin : thorax . In humans and other hominids , the thorax is the chest region of the body between the neck and the abdomen , along with its internal organs and other contents. It is mostly protected and supported by the rib cage , spine , and shoulder girdle . The contents of

1512-408: Is monoclinic form I. The hydrogen bonding mechanisms are the same for all three phases; however, they differ strongly in their pi-pi interactions. In 2006 a new polymorph of maleic acid was discovered, 124 years after the first crystal form was studied. Maleic acid is manufactured on an industrial scale in the chemical industry. It forms salt found in medicine. The new crystal type is produced when

1596-401: Is blocked and traps air. The blockage may be caused by something inside the bronchus , such as a plug of mucus, a tumour , or an inhaled foreign object such as a coin, piece of food, or a toy. It is possible for something outside of the bronchus to cause the blockage. Pneumothorax is the condition where air or gas can build up in the pleural space. It can occur without a known cause or as

1680-404: Is generally felt when deep breaths are taken or during a cough. Another non-cardiac cause of chest pain is atelectasis . It is a condition that occurs when a portion of the lung collapses from being airless. When bronchial tubes are blocked, this condition develops and causes patients to feel shortness of breath. The most common cause of atelectasis is when a bronchi that extends from the windpipe

1764-526: Is important. Some people, especially those who are elderly or have diabetes, may not have typical chest pain but may have many of the other symptoms of a heart attack. It is important that these patients and their caregivers have a good understanding of heart attack symptoms. Just like with a heart attack, not all chest pain is caused by conditions involving the heart . Chest wall pain can be experienced after an increase in activity. Persons who add exercise to their daily routine generally feel this type of pain at

SECTION 20

#1732779533651

1848-432: Is not a polymorph of diamond and londsdaleite. Isomerization and allotropy are only two of the phenomena linked to polymorphism. For additional information about identifying polymorphism and distinguishing it from other phenomena, see the review by Brog et al. It is also useful to note that materials with two polymorphic phases can be called dimorphic , those with three polymorphic phases, trimorphic , etc. Polymorphism

1932-525: Is of practical relevance to pharmaceuticals , agrochemicals , pigments , dyestuffs , foods , and explosives . Early records of the discovery of polymorphism credit Eilhard Mitscherlich and Jöns Jacob Berzelius for their studies of phosphates and arsenates in the early 1800s. The studies involved measuring the interfacial angles of the crystals to show that chemically identical salts could have two different forms. Mitscherlich originally called this discovery isomorphism. The measurement of crystal density

2016-480: Is the main cause of pleural mesothelioma . Chrysotile has been recommended for inclusion in the Rotterdam Convention on Prior Informed Consent , an international treaty that restricts the global trade in hazardous materials. If listed, exports of chrysotile would only be permitted to countries that explicitly consent to imports. Canada , a major producer of the mineral, has been harshly criticized by

2100-481: Is the pair of minerals calcite , which is rhombohedral, and aragonite , which is orthorhombic. Both are forms of calcium carbonate . A third form of calcium carbonate is vaterite, which is hexagonal and relatively unstable. β-HgS precipitates as a black solid when Hg(II) salts are treated with H 2 S . With gentle heating of the slurry, the black polymorph converts to the red form. According to Ostwald's rule , usually less stable polymorphs crystallize before

2184-562: Is the phenomenon where a compound or element can crystallize into more than one crystal structure . The preceding definition has evolved over many years and is still under discussion today. Discussion of the defining characteristics of polymorphism involves distinguishing among types of transitions and structural changes occurring in polymorphism versus those in other phenomena. Phase transitions (phase changes) that help describe polymorphism include polymorphic transitions as well as melting and vaporization transitions. According to IUPAC ,

2268-477: Is the surface energy, and the second term − b r 3 {\displaystyle -br^{3}} is the volume energy. Both parameters a , b > 0 {\displaystyle a,b>0} . The function G ( r ) {\displaystyle G(r)} rises to a maximum before dropping, crossing zero at r c r i t {\displaystyle r_{crit}} . In order to crystallize,

2352-425: Is thermodynamically competitive to the experimentally obtained structure. Whilst in case of olanzapine , crystal energy landscape highlights that the extensive experimental screening has probably not found all possible polymorphs of olanzapine , and further solid form diversity could be targeted with a better understanding of the role of kinetics in its crystallisation. CSP studies were able to offer an explanation for

2436-404: Is thermodynamically stable at room temperature. Polytypes are a special case of polymorphs, where multiple close-packed crystal structures differ in one dimension only. Polytypes have identical close-packed planes, but differ in the stacking sequence in the third dimension perpendicular to these planes. Silicon carbide (SiC) has more than 170 known polytypes , although most are rare. All

2520-472: Is thus stable in high pH pore water of Portland cement ), but when the fibres are attacked by acids, the magnesium ions are selectively dissolved, leaving a silica skeleton. It is thermally stable up to around 550 °C (1,022 °F), at which temperature it starts to dehydrate . Dehydration is complete at about 750 °C (1,380 °F), with the final products being forsterite (magnesium silicate), silica and water. The global mass balance reaction of

2604-530: The Canadian Medical Association for its opposition to including chrysotile in the convention. According to EU Regulation 1907/2006 ( REACH ) the marketing and use of chrysotile, and of products containing chrysotile, are prohibited. As of March 2024, the U.S. Environmental Protection Agency finalized regulations banning imports of chrysotile asbestos (effective immediately) due to its link to lung cancer and mesothelioma . However,

Chrysotile - Misplaced Pages Continue

2688-688: The WTO and the European Commission concerning France's 1996 prohibition of the importation and sale of all forms of asbestos. Canada said that the French measures contravened provisions of the Agreements on Sanitary and Phytosanitary Measures and on Technical Barriers to Trade, and the GATT 1994. The EC claimed that safer substitute materials existed to take the place of asbestos. It stressed that

2772-483: The amoxapine . A combined experimental and computational study demonstrated that the methyl group in loxapine has a significant influence in increasing the range of accessible solid forms and favouring various alternate packing arrangements. CSP studies have again helped in explaining the observed solid-state diversity of loxapine and amoxapine. PIXEL calculations showed that in absence of strong H-bonds, weak H-bonds such as C–H...O, C–H...N and dispersion interactions play

2856-528: The enthalpy of polymorphic transitions. In the 20th century, X-ray crystallography became commonly used for studying the crystal structure of polymorphs. Both single crystal x-ray diffraction and powder x-ray diffraction techniques are used to obtain measurements of the crystal unit cell. Each polymorph of a compound has a unique crystal structure. As a result, different polymorphs will produce different x-ray diffraction patterns. Vibrational spectroscopic methods came into use for investigating polymorphism in

2940-458: The "thoracic skeleton" is a component of the axial skeleton . It consists of the ribs and sternum . The ribs of the thorax are numbered in ascending order from 1–12. 11 and 12 are known as floating ribs because they have no anterior attachment point in particular the cartilage attached to the sternum, as 1 through 7 are, and therefore are termed "floating". Whereas ribs 8 through 10 are termed false ribs as their costal cartilage articulates with

3024-404: The 1T polytype is metallic in character, while the 2H form is more semiconducting. Another example is tantalum disulfide , where the common 1T as well as 2H polytypes occur, but also more complex 'mixed coordination' types such as 4Hb and 6R, where the trigonal prismatic and the octahedral geometry layers are mixed. Here, the 1T polytype exhibits a charge density wave , with distinct influence on

3108-500: The French measures were not discriminatory under the terms of international trade treaties, and were fully justified for public health reasons. The EC further claimed that in the July consultations, it had tried to convince Canada that the measures were justified, and that just as Canada broke off consultations, it (the EC) was in the process of submitting substantial scientific data in favour of

3192-834: The ability to either breathe deeply or to cough is considered a chest disease or condition. Injury to the chest (also referred to as chest trauma, thoracic injury, or thoracic trauma) results in up to ⁠ 1 / 4 ⁠ of all deaths due to trauma in the United States. The major pathophysiologies encountered in blunt chest trauma involve derangements in the flow of air, blood, or both in combination. Sepsis due to leakage of alimentary tract contents, as in esophageal perforations, also must be considered. Blunt trauma commonly results in chest wall injuries (e.g., rib fractures). The pain associated with these injuries can make breathing difficult, and this may compromise ventilation. Direct lung injuries, such as pulmonary contusions (see

3276-480: The absence of the centrosymmetric dimer in anhydrous clozapine . PIXEL calculations on all the crystal structures of clozapine revealed that similar to olanzapine , the intermolecular interaction energy in each structure is also dominated by the Ed. Despite the molecular structure similarity between amoxapine and loxapine (molecules in group 2), the crystal packing observed in polymorphs of loxa differs significantly from

3360-475: The asbestos ban. In the late 1990s and early 2000s, the Government of Canada continued to claim that chrysotile was much less dangerous than other types of asbestos. Chrysotile continued to be used in new construction across Canada, in ways that are very similar to those for which chrysotile was exported. Similarly, Natural Resources Canada once stated that chrysotile, one of the fibres that make up asbestos,

3444-429: The beginning. It is important to monitor the pain to ensure that it is not a sign of something more serious. Pain can also be experienced in persons who have an upper respiratory infection . This virus is also accompanied by a fever and cough. Shingles is another viral infection that can give symptoms of chest or rib pain before a rash develops. Injuries to the rib cage or sternum is also a common cause of chest pain. It

Chrysotile - Misplaced Pages Continue

3528-421: The cause of the pain. While cardiac issues cause feelings of sudden pressure in the chest or a crushing pain in the back, neck, and arms, pain that is felt due to noncardiac issues gives a burning feeling along the digestive tract or pain when deep breaths are attempted. Different people feel pains differently for the same condition. Only a patient truly knows if the symptoms are mild or serious. Chest pain may be

3612-418: The chest does not correspond to that part of the thoracic skeleton that encloses the heart and lungs . All the breadth of the shoulders is due to the shoulder girdle, and contains the axillae and the heads of the humeri . In the middle line the suprasternal notch is seen above, while about three fingers' breadth below it a transverse ridge can be felt, which is known as the sternal angle and this marks

3696-519: The chrysotile dehydration can be written as follows: The chrysotile (serpentine) dehydration reaction corresponds to the reverse of the forsterite (Mg- olivine ) hydrolysis in the presence of dissolved silica ( silicic acid ). Previously, in the 1990s it was used in asbestos-cement products (like pipes and sheets). Magnesium sulfate (MgSO 4 ) may be produced by treating chrysotile with sulfuric acid (H 2 SO 4 ). Chrysotile has been included with other forms of asbestos in being classified as

3780-425: The conductivity as a function of temperature, while the 2H polytype exhibits superconductivity . ZnS and CdI 2 are also polytypical. It has been suggested that this type of polymorphism is due to kinetics where screw dislocations rapidly reproduce partly disordered sequences in a periodic fashion. In terms of thermodynamics , two types of polymorphic behaviour are recognized. For a monotropic system, plots of

3864-418: The costal cartilage of the rib above. The thorax bones also have the main function of protecting the heart, lungs, and major blood vessels in the thorax area, such as the aorta . The anatomy of the chest can also be described through the use of anatomical landmarks . The nipple in the male is situated in front of the fourth rib or a little below; vertically it lies a little external to a line drawn down from

3948-570: The details of crystallisation . The solvent in all respects affects the nature of the polymorph, including concentration, other components of the solvent, i.e., species that inhibiting or promote certain growth patterns. A decisive factor is often the temperature of the solvent from which crystallisation is carried out. Metastable polymorphs are not always reproducibly obtained, leading to cases of " disappearing polymorphs ", with usually negative implications on law and business. Drugs receive regulatory approval and are granted patents for only

4032-455: The differences in their optical properties in some cases. The known cases up to 2015 are discussed in a review article by Bučar, Lancaster, and Bernstein. Dibenzoxazepines Multidisciplinary studies involving experimental and computational approaches were applied to pharmaceutical molecules to facilitate the comparison of their solid-state structures. Specifically, this study has focused on exploring how changes in molecular structure affect

4116-406: The extinct trilobites , the thorax is one of the three main divisions of the body, each in turn composed of multiple segments. The human thorax includes the thoracic cavity and the thoracic wall . It contains organs including the heart , lungs , and thymus gland, as well as muscles and various other internal structures. Many diseases may affect the chest, and one of the most common symptoms

4200-414: The extinct trilobites , the thorax is one of the three main divisions of the creature's body, each of which is in turn composed of multiple segments. It is the area where the wings and legs attach in insects, or an area of multiple articulating plates in trilobites. In most insects, the thorax itself is composed of three segments; the prothorax , the mesothorax , and the metathorax . In extant insects,

4284-409: The fibre bundles is 0.1–1  μm , and the individual fibrils are even finer, 0.02–0.03 μm, each fibre bundle containing tens or hundreds of fibrils. Chrysotile fibres have considerable tensile strength , and may be spun into thread and woven into cloth. They are also resistant to heat and are excellent thermal, electrical and acoustic insulators. The idealized chemical formula of chrysotile

SECTION 50

#1732779533651

4368-442: The field of metallurgy. Some (but not all) allotropes are also polymorphs. For example, iron has three allotropes that are also polymorphs. Alpha-iron, which exists at room temperature, has a bcc form. Above 910 degrees gamma-iron exists, which has a fcc form. Above 1390 degrees delta-iron exists with a bcc form. Another metallic example is tin, which has two allotropes that are also polymorphs. At room temperature, beta-tin exists as

4452-428: The formation of crystals and predict the existence of specific polymorphs of a compound before they have been observed experimentally by scientists. Many compounds exhibit polymorphism. It has been claimed that "every compound has different polymorphic forms, and that, in general, the number of forms known for a given compound is proportional to the time and money spent in research on that compound." The phenomenon

4536-403: The free energies of the various polymorphs against temperature do not cross before all polymorphs melt. As a result, any transition from one polymorph to another below the melting point will be irreversible. For an enantiotropic system, a plot of the free energy against temperature shows a crossing point before the various melting points. It may also be possible to convert interchangeably between

4620-434: The functional thorax is composed of four segments, and is therefore typically called the mesosoma to distinguish it from the "thorax" of other insects. Each thoracic segment in an insect is further subdivided into various parts, the most significant of which are the dorsal portion (the notum ), the lateral portion (the pleuron ; one on each side), and the ventral portion (the sternum ). In some insects, each of these parts

4704-448: The hydrogen bonds formed by the carboxylic acid groups, both polymorphs form identical dimer structures. The aspirin polymorphs contain identical 2-dimensional sections and are therefore more precisely described as polytypes. Pure Form II aspirin could be prepared by seeding the batch with aspirin anhydrate in 15% weight. Paracetamol powder has poor compression properties, which poses difficulty in making tablets. A second polymorph

4788-445: The image below), are frequently associated with major chest trauma and may impair ventilation by a similar mechanism. Chest pain can be the result of multiple issues, including respiratory problems, digestive issues, and musculoskeletal complications. The pain can trigger cardiac issues as well. Not all pain that is felt is associated with the heart, but it should not be taken lightly either. Symptoms can be different depending on

4872-402: The junction between the manubrium and body of the sternum . Level with this line the second ribs join the sternum, and when these are found the lower ribs can often be counted. At the lower part of the sternum, where the seventh or last true ribs join it, the ensiform cartilage begins, and above this there is often a depression known as the pit of the stomach . The bones of the thorax, called

4956-403: The largest number of well-characterised polymorphs is held by a compound known as ROY . Glycine crystallizes as both monoclinic and hexagonal crystals . Polymorphism in organic compounds is often the result of conformational polymorphism . Elements including metals may exhibit polymorphism. Allotropy is the term used when describing elements having different forms and is used commonly in

5040-512: The long axis of the fibres (as for clinochrysotile); whereas for parachrysotile the higher refractive index is measured perpendicular to the long axis of the fibres. Bulk chrysotile has a hardness similar to a human fingernail and is easily crumbled to fibrous strands composed of smaller bundles of fibrils. Naturally-occurring fibre bundles range in length from several millimetres to more than ten centimetres, although industrially-processed chrysotile usually has shorter fibre bundles. The diameter of

5124-514: The lung or as a shortness of breath. In mammals , the thorax is the region of the body formed by the sternum , the thoracic vertebrae , and the ribs . It extends from the neck to the diaphragm , and does not include the upper limbs . The heart and the lungs reside in the thoracic cavity , as well as many blood vessels . The inner organs are protected by the rib cage and the sternum. Thoracic vertebrae are also distinguished in birds , but not in reptiles . In insects , crustaceans , and

SECTION 60

#1732779533651

5208-539: The market licensed as Noxafil were formulated utilising form I of posaconazole . The discovery of polymorphs of posaconazole increased rapidly and resulted in much research in crystallography of posaconazole . A methanol solvate and a 1,4-dioxane co-crystal were added to the Cambridge Structural Database (CSD). The antiviral drug ritonavir exists as two polymorphs, which differ greatly in efficacy. Such issues were solved by reformulating

5292-469: The medicine into gelcaps and tablets, rather than the original capsules. There was only one proven polymorph Form I of aspirin , though the existence of another polymorph was debated since the 1960s, and one report from 1981 reported that when crystallized in the presence of aspirin anhydride , the diffractogram of aspirin has weak additional peaks. Though at the time it was dismissed as mere impurity, it was, in retrospect, Form II aspirin. Form II

5376-406: The middle of the clavicle ; in the female it is not so constant. A little below it the lower limit of the great pectoral muscle is seen running upward and outward to the axilla; in the female this is obscured by the breast , which extends from the second to the sixth rib vertically and from the edge of the sternum to the mid-axillary line laterally. The female nipple is surrounded for half an inch by

5460-704: The molecular conformation, packing motifs, interactions in the resultant crystal lattices and the extent of solid-state diversity of these compounds. The results highlight the value of crystal structure prediction studies and PIXEL calculations in the interpretation of the observed solid-state behaviour and quantifying the intermolecular interactions in the packed structures and identifying the key stabilising interactions. An experimental screen yielded 4 physical forms for clozapine as compared to 60 distinct physical forms for olanzapine . The experimental screening results of clozapine are consistent with its crystal energy landscape which confirms that no alternate packing arrangement

5544-591: The new rules can allow up to a dozen years to phase out the use of chrysotile asbestos in some manufacturing facilities. The long phase-out period was a result of a strong lobby by Olin Corporation , a major chemical manufacturer, as well as trade groups like the U.S. Chamber of Commerce and the American Chemistry Council . Chrysotile asbestos is now banned in more than 50 other countries. In May 1998, Canada requested consultations before

5628-452: The origins of vibrations within crystals. The combination of techniques provides detailed information about crystal structures, similar to what can be achieved with x-ray crystallography. In addition to using computational methods for enhancing the understanding of spectroscopic data, the latest development in identifying polymorphism in crystals is the field of crystal structure prediction . This technique uses computational chemistry to model

5712-480: The polytypes of SiC have virtually the same density and Gibbs free energy . The most common SiC polytypes are shown in Table 1. Table 1 : Some polytypes of SiC. A second group of materials with different polytypes are the transition metal dichalcogenides , layered materials such as molybdenum disulfide (MoS 2 ). For these materials the polytypes have more distinct effects on material properties, e.g. for MoS 2 ,

5796-417: The prothorax never has wings, though legs are always present in adults; wings (when present) are restricted to at least the mesothorax, and typically also the metathorax, though the wings may be reduced or modified on either or both segments. In the apocritan Hymenoptera , the first abdominal segment is fused to the metathorax, where it forms a structure known as the propodeum . Accordingly, in these insects,

5880-413: The result of a lung disease or acute lung injury. The size of the pneumothorax changes as air or gas builds up, so a medical procedure can release the pressure with a needle. If it is untreated, blood flow can be interrupted and cause a drop in blood pressure known as tension pneumothorax. It is possible for smaller cases to clear up on their own. Symptoms of this condition are often felt only on one side of

5964-824: The second half of the twentieth century and have become more commonly used as optical, computer, and semiconductor technologies improved. These techniques include infrared (IR) spectroscopy , terahertz spectroscopy and Raman spectroscopy . Mid-frequency IR and Raman spectroscopies are sensitive to changes in hydrogen bonding patterns. Such changes can subsequently be related to structural differences. Additionally, terahertz and low frequency Raman spectroscopies reveal vibrational modes resulting from intermolecular interactions in crystalline solids. Again, these vibrational modes are related to crystal structure and can be used to uncover differences in 3-dimensional structure among polymorphs. Computational chemistry may be used in combination with vibrational spectroscopy techniques to understand

6048-510: The sheets are oriented in the same direction. After 125 years of study, 1,3,5-trinitrobenzene yielded a second polymorph. The usual form has the space group Pbca , but in 2004, a second polymorph was obtained in the space group Pca 2 1 when the compound was crystallised in the presence of an additive, trisindane . This experiment shows that additives can induce the appearance of polymorphic forms. Acridine has been obtained as eight polymorphs and aripiprazole has nine. The record for

6132-500: The small substituents on shape and electron distribution can also be investigated by mapping the total electron density on the electrostatic potential for molecules in the gas phase. This allows straightforward visualisation and comparison of overall shape, electron-rich and electron-deficient regions within molecules. The shape of these molecules can be further investigated to study its influence on diverse solid-state diversity. Posaconazole The original formulations of posaconazole on

6216-574: The stable form. The concept hinges on the idea that unstable polymorphs more closely resemble the state in solution, and thus are kinetically advantaged. The founding case of fibrous vs rhombic benzamide illustrates the case. Another example is provided by two polymorphs of titanium dioxide . Nevertheless, there are known systems, such as metacetamol , where only narrow cooling rate favors obtaining metastable form II. Polymorphs have disparate stabilities. Some convert rapidly at room (or any) temperature. Most polymorphs of organic molecules only differ by

6300-453: The thorax include the heart and lungs (and the thymus gland); the major and minor pectoral muscles, trapezius muscles , and neck muscle; and internal structures such as the diaphragm , the esophagus , the trachea , and a part of the sternum known as the xiphoid process . Arteries and veins are also contained – ( aorta , superior vena cava , inferior vena cava and the pulmonary artery ); bones (the shoulder socket containing

6384-455: The two polymorphs by heating or cooling, or through physical contact with a lower energy polymorph. A simple model of polymorphism is to model the Gibbs free energy of a ball-shaped crystal as G = a r 2 − b r 3 {\displaystyle G=ar^{2}-br^{3}} . Here, the first term a r 2 {\displaystyle ar^{2}}

6468-414: The upper part of the humerus , the scapula , sternum , thoracic portion of the spine , collarbone , and the rib cage and floating ribs ). External structures are the skin and nipples . In the human body , the region of the thorax between the neck and diaphragm in the front of the body is called the chest. The corresponding area in an animal can also be referred to as the chest. The shape of

6552-1112: The workplace. However, under increasing criticism by environmental groups, in May, 2012, the Canadian government stopped funding the Chrysotile Institute. As a result, the Chrysotile Institute has now closed. The Canadian government continues to draw both domestic and international criticism for its stance on chrysotile, most recently in international meetings about the Rotterdam Convention hearings regarding chrysotile. The CFMEU pointed out that most exports go to developing countries. Canada has pressured countries, including Chile , and other UN member states to avoid chrysotile bans. In September 2012, governments in Quebec and Canada ended official support for Canada's last asbestos mine in Asbestos, Quebec , now renamed as Val-des-Sources . Polytype In crystallography, polymorphism

6636-620: Was also used by Wilhelm Ostwald and expressed in Ostwald's Ratio. The development of the microscope enhanced observations of polymorphism and aided Moritz Ludwig Frankenheim ’s studies in the 1830s. He was able to demonstrate methods to induce crystal phase changes and formally summarized his findings on the nature of polymorphism. Soon after, the more sophisticated polarized light microscope came into use, and it provided better visualization of crystalline phases allowing crystallographers to distinguish between different polymorphs. The hot stage

6720-425: Was discovered in 1832 by Friedrich Wöhler and Justus von Liebig . They observed that the silky needles of freshly crystallized benzamide slowly converted to rhombic crystals. Present-day analysis identifies three polymorphs for benzamide: the least stable one, formed by flash cooling is the orthorhombic form II. This type is followed by the monoclinic form III (observed by Wöhler/Liebig). The most stable form

6804-453: Was found with more suitable compressive properties. Cortisone acetate exists in at least five different polymorphs, four of which are unstable in water and change to a stable form. Carbamazepine , estrogen , paroxetine , and chloramphenicol also show polymorphism. Pyrazinamide has at least 4 polymorphs. All of them transforms to stable α form at room temperature upon storage or mechanical treatment. Recent studies prove that α form

6888-510: Was invented and fitted to a polarized light microscope by Otto Lehmann in about 1877. This invention helped crystallographers determine melting points and observe polymorphic transitions. While the use of hot stage microscopes continued throughout the 1900s, thermal methods also became commonly used to observe the heat flow that occurs during phase changes such as melting and polymorphic transitions. One such technique, differential scanning calorimetry (DSC), continues to be used for determining

6972-437: Was not as dangerous as once thought. According to a fact sheet from 2003, "current knowledge and modern technology can successfully control the potential for health and environmental harm posed by chrysotile". The Chrysotile Institute, an association partially funded by the Canadian government, also prominently asserted that the use of chrysotile did not pose an environmental problem and the inherent risks in its use were limited to

7056-417: Was reported in 2005, found after attempted co-crystallization of aspirin and levetiracetam from hot acetonitrile . In form I, pairs of aspirin molecules form centrosymmetric dimers through the acetyl groups with the (acidic) methyl proton to carbonyl hydrogen bonds . In form II, each aspirin molecule forms the same hydrogen bonds, but with two neighbouring molecules instead of one. With respect to

#650349