Silicon dioxide polymorphs

Stevens-Kalceff, M. A., Electron-irradiation-induced radiolytic oxygen generation and microsegregation in silicon dioxide polymorphs, Phys. Rev. Lett. 84, 3137 (2000). 

Silicon dioxide [7631-86-9] Si02, exists in both crystalline and glassy forms. In the former, the most common polymorph is a-quartz (low quartz). All commercial appHcations of crystalline quartz use a-quartz, which is stable only below ca 573°C at atmospheric pressure. Some of the properties of a-quartz are Hsted in Table 1. 

Any two samples of a particular mineral, whatever their source or place of origin, have the same basic composition and characteristic crystal structure moreover, no two different minerals have identical chemical composition and crystal structure (see Textboxes 8 and 21). Quartz, for example, is a common and abundant mineral composed of silicon dioxide, a compound that occurs naturally not only as quartz but also in other crystal structures, known as polymorphs (polymorphs are minerals that have the same chemical composition but different crystal structure), some of which, listed in Table 23, have been used for a variety of purposes. The crystal structure, which is essential for the characterization of solid materials, is just one of a wide range of physical properties, that is, properties not involving chemical differences, which provide convenient criteria for characterizing and identifying solids. 

Li D, Bancroft GM, Kasrai M, Fleet ME, Secco RA, Feng XH, Tan KH, Yang BX (1994) X-ray absorption spectroscopy of silicon dioxide (Si02) polymorphs The stractrrral characterization of opal. Am Mineral 79 622-632... 

The formal systematic replacement of Si02 units in quartz or other polymorphs of silicon dioxide by AIO(OH) or M A102 (M = monovalent metal) is well known to lead either to sheet silicates or to tectosilicates, the zeolites being the most remarkable representatives.All these solid-state compounds have as a common feature interstitial holes, which are occupied by easily extractable cations, or have three-dimensional frameworks which can be modified and used for many applications (ion exchange, catalysts, specific coordination sites,... 

Finally, new polymorphic structures can arise from different ways of assembling closely related coordination polyhedra. Three possibilities are known that lead to polymorphism of this type, which are most commonly found in oxidic lattices. First, the coordination polyhedron of one lattice particle is the same, while that of the other is only slightly changed (e g., silicon dioxide). Second, the coordination polyhedron of one lattice particle is the same, and that of the other is considerably changed (e.g., titanium dioxide). Finally, the coordination polyhedra are both deformed to varying degrees, with the basic lattice type being the same but the type of distortion different (e.g., zirconium dioxide). 

Quartz is one of the predontinant polymorphs of silicon dioxide (Si02) and one of the most abundant minerals. It occurs as an essential constituent of many igneous, sedimentary, and metamorphic rocks (Deer et al. 1992) and is used in many electronic devices, in the ceramic industry, and in the synthesis of zeolites. a-CJuartz has a hexagonal crystal structure with space group P3i21 (left side)... 

The pyroxene was composed principally of sodium and calcium, followed by silicon dioxide because of its ability to be transformed into stishovite. When undergoing a polymorphic transition to stishovite, silicon dioxide iiKreases in volume and therefore can be quickly destroyed. It is important to establish why some elements were not detected in the samples before impact, particularly carbon. To understand where it came from, the process through which siminals are obtained should be analyzed. 

Perhaps significantly in the context of this book there are three rare polymorphs of silicon dioxide all found after the application of high pressure coesite, keatite, and stishovite. The possible production of these in the zone immediate to the indenter and the energy this would absorb do not appear to have been considered. 

The 1 1 compound between RjOj and SiOj is called rare earth oxyorthosilicate with a formula R2(8104)0 because it contains isolated Si04 tetrahedra and extra oxygen atoms that are not bonded to silicon. The oxyorthosilicates can be prepared directly by a solid state reaction between rare earth oxides and silicon dioxide. The reaction is slow and high temperatures are needed (1700°C) to get 100% conversion in a reasonable time (Keler and Kuznetsov, 1962). The kinetics of the reaction between Y2O3 and Si02 have been studied and the reaction appears to proceed via diffusion (Keler and Kuznetsov, 1962 Leskela and Niskavaara, 1982). The polymorphic transitions of Si02 complicate the interpretation of the reaction mechanism, however. 

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