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Silica and Silicates
Published in Shamil Shaikhutdinov, Introduction to Ultrathin Silica Films Silicatene and Others, 2022
A phase diagram of silica is depicted in Fig. 1.1a. Although many crystalline silica polymorphs have been described in the literature, the basic building block is unique, and it is a SiO4 tetrahedron (Fig. 1.1b). In the great majority of cases, exemplified by quartz, tridymite, and cristobalite, the SiO4 tetrahedra are connected by sharing corner oxygen atoms such that every Si atom is surrounded by four oxygen atoms, and every O atom binds to two Si atoms. Stishovite is the only known form of silica that shares the rutile crystal structure, in which each Si atom is coordinated by six O atoms (Fig. 1.1c), and each oxygen atom is bonded to three Si atoms. Therefore, the crystal structure contains distorted SiO6 octahedra sharing edges and corners. Interestingly, the above-mentioned stishovite as well as coesite have been synthesized even before they were discovered in nature, in experiments using very high pressures. It is, therefore, believed that stishovite and coesite are formed on Earth only due to the high-pressure conditions generated, for example, by a meteorite impact. Actually, its presence in the samples is considered as an indicator of metamorphic reactions at extremely high pressures.
Minerals
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
Stishovite and coesite, rare dense varieties of SiO2, are found in some rocks shocked by meteorite impacts. Less commonly, coesite is found in very high-pressure metamorphic rocks. Metastable cristobalite and tridymite occur in some glassy volcanic rocks, but over time may react to form α-quartz. β-quartz is never seen at normal temperatures because it reacts to form α-quartz instantaneously upon cooling. β-quartz is, however, stable at high temperature, and consequently is useful in some high-temperature applications.
Physical properties of magnesium fluoride: FPLAPW Approach
Published in Phase Transitions, 2023
Abdeldjalil Arroussi, Mohamed Ghezali, Abdelbasset Baida, Belkacem Mahfoud, Benameur Amiri
Alkaline earth metal fluorides include MgF2 crystallizes in the rutile structure [4–7] while CaF2, SrF2 and BaF2 crystallize in the fluorite-type structure [8–11]. Magnesium fluoride crystallizes under ambient conditions in a rutile-like tetragonal structure with a space group of P42/mnm and is isomorphic to rutile and stishovite. The phase transition of Mgf2 is obtained at a lower pressure than many dioxides. This is due to the high compressibility of this material and the large relative size of the ions. These phase transitions can be investigated under hydrostatic or quasi-hydrostatic conditions [12].