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Petroleum Geochemical Survey
Published in Muhammad Abdul Quddus, Petroleum Science and Technology, 2021
The three-dimensional network silicates do not contain any metallic cations; they only have silicon and oxygen atoms. The group may be classified as silicate as well as oxide minerals. Three-dimensional frameworks of silicates are produced when all the four oxygen atoms of each tetrahedron are shared. The silicon/oxygen ratio is 1:2. Silica (SiO2) is an example of a three-dimensional network silicate. With different structural forms, silica produces minerals like quartz and tridymite.
Semi-precious stones
Published in Francis P. Gudyanga, Minerals in Africa, 2020
High-temperature polymorphs of SiO2 that are found in high-silica volcanic rocks include tridymite and cristobalite. Some meteorite impact sites and metamorphic rocks formed at pressures greater than those typical of the Earth’s crust have been found to host coesite and stishovite which are denser polymorphs of SiO2. Lightning strikes in quartz sand may cause the formation of lechatelierite which is an amorphous silica glass SiO2.
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
For another example of polymorphs, we may consider minerals made of SiO2. Quartz and tridymite, shown in Figure 3.28, are only two of many possible minerals of the same composition, and they have distinctive crystal shapes. Quartz is very common, but tridymite is only stable at high temperature and thus is rare under normal Earth surface conditions. In all, mineralogists have described more than a dozen crystalline forms of SiO2. They are all polymorphs, meaning that they have the same composition but different arrangements of atoms, and are stable under different conditions.
The role of SiO2 and silica-rich amorphous materials in understanding the origin of uncommon archeological finds
Published in Materials and Manufacturing Processes, 2020
The absence of amorphous materials in XRD analysis indicates that melting did not occur, and therefore a temperature of <1000°C. Thus, the development of cristobalite is related to the recrystallization of the original material, most probably a rhyolite tuff, rich in volcanic glass and Na+K, suitable to extract the SiO2, required for glass production. The existence of cubic (high-temperature) and orthorhombic (low-temperature) cristobalite also requires a slow cooling and the stabilization of cubic cristobalite by alkaline cations,[31] in contrast to ceramic materials.[32] Tridymite is formed at the 870°C transition in the SiO2 polymorph recrystallization reactions. Its appearance could be genetically related to the missing of quartz. The original material was probably also lacking quartz, and cristobalite was recrystallized from volcanic glass, containing Al, K, Na. These cations can be accommodated in the cristobalite and tridymite crystal structures as interstitial stabilizers for Al3+ → Si4+ substitution, and they prevent the crystallization of quartz.[33]
Correlation between thermoluminescence emission and phase transitions of tridymite
Published in Phase Transitions, 2018
S. Balci-Yegen, C. Boronat, J. Garcia-Guinea, M. Topaksu, V. Correcher
Tridymite is a high–temperature and low-pressure SiO2 polymorph (just with cristobalite) that is thermally treated to be used in ceramics, molds for foundries, refractories or furnace coatings. This silicon compound is also of interest from a mineralogical viewpoint to determine events concerning to the Earth formation since appears in nature in environments as diverse as volcanic rocks [1] or meteorites [2]. It displays the simplest SiO2 structure that is kinetically favored at lower temperatures, mainly the β-tridymite, consisting of tetrahedral giving rise to hexagonal rings in a hexagonal lattice. It is arranged in the form of layers sharing apices and forming void channels where the impurities are hosted. There are two reversible phases, α- and β-tridymite, in the range of 150–200°C [3] with space groups of symmetry of C2221 (orthorhombic) for α-tridymite and P63/mmc (hexagonal) for β-tridymite [4,5]. The thermal effect and the chemical composition of natural tridymite have been well-characterized by Raman, X-ray diffraction, differential thermal analysis [6], cathodoluminescence [7], electron spin resonance [8], etc.; however, to the best of our knowledge, the thermoluminescence (TL) response has been scarcely studied. Only few results pointed by Garcia-Guinea et al. [9], described how the thermal effect could induce non-bridging oxygen hole centers and/or silicon vacancies associated with the 340 nm emission.