Alpha quartz

Table 58. SHG intensity values (hdho, (SiO2)) °f some tantalum and niobium fluorides and oxyfluorides after normalization by alpha-quartz signal fu (SiO]). Measurements were taken before and after thermal treatment, up to extinction of signal.

Fig. 100. Irreversible extinction of SHG signal during heating of compounds that crystallize in centrosymmetric space groups. SHG signal is normalized by signal of alpha-quartz (a-SiO/f.

Alpha quartz (aSi02) is found throughout the boiler and post-boiler sections. It is a very hard, adherent scale, found in the mud drum, on boiler tubes, and on turbine blades as a hexagonal crystal structure. 

Alpha quartz aSi02 Hard scale found in drum, tubes, and turbine blades. 

Deposition in the 300 to 500 °F (149-260 °C) range is alpha-quartz (a-Si02), chalcedony (Si02), and crystabalite (a- and 0-Si02)... 

A series of calculations on defect centers induced by radiation damage in alpha-quartz is reported. Ab initio SCF-MO calculations were carried out on a 21 atom cluster, Si50i6 % surrounded by 956 point-ions, designed to simulate alpha-quartz. This two-region approach made it possible to represent the long-range electrostatic effects, present in the crystal, in the SCF-MO cluster. 

In the calculations presented here, the long-range effects present in a crystal were introduced explicitly for the SCF-MO treated cluster, by surrounding it with point-ions situated at the X-ray determined atomic positions of alpha-quartz. This method has been used for the more ionic systems of alpha-NaOH, and MgO with some success and the calculations described in this paper show that it is equally applicable for semi-covalent materials. 

Alpha-quartz has many useful properties which lead to its wide use in industry as a glass, ceramic and molecular sieve. However, undoubtedly its most technically important use occurs by virtue of its piezo-electric properties, which allow it to be used as a frequency regulating device in satellites, computers, and the ubiquitous quartz-watch . Unfortunately, it has been found that quartz crystals are susceptible to damage by radiation, and that this is associated with the presence of defects in the crystal lattice. These defects, particularly aluminum and hydrogen, are grown into the crystal and so far have proved impossible to remove. This problem has been the cause of intensive research, which has led to some information on the possible types of defects involved, but has failed to produce details of their geometries, and the way in which they interact. 

Table I. The electrostatic energies of the central silicon site and one nearest-neighbor oxygen site for a 977 point-ion cluster, and the Madelung energies for silicon and oxygen in alpha-quartz...

Transient blue alpha-quartz In aquecus scluticns ED, GB 1,3... 

Chang EK, Rohlfing M, Louie SG (2000) Excitons and optical properties of alpha-quartz. Phys Rev Lett 85 2613... 

Cohen, A. J. Hassan, F. (1974) Ferrous and ferric ions in synthetic alpha-quartz and natural amethyst. Amer. Mineral., 59, 719-28... 

Monje, P.V. and Baran, E.J., First evidences of the bioaccumulation of alpha-quartz in cactaceae, J. Plant Physiol., 157, 457, 2000. 

NCIO nitrosyl chloride 2696-92-6 3.880E-h09 24.620 4265 Si02 silicon dioxide (alpha quartz) 14808-60-7 5.030E+09 33.580... 

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