Silicates crystallization

The chemionl changes observed in the devitrification of iotlU-glasi consist, therefore,—1st. In-the loss of ft part or the whole of the -potassa. -2nd. In the passing of a part or the whole of the protoxide of iron and of manganese to the state of sesquioxides. 3rd. In the formation of one or mors definite and crystallized silicates, by means of tbe remaining products. 

The processes which take place according to the mentioned stages are partially overlapped that brings to complications into the mechanism of silica gel formation and into the preparation of products with prescribed properties. In particular, it is impossible to obtain crystal silicic acid from gels under usual conditions. 

In order to crystallize silicate glasses it is not necessary to remove the modifier cations since they can interact with more than one oxygen tetrahedron due to their ionic bonding character. Thus, these cations act as connecting links between the non-bonding oxygen atoms. First of aU one deals with a kinetic problem, and often a previous phase separation process is needed to get crystallizable components. 

Rosholt J. N. Prijana and Noble D. C. Mobility of uranium and thorium in glassy and crystallized silicic volcanic rocks. Econ. Geol., 66, 1971, 1061-9. 

More recently, the Hoechst firm has put out a lamellar crystallized silicate called SKS-6. This product is anhydrous and is a very powerful calcitim sequesterer. However, its dissolution kinetic in a detergent medium is rather slow. It causes a steep rise in pH, which subsequently goes back down by way of a delayed depolymerization-dissolution effect, typical of all the silicates, but particularly aggravated for lamellar crystallized silicate (cf. the following subsection). 

Single molecules also have promise as probes for local stmcture when doped into materials tliat are tliemselves nonfluorescent. Rlrodamine dyes in botli silicate and polymer tliin films exliibit a distribution of fluorescence maxima indicative of considerable heterogeneity in local environments, particularly for the silicate material [159]. A bimodal distribution of fluorescence intensities observed for single molecules of crystal violet in a PMMA film has been suggested to result from high and low viscosity local sites witliin tire polymer tliat give rise to slow and fast internal conversion, respectively [160]. 

Silicon makes up 25.7% of the earth s crust, by weight, and is the second most abundant element, being exceeded only by oxygen. Silicon is not found free in nature, but occurs chiefly as the oxide and as silicates. Sand, quartz, rock crystal, amethyst, agate, flint, jasper, and opal are some of the forms in which the oxide appears. Granite, hornblende, asbestos, feldspar, clay, mica, etc. are but a few of the numerous silicate minerals. 

Glass-Ceramics Based on Silicate Crystals. The principal commercial glass-ceramics fall into this category. These can be grouped by composition, simple siUcates, fluorosiUcates, and aluminosihcates, and by the crystal stmctures of these phases. 

Aluminosilicates. These silicates consist of frameworks of silica and alumina tetrahedra linked at all corners to form three-dimensional networks familiar examples are the common rock-forming minerals quartz and feldspar. Framework silicates generally form blocky crystals, more isotropic... 

Pigment Systems. Most of the crystals used for ceramic pigments are complex oxides, owing to the great stability of oxides in molten silicate glasses. Table 3 fists these materials. The one significant exception to the use of oxides is the family of cadmium sulfoselenide red pigments. This family is used because the colors obtained caimot be obtained in oxide systems thus it is necessary to sustain the difficulties of a nonoxide system. 

Ahrens, T.J., Anderson, D.L., and Ringwood, A.E. (1969), Equation of State and Crystal Structures of High-Pressure Phases of Shocked Silicates and Oxides, Rev. Geophys. 7, 667-707. 

Raman spectroscopy is particularly useful for investigating the structure of noncrystalline solids. The vibrational spectra of noncrystalline solids exhibit broad bands centered at wavenumbers corresponding to the vibrational modes of the corresponding crystals (Figure 5). In silicate glasses shifts in the high-wavenumber bands... 

Together with the structural principles established by the Bragg school concerning the many types of silicates, Goldschmidt s ideas were taken further by Linus Pauling in California to establish the modern science of crystal chemistry. A good early overview of the whole field can be found in a book by Evans (1939, 1964). 

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