Silicates, molten

NMR can be carried out over a wide range of temperatures, although there is a time and often a resolution penalty in using temperatures other than ambient. An effective lower limit of - 150 °C is set by the lack of solvents that are liquid below this. Temperatures above 130 °C require special thennal protection devices, although measurements have even been made on molten silicates. 

Cerium(IV) oxidizes ferrous ion to ferric and the cerium ions are stable under the conditions of a molten silicate—glass bath. Furthermore, cerium itself has no absorption ia the visible region. Economical additions of cerium, as cerium concentrate, enable the efficient use of raw materials containing trace quantities of iron (26). 

The solubility of Ti02 in molten silicates is around 8—10%. At room temperature this solubility is reduced to around 5%. Thus when using Ti02 as an opacifier, substantial amounts, about 15%, must be used. 

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. 

Some metals are soluble as atomic species in molten silicates, the most quantitative studies having been made with Ca0-Si02-Al203(37, 26, 27 mole per cent respectively). The results at 1800 K gave solubilities of 0.055, 0.16, 0.001 and 0.101 for the pure metals Cu, Ag, Au and Pb. When these metal solubilities were compared for metal alloys which produced 1 mm Hg pressure of each of these elements at this temperature, it was found drat the solubility decreases as the atomic radius increases, i.e. when die difference in vapour pressure of die pure metals is removed by alloy formation. If the solution was subjected to a temperature cycle of about 20 K around the control temperamre, the copper solution precipitated copper particles which grew with time. Thus the liquid metal drops, once precipitated, remained stable thereafter. 

Table 6.9 also lists the parameters of Richet and Bottinga s (1985) model, which, devised primarily for Al-free melts, accounts for the nonideality of molten silicate systems containing K by introducing an excess term that represents Si-K interactions ... 

Fraser D. G, Rammensee W., and Hardwick A. (1985). Determination of the mixing properties of molten silicates by Knudsen cell mass spectrometry, II The system (Na-K)AlSi40io and (Na-K)AlSi50i2. Geochim. Cosmochim. Acta, 49 349-359. 

Table 5.1 summarizes the uses of lime. Lime is used as a basic flux in the manufacture of steel. Silicon dioxide is a common impurity in iron ore that cannot be melted unless it combines with another substance first to convert it to a more fluid lava called slag. Silicon dioxide is a Lewis acid and therefore it reacts with the Lewis base lime. The molten silicate slag is less dense than the molten iron and collects at the top of the reactor, where it can be drawn off. Over 100 lb of lime must be used to manufacture a ton of steel. 

Dissolution in molten silicates effects of solid solution. Geochim. Cosmo-... 

Spera F.J. and Trial A.F. (1993) Verification of Onsager s reciprocal relations in a molten silicate solution. Science 259, 204—206. 

Whereas electric conduction in the molten silicates is predominantly a cationic process, viscous flow involves the motion of both ionic species and is expected to furnish more information on the larger anions. Some recent systematic studies have indeed confirmed this (5, 6). The addition of even small amounts of metal oxides to silica can reduce the viscosity very appre-... 

The interiors of planets, moons, and many asteroids either are, or have been in the past, molten. The behavior of molten silicates and metal is important in understanding how a planet or moon evolved from an undifferentiated collection of presolar materials into the differentiated object we see today. Basaltic volcanism is ubiquitous on the terrestrial planets and many asteroids. A knowledge of atomic structure and chemical bonding is necessary to understand how basaltic melts are generated and how they crystallize. Melting and crystallization are also important processes in the formation of chondrules, tiny millimeter-sized spherical obj ects that give chondritic meteorites their name. The melting, crystallization, and sublimation of ices are dominant processes in the histories of the moons of the outer planets, comets, asteroids, and probably of the Earth. 

Figure 15.11 Enthalpies of mixing in binary molten silicates. Reproduced by permission from A. Navrotsky, Am. Mineral., 79, 589-605 (1994).

The last part of ionic electrochemistry, ionics, is about pure electrolytes. A few decades back this electrochemistry would have been all about high-temperature liquids (liquid common salt at 850 °C was the role model). However, this has changed, and the temperatures for eliminating the solvent have deaeased considerably. Some molten salts are now room temperature liquids. At the other end of the temperature scale are the molten silicates, where large polyanions predominate. These are important not only in the steel industry, where molten silicate mixtures form blast furnace slags, but also in the corresponding frozen liquids, the glasses. 

One can quote exceptions to these generalizations. The tetraalkylammonium salts as a class are liquid at temperatures below 300 K. There are liquid electrolytes— produced from dissolving AICI3 into some complex organics—which are liquid at room temperature (Tables 5.3 and 5.4). Above the normal range of 300-1300 K is another set of molten electrolytes, the molten silicates, borates, and phosphates, for which the characteristic temperature range is 1300-2300 K (Tables 5.5 and 5.6). 

Faman and Stebbins performed NMR measurements on simple molten silicates in 1990 and managed to pull more out of their data than other workers in this area. 

Magma molten fluid formed within the Earth s crust or mantle containing molten silicates, water, and gases magma that is extruded on to the surface of the Earth is called lava Metamorphism change created in rocks by heat, pressure, and chemically active fluids within the Earth s crust Meteoric (water) water that percolates rocks from above (e.g. from rivers, rain, snow, etc.)... 

Although the bulk of detailed work has been undertaken on v-SiOj, in situ x-ray studies of molten silicates at high temperature have confirmed that Si is coordinated to four oxygens in the molten as well as the glassy state (Waseda and Toguri, 1977, 1978) and yielded information on Si-0 bond lengths and some angular information (see Table 8.1). 

Fig. 8.9. Typical NMR spectra for molten silicates. Data are from Stebbins et al. (1985) and are for NaAlSijO at 1300°C. Relative location of the spectra is arbitrary, but they all have the same frequency scale (after Stebbins et al., 1985 reproduced with the publisher s permission).

F. Marumo and M. Okuno, X-ray structural studies of molten silicates Anorthite and albite melts, in Material Science of the Earth s Interior (ed. I. Sunagawa), Terra Scientific Pub. Tokyo, pp. 25-38 (1984). 

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