Silicate equilibrium data

A slightly different, but still a high pressure and temperature scenario, has been proposed recently by several groups, based on a subset of the above elements. With additional data for nickel and cobalt, Li and Agee (2001) proposed that the concentrations of these elements in Earth s upper mantle could be explained by metal-silicate equilibrium in the deep mantle (40-50 GPa and —3,000 °C). Studying nickel, cobalt, manganese, vanadium, and chromium in (Mg, Ee)0-metal systems, Gessmann and Rubie (2000) concluded that the depletions of these five elements could be... 

Three different approaches have been used. Firstly, the distribution of the major elements between mineral phases and a coexisting silicate melt may be calculated from experimental phase equilibrium data using regression techniques. Secondly, mineral-melt equilibria can be determined from mineral-melt distribution coefficients. A third, less empirical and more complex, approach is to use equilibrium thermodynamic models for magmatic systems. These require a thermodynamically valid mixing model for the liquid and an internally consistent set of solid-liquid thermodiemical data. 

Viscosity of NBS7I0 soda-lime silicate glass, based on equilibrium data of Napolitano el al. 5 and isostructural data of Mazurin el a . [6] from Scherer (7]. Along the isostructural curve the atomic configuration is fixed (see discussion on following pages). 

Patel A, Price GD, Mendelssohn MJ (1991) A computer simulation approach to modeling the structure, thermodynamics and oxygen isotope equilibria of silicates. Phys Chem Min 17 690-699 Polyakov VB (1997) Equilibrium fractionation of the iron isotopes estimation from Mossbauer spectroscopy data. Geochim Cosmochim Acta 61 4213-4217... 

Probably the only general statement which can be made about the experimental studies on zeolites is that the majority of published data is inapplicable directly to natural minerals. This is due either to the excessively high temperatures under which the experiments are performed, outside of the physical limits of zeolite stability, or to short time spans of observation which do not allow the silicates to come to equilibrium with the fluids of the experiments. Those studies designed to determine zeolite stability indicate that the most silica-poor alkali zeolite, analcite, is not stable above 180°C. More silica-rich species will be found below this temperature. However, the reasons for the crystallization of one or another of the silica-rich alkali zeolites are not yet elucidated. 

Figure 1 gives results obtained by Alexander et al. (I) and Baumann (2) by dissolving fine particles of commercially available vitreous silica powders in aqueous solutions. Similar data obtained in polymerization and depolymerization experiments by Scheel et al. (15) and Friedberg (10) indicate that the curve shown in Figure 1 represents an equilibrium concentration for oligomeric acid. It can be approached from the supersaturated state of monomeric silicic acid as well as from a solution of pure polymeric silicic acid. 

Owing to this activation threshold, the first precipitation product from aqueous solutions of silicic acids will be an amorphous silica of some degree of hydration, while at room temperature the growth of vitreous and crystalline forms of silica from the precipitate (and thus the approach toward the absolute equilibrium) will proceed extremely slowly. With this understanding the data in Figure 1 are said to represent, an equilibrium—i.e., the reversible equilibrium between silicic acids in aqueous solution and metastable hydrated silica or polymeric silicic acid as precipitate. 

Figure 9 gives some results obtained with vitreous silica. Three runs were made with 5, 15, and 45 sq. meters of total surface area in suspension. Apparently, the data indicate again the approach toward the two-phase equilibrium for polymeric silicic acid—i.e., independent of the total amount of solid material, the equilibrium concentration of about 110 //grams Si02/ml., as indicated in Figure 1, is approached in all three cases. Only the rate of dissolution is different, being determined by the size of the exposed surface area. 

There are several explanations for these deviations. As Figure 11 indicates, it takes some time to establish the adsorption equilibrium. Thus, the theoretical assumption concerning the adsorption equilibrium and resulting in Equation 3 may not be satisfied adequately. Here a higher initial silicic acid concentration would result. On the other hand, at high silicic acid concentrations the Langmuir isotherm is an idealization which will not properly represent experimental data of extensive... 

In a nuclear waste repository located in basalt, solution pH is controlled by interactions between groundwater and the reactive glassy portion of the Grande Ronde basalt (10). In situ measurements and experimental data for this system indicate that equilibrium or steady-state solutions are saturated with respect to silica at ambient temperatures and above. Silica saturation and the low, total-dissolved carbonate concentration indicate the pH may be controlled by the dissolution of the basalt glass (silica-rich) with subsequent buffering by the silicic acid buffer. At higher temperatures, carbonate, sulfate, and water dissociation reactions may contribute to control the final pH values. 

Using coal-based sorbents, Sivasamy et al. [62] evaluated their ability to remove fluoride from water. On equilibrium basis, Langmuir and Freundlich models were used to describe the data points, while the kinetic data points were interpreted in terms of reaction and mass transfer processes. Kaolinite, adioctahedral two-layered (silica and alumina) silicate (1 2 type), has also been tested in drinking water defluoridation. Recently, Sugita etal. [58] and earlier Kau etal. [63] and Weerasooriya et al. [10] presented fluoride adsorption results of kaolinite. The fluoride-binding sites in kaolinite consist of aluminol and silinol sites. The authors explained that the fluoride-kaolinite interaction led to the formations of both the inner- and outer-sphere complexes. 

The thermodynamics of corrosive alkali salt-oxide interaction is not well established. In an assessment of research needs for materials in coal conversion, the need for carbonate-silicate melt studies, including activity and phase equilibrium measurements, was stressed (31 ). The lack of thermodynamic data for fused salts, and their reactions with oxides and alloys leading to models of hot corrosion, was also indicated. 

From experimental data on the synthesis of hydrous iron silicates from gels (Grubb, 1971) and from petrographic observations (Dimroth and Chauvel, 1973 French, 1973 Klein and Bricker, 1977) it follows that in late diagenesis or early (low-rank) metamorphism, greenalite converts to minne-sotaite. The equilibrium parameters of the conversion reaction ... 

Elemental and isotopic fractionations by evaporation of silicate liquids, in particular limiting circumstances, can be simulated by equilibrium calculations, provided that an adequate thermodynamic model of the melt is available. In this approach, a particular starting temperature, pressure, and initial composition of condensed material are chosen and the gas in equilibrium with the melt is calculated from thermodynamic data. The gas is then removed from the system and equilibrium is recalculated. Repeated small steps of this sort can simulate the kinetic behavior during vacuum evaporation (i.e., the limit of fast removal of the gas relative to the rate it is generated by evaporation). This approach has been taken by Grossman et al. (2000, 2002) and Alexander (2001, 2002). 

Although HSE concentrations are low in the Earth s mantle, they are not as low as one would expect from equilibrium partitioning between core forming metal and residual mantle silicate, as emphasized by new data on metal/silicate partition coefficients for these elements (Borisov and Palme, 1997 Borisov et al., 1994). Murthy (1991) suggested that partition coefficients are dependent on temperature and pressure in such a way that at the high P-T conditions where core formation may have occurred, the observed mantle concentrations of HSEs would be obtained by metal/silicate equilibration. This hypothesis has been rejected on various grounds (O Neill, 1992), and high P-T experiments have not provided support for the drastic decrease of metal/silicate partition coefficients of HSE required by the Murthy model (Holzheid et al., 1998). 

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