Silicate phases

The failure to identify the necessary authigenic silicate phases in sufficient quantities in marine sediments has led oceanographers to consider different approaches. The current models for seawater composition emphasize the dominant role played by the balance between the various inputs and outputs from the ocean. Mass balance calculations have become more important than solubility relationships in explaining oceanic chemistry. The difference between the equilibrium and mass balance points of view is not just a matter of mathematical and chemical formalism. In the equilibrium case, one would expect a very constant composition of the ocean and its sediments over geological time. In the other case, historical variations in the rates of input and removal should be reflected by changes in ocean composition and may be preserved in the sedimentary record. Models that emphasize the role of kinetic and material balance considerations are called kinetic models of seawater. This reasoning was pulled together by Broecker (1971) in a paper called "A kinetic model for the chemical composition of sea water."... 

A short review on the development of laser heating in special applications under pressure has been published by Bassett (2001). A heating system to be used, with either ruby or YAG laser, under pressure in a diamond anvil cell has been described. Graphite to diamond and several silicate phase transformations have been studied. 

Figure 8. Figure (a) after Clayton et al. (1976, 1977). The scales are as in Figure 1. The O isotopic compositions of the different meteorite classes are represented ordinary chondrites (H, L, LL), enstatite chondrites (EFl, EL), differentiated meteorites (eucrites, lAB irons, SNCs) and some components of the carbonaceous chondrites. As the different areas do not overlap, a classification of the meteorites can be drawn based on O isotopes. Cr (b) and Mo (c) isotope compositions obtained by stepwise dissolution of the Cl carbonaceous chondrite Orgueil (Rotaru et al. 1992 Dauphas et al. 2002), are plotted as deviations relative to the terrestrial composition in 8 units. Isotopes are labeled according to their primary nucleosynthetic sources. ExpSi is for explosive Si burning and n-eq is for neutron-rich nuclear statistical equilibrium. The open squares represent a HNOj 4 N leachate at room temperature. The filled square correspond to the dissolution of the main silicate phase in a HCl-EIF mix. The M pattern for Mo in the silicates is similar to the s-process component found in micron-size SiC presolar grains as shown in Figure 7.

Holdaway M. J. (1971). Stability of andalusite and the aluminium silicate phase diagram. Amer. Jour. Set, 271 97-131. 

Schreiber H. D. (1976). The experimental determination of redox states, properties, and distribution of Chromium in synthetic silicate phases and application to basalt petrogenesis. Ph.D. diss., University of Wisconsin. 

SMF and SNF superplasticizers are adsorbed rapidly onto hydrating cement but this net effect is made up of very rapid adsorption by C3A and slower adsorption by the silicate phases, as shown in Fig. 2.9. [39],... 

Douglas et al. (2003) prepared Np-doped U6+ silicate phases under different pH conditions. In Figure 12, EELS spectra from a synthetic urano-phane crystallite clearly illustrate the presence of... 

Bulakh et al. (1998) and Williams et al. (2001) have reported on the alteration of zirconolite from carbonatites. In these examples, the zirconolite has been replaced along cracks and within micron-sized domains by an unidentified Ba-Ti-Zr-Nb-ACT silicate phase, suggesting that zirconolite may not be stable in the presence of hydrothermal fluids rich in Si. Figure 3 presents an example of this type of alteration, where one may see additional evidence for enhanced alteration along zones enriched in Th and U. In... 

Fig. 3. SEM backscattered electron image of alteration in zirconolite from the Afrikanda alkaline complex, Kola Peninsula, Russia. This crystal exhibits complex magmatic zoning, late-stage replacement by an unknown Ba-Zr-Ti-silicate phase, and preferential alteration along cracks and Th-U-rich zones.

It should be possible, through a unification of chemical and mineral structure data and the results of experimental studies on silicate phase equilibria, to develop a general picture of clay mineralogy based upon the known chemical behavior of phyllosilicates under various physical conditions. The major elements for such a study are presently available in a rough outline. It is fact the purpose of this essay to summarize the available information and create a general outline of clay mineral petrology. It is hoped that such an attempt meets with some success and, more important in the long run, that such an attempt will interest others in similar exercises, especially those of precision and revision. 

The two major sources of information available are quite different and, as a first approximation, require different interpretations. We will first consider the composition and chemiographic relations of the silicate phases involved as they can be deduced from analyses of natural minerals. Then we will look at the mineral assemblages reported in soils, sediments... 

It is important to note two things in this analysis first, the reactions which govern silicate phase equilibria occur in a system closed to large-scale chemical migration. This corresponds to a pore-water sediment system of local equilibrium. Second, the most striking mineralogical change—the crystallization of feldspar—is, in fact, the result of the instability of another phase, montmorillonite. The use of... 

The initial increase in hydrostatic pressure in a sedimentary basin appears not to change mineral stabilities from those of the weathering environment. The formation of potassic, iron-rich micas such as ferric illite and glauconite both in lacustrine and shallow ocean basins demonstrates their stability at low pressures and temperatures. The same is true of the 7 8 chlorite chamosite or berthierine. Most likely the chemical variables of pH, Eh and the activity of the various ions in solution are predominant in silicate phase equilibria in sedimentary environments. 

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