Silicate equilibria

CO, SC ), or the occurrence of the minerals is sufficiently rare to represent a special case—the various halide salts, for example. However, dolomite presents a special problem in that the existence of Mg is important to silicate equilibria under consideration. The main trouble here is that the conditions of crystallization and stability of dolomite in sediments and sedimentary rocks is imperfectly known, thus leaving a question as to its influence on silicates or the influence of silicates on its presence. One is forced more or less to ignore the importance of dolomite at present. This does not mean that it can be ultimately excluded from a complete discussion of clay mineral stability. 

In this context it is interesting to note the persistently low levels of silica concentration in river and sea water. This suggests that the concentration of silica is controlled by other silicate equilibria or biologic processes (Garrels, 1965 Wollast and de Broeu, 1971). 

One can imagine two types of chemical systems in which sepiolite and palygorskite appear, "closed" and "open"—systems where chemical components are defined by their relative masses (Figure 41) and systems where their chemical activity in solution determines their role in silicate equilibria (Figure 40). What criteria are necessary to allow the use of one or the other (or an intermediate system with fewer components "inert") to be used to explain clay mineral assemblages in a given geologic situation ... 

In summary there is not enough information published regarding the production of light hydrocarbon molecules (petroleum) or very heavy ones (coal) in geologic time. It is not possible at present to determine whether time, temperature or pressure plays a role, predominant or minor, in the evolution of the organic debris incorporated in sediments. It is certain that the interaction of silicate equilibria and transformations... 

Ca ) and the Fe -Fe balance in solution is certainly of primary importance for sedimentary silicate equilibria. 

We may now examine specific information from chemical analyses of the Great Lakes (7, 8, 13) to determine to what degree the variations of the proposed model fit the actual data. Rather than consider all of the variables at once, it is simpler to consider smaller portions to get a better idea of what actually is happening. We shall look at calcium carbonate equilibria, dolomite equilibria, phosphate equilibria, and silicate equilibria. 

Silicate Equilibria. The following equilibria are depicted at constant temperature and pressure (25°C., 1 atm. total pressure) in Figure 4. 

The next stage in the development of the atmosphere began when formation of oxygen exceeded its consumption (Mason, 1971). This occurred in the period between 1.7 and 1.2 b.y. ago at about 1 b.y., at the boundary of the Proterozoic and Paleozoic, the amounts of Oj reached the level of the Pasteur point ( 1% of the present oxygen content), as a result of which more complex organisms could appear in the biosphere. The CO2 content at this stage was controlled by carbonate-silicate equilibria in the hydrosphere. 

Fig. 23. Diagram of carbonate and carbonate-silicate equilibria in deposition of BIF solid lines bound field of deposition of crystalline minerals dashed lines— field of deposition of amorphous sediments thin lines—isobars of C02 in primordial atmosphere. Sid = siderite Cree = greenalite.

Ionic form of transport. This is possible only in very acid solutions (pH < 3), the long existence of which is not likely at the Earth s surface due to interaction with rocks, due to dilution by surface waters, and due to the buffer effect of carbonate and silicate equilibria controlling the pH in the ocean. 

A strong effect of temperature on metal/silicate partition coefficients is expected on thermodynamic grounds for those metal/silicate equilibria possessing a large entropy change (e.g., Capobianco et aL, 1993). Although Murthy... 

Arculus R. J., Holmes R. D., Powell R., and Righter K. (1990) Metal/silicate equilibria and core formation. In The Origin of the Earth (eds. H. Newsom and J. H. Jones). Oxford University Press, London, pp. 251-271. 

Sheldon N. D., Retallack G. J., and Reed M. H. (2001) Siderite-iron-silicate equilibria in paleosols as an atmospheric CO2 paleobarometer or paleoproductivity index Abstr. Geol Soc. Am. Geol Soc. London Global Meet. Edinburgh, 42. 

The pH chauges iu wetlaud soils siguificautly iuflueuce hydroxide, carbonate, sulfide, and silicate equilibria. These equilibria regulate the precipitation and dissolution of solids, the sorption and desorption of ions, and the concentrations of nutritionally significant ions or substances such as AT+, Fe, H2S, H2CO3, and undissociated organic acids. 

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