Mineral stability diagrams

Figure 2.6 shows P-T stability diagrams for several components exhibiting polymorphism in geology (the Co2Si04 orthosilicate, which is not a major constituent of rock-forming minerals, is nevertheless emblematic of phase transitions observed in the earth s mantle cf section 5.2.3). 

The stability diagrams of oxide/hydroxide of aluminum and silicon minerals/ aqueous solution systems are illustrated in Figures 1.6 and 1.7. In these diagrams, the following reactions were taken into account. 

Of course, stability diagrams can be constructed not only for the oxides but for other more complex minerals as well. From these diagrams, the stabilities of the minerals under a given condition can be predicted. 

Diagenesis of iron cherts does not lead to any changes in principle in the relationships between minerals and mineral associations. As a result of diagenesis, no new compounds arise which could not have been formed by chemical precipitation. From a comparison of the stability diagrams of the sediments and minerals it is seen that only the numerical values of the... 

The coincidence of the stability diagrams of Fe-Mg pyroxenes (see Fig. 92) and amphiboles (see Fig. 93) makes it possible to plot a schematic diagram of mineral equilibria for (Fig. 93A) which is divided into two... 

Garrei s, R. M. 1984. Montmorillonite/illite stability diagrams. Clays Clay Minerals 32 161-66. 

It is necessary to conclude that many of the mineral stability diagrams that are commonly constructed to explain primary mineral weathering (such as the one for feldspar in Figure 6.12) have no quantitative value—they are useful only to the extent that they gauge the tendency of the weathering reaction to proceed in a forward direction. If, for example, feldspar and kaolinite coexist in a soil, overall equilibrium between the two minerals is not possible when a realistic temperature and time frame is considered. The reaction is irreversible if it proceeds, feldspar must decompose and kaolinite must precipitate. It is true that back reactions such as the... 

Surface conversion due to reactions of the dissolved species with the mineral surface can be predicted using thermodynamic stability diagrams for heterogeneous mineral systems based on relevant mineral dissociation equilibria. This is illustrated in Fig. 3.10 for the calcite/apatite/dolomite system. The activities of Ca + species in equilibrium with various solid phases show that the singular point for calcite and apatite is 9.3. Above this pH, apatite is less stable than calcite and hence conversion of apatite surface to that of calcite can be expected in calcite-apatite system. Similarly, the calcite-dolomite and apatite-dolomite singular points occur at pH 8.2 and 8.8, respectively. 

Strictly speaking, soils are always nonequilibrium systems. With care, however, a partial equilibrium or steady state can be attained by assuming that the soil solids do not change. This is the usual assumption in cation exchange and adsorption studies. Kittrick and co-workers were able to obtain near-equilibrium measurements of some soil minerals in studies requiring -several years. From the resulting ion activities in solution, they were able to calculate some of the equilibrium constants used for the mineral stability diagrams shown later in this book. 

The K and Na stability diagrams are similar, but the values of M/H vary. The Ca feldspars are the most unstable, and the K feldspars are the most stable, with respect to weathering. At higher M/H ratios (more basic solutions) other minerals are stable and would precipitate before the igneous feldspars, but free energy data for other silicates are lacking. 

Stability diagrams such as Fig. 7.5 should not be taken too literally. Free energy data for most minerals are uncertain. Even small errors are magnified by calculating the equilibrium constant, which is the antilogarithm of a small difference between large AG s of formation. In addition, the activities of the solid phases are assumed... 

Phase relationships in the two systems NaaO-CaO-AlgOa-SiOa and CaMgSiaOe—CaAlgSiOg have been defined, and the stability diagrams of some clay minerals in aqueous solutions determined. ... 

Fig. 2 Pressure (depth) versus temperature stability diagram for methane hydrate. (From "Oceanic Gas Hydrate Research and Activities Review. U.S. Department of the Interior. Minerals Management Service. Gulf of Mexico OCS Region. OCS Report MMS 2000-017.)...

Figure 1.96. Log /oj-pH diagram constructed for temperature = 200°C, ionic strength = 1, ES = 10 m, and EC = 10 m. Solid line represents aqueous sulfur and carbon species boundaries which are loci of equal molalities. Dashed lines represent the stability boundaries for some minerals. Ad adularia. Bn bomite, Cp chalcopyrite, Ht hematite, Ka kaolinite, Mt magnetite, Po pyrrhotite, Py pyrite, Se sericite. Heavy dashed lines (1), (2), and (3) are iso-activity lines for ZnCOs component in carbonate in equilibrium with sphalerite (1) 4 co3=0-1- (2) 4 ,co3=0-01- (3) 4 co3 =0-001 (Shikazono, 1977b).

Fig. 12.2. Redox-pH diagram for the Fe-S-H20 system at 100 °C, showing speciation of sulfur (dashed line) and the stability fields of iron minerals (solid lines). Diagram is drawn assuming sulfur and iron species activities, respectively, of 10-3 and 10-4. Broken line at bottom of diagram is the water stability limit at 100 atm total pressure. At pH 4, there are two oxidation states (points A and B) in equilibrium with pyrite under these conditions.

Redox diagrams are used to express the stability of dissolved species and minerals. An example diagram is presented in Fig. 2.4, where the redox potentials of various types of aqueous systems are shown as a function of pH. It can be seen that at acidic pH, a mine water system has a very high oxidation potential (Eh > 500 mv). In... 

Holdaway M. J. and Mukhopadhyay B. (1993). A reevaluation of the stability relations of andalusite Thermochemical data and phase diagram for the aluminum silicates. Amer Mineral, 78 298-315. 

Fig.1. Eh-pH diagram for the system Fe-U-S-C-H2O at 25 °C showing the mobility of uranium under oxidizing conditions, the relative stability of iron minerals, and the distribution of aqueous sulfur species. Heavy line represents the boundary between soluble uranium (above), and insoluble conditions (below), assuming 1 ppm uranium in solution.

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