Feldspars stability

Nesbitt H. W., Fedo C. M., and Young G. M. (1997) Quartz and feldspar stability, steady and non steady state weathering and petrogenesis of siliciclastic sands and muds. J. Geol. 105, 173-191. 

A solids-stabilized water-in-oil emulsion may be used either as a drive fluid for displacing hydrocarbons from the formation or to produce a barrier for diverting the flow of fluids in the formation. The solid particles may be formation solid particles or nonformation solid particles, obtained from outside the formation (e.g., clays, quartz, feldspar, gypsum, coal dust, asphaltenes, polymers) [228,229]. 

The P-T stability curve of muscovite intersects the incipient melting curve of granite in hydrous conditions at about 2.3 kbar total pressure and T = 650 °C. Thus, muscovite may crystallize as a primary phase from granitic melts above these P and T conditions (interstitial poikihtic crystals) or may form by reaction with pristine solid phases at lower P and T (muscovite as dispersed phase within feldspars, for instance). In this second type of occurrence, the following two equilibria are of particular importance ... 

Burnham, 1962 Keller and Hanson, 1968 Rose, 1970 Lowell and Guilbert, 1970 Keller, 1963). These studies indicate that kaolinite can be formed by hydrothermal alteration at the surface as well as to depths of several kilometers. Although information is lacking for low temperatures, intermediate conditions of pressure and temperature are known to permit the stability of the potassic mica-beidellite mixed layered composition series which excludes the stable coexistence of K-feldspar and kaolinite. If one accepts the argument that both beidellite-sodic and potassic are... 

The studies cited above deal with deeply buried sediments which occur in areas of low geothermal gradients. Further the rocks are basically sodi-potassic, at least in the silicate aggregates, and most often contain a potassic phase such as illite, feldspar or zeolites. In instances where these chemical conditions prevail and where the geothermal gradient is high (Muffler and White, 1969 Browne and Ellis, 1970) the same temperature-mineralogy relations seem to hold, 100-120°C appears to be the upper limit of fully expandable montmorillonite stability. However, R. 0. 

Frequently two or more species are found together in the same geologic sample. As pressure-temperature conditions become more severe, the mineralogy becomes more simple, feldspar appears and finally within the limits of clay mineral stabilities only calcic zeolites are found. However, the calcic minerals are generally confined to rocks of basic... 

A consideration of natural occurrence and chemical composition of alkali zeolites allows a certain refinement of the zeolite facies concept previously proposed. The key factor is the grouping of the alkali zeolites into a continuous solid solution series. Other possible coexisting phases of similar composition are sodium and potassium feldspar, natrolite and analcite. The extent of solid solution decreases with temperature, possibly also with pressure. This effect allows the sequential series zeolite-K feldspar, zeolite-analcite-K feldspar, analcite-K feldspar-albite and eventually two feldspars to the exclusion of analcite, the alkali zeolite with the highest stability limits. 

Next, when the content of dissolved ionic species due to weathering increases in the fluid, the alkali activity in solution will dictate the formation of an expanding phase then at high activity of K+ the stability of mica. Finally, the feldspar-quartz assemblage is stable. This would happen where the solutions migrate slowly enough so that the original minerals are not dissolved and remain stable in the chemical system. 

To increase the sulfur content and thereby improve color quality, the lattice aluminum content can be decreased by including high-silicon feldspar in the manufacturing recipe. This reduces the number of sodium ions needed for lattice stabilization and leaves more for sulfur group equivalence. A typical product would be Na6.9Al5.68i6.4 024 84.2 with a stronger, redder shade of blue than the simpler type. An... 

Are there Earth-surface conditions for which the above minerals are thermodynamically stable and so would not weather The answer is rarely for the olivine minerals (cf. Bath et al. 1987), which rapidly disappear in the volcanic soils and beach sands of Hawaii, for example. What about the stability of the plagioclase feldspar Gibbs free energies of the reactants and products (in kcal/mol) given under weathering reaction (7.3) indicate that AG° = -23.53 kcal/mol, and = 1017.25 If we assume a typical soil pH of 6, then we can compute that Ca- would have... 

Figure 7.1 Goldich s sequence of increasing weatherability of common minerals (cf. Loughnan 1969 Faure 1991). In parentheses are the lifetimes in years from Table 7.1, assuming olivine = forsterite, augite = diopside, hornblende = tremolite, Ca-plagioclase = anorthite, Na-plagioclase = albite, K-feldspar = microcline, and the stability of muscovite is comparable to that of the related clay, illite.

In our weathering example (Table 7,2) kaolinite, rather than the aluminum oxyhydroxides, was the chief weathering product of the feldspars. This reflects the fact that the silica present in soil moisture and natural waters, generally, is high enough to stabilize kaolinite relative to the aluminum oxyhydroxides. This observation is better understood if we write the kaolinite dissolution reaction ... 

Figure 9.8 Log([K+]/lH ]) versus log[H4SiO ] diagram at 25°C and 1 bar pressure, showing the stability fields of gibbsite, K-mica, K-feldspar, and kaolinite. Circled numbers are the same numbers that have been assigned to the equilibrium reactions between these phases in the text.

Figure 9.10 Log([Na ]/[H+]) versus log[H4SiO J diagram at 25 C and 1 bar pressure. The figure shows a stability field for an idealized sodic montmorillonite. Plotted on the diagram are analyses of groundwaters from various rock types. A lutite is a shale or mudstone that probably contains illite and kaolinite, with smaller amounts of smectite clays such as montmorillonite. Sandstones include feldspars as well as quartz. Note that most of the water analyses fall in the kaolinite field. After O. P. Bricker and R. M. Garrels, Mineralogic factors in natural water equilibria. In Principles and applications of natural water chemistry, ed. S. Faust and J. V. Hunter. Copyright 1965. Reprinted by permission.

In their study of the Canadian uranium deposit at Cigar Lake, Cramer and Smellie (1994) have plotted data for K, Na+, Ca +, and Mg +, in site waters on log([M"]/[H+]") versus log[H4Si04] diagrams. In Fig. 9.15, the illite phase field is contoured to show the stabilities of different illite fractions in I/S. The plot describes the evolution of water chemistry from atmospheric precipitation and surface-waters (lakes and streams) to infiltrating soil water and groundwater above, and then in contact with, the orebody. In the soil, kaolinite and illite (the dominant clay), quartz, and feldspars are... 

Montoya, J.W. Hemley, J.J. "Activity Relations and Stabilities in Alkali Feldspar and Mica Alteration Reactions," Econ. Geol. 1975, 70, 577-582. 

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