Illite equilibria

To test whether the mixing hypothesis might explain the diagenetic alteration observed, we begin by equilibrating the fresh water, assuming equilibrium with the potassium feldspar ( maximum microcline in the database), quartz, and muscovite (a proxy for illite) in the formation. In REACT, we enter the commands... 

In a final application of kinetic reaction modeling, we consider how sodium feldspar (albite, NaAlSisOs) might dissolve into a subsurface fluid at 70 °C. We consider a Na-Ca-Cl fluid initially in equilibrium with kaolinite [Al2Si20s (OF )/ ], quartz, muscovite [KAl3Si30io(OH)2, a proxy for illite], and calcite (CaC03), and in contact with a small amount of albite. Feldspar cannot be in equilibrium with quartz and kaolinite, since the minerals will react to form a mica or a mica-like... 

Fig. 4.18 shows the result of Cd2+ adsorption on illite in presence of Ca2+ (Comans, 1987). The data are fitted by Freundlich isotherms after an equilibration time of 54 days. It was shown in the experiments leading to these isotherms that adsorption approaches equilibrium faster than desorption. Comans has also used 109Cd to assess the isotope exchange he showed that at equilibrium (7-8 weeks equilibration time) the isotopic exchangeabilities are approximately 100 % i.e., all adsorbed Cd2+ is apparently in kinetic equilibrium with the solution. The available data do not allow a definite conclusion on the specific sorption mechanism. 

Adsorption-desorption equilibrium for Cd(II) on illite after 54 days of equilibration. The solution contains HCO3, 2 x 10 3 M Ca2+ and has a pH = 7.8. Freundlich isotherms based on separate adsorption ( ) and desorption (O) data are given from Comans (1987). 

Comans RNJ, Haller M, De Preter P (1991) Sorption of cesium on illite Non-equilibrium behavior and reversibility. Geochim Cosmochim Acta 55 433-440... 

When an expanding mineral is no longer stable, the iron content of the chlorite in equilibrium with illite will become more variable (Figure 49b). If chlorite is present due to a relatively high Fe/Fe + Mg content of a rock, it can occur with three other aluminous phases such as illite-montmorillonite and kaolinite. Thus the four-phase phyllosilicate assemblage common to argillaceous rocks can be accounted for by dividing the... 

A common way to determine Kid values is to measure sorption isotherms in batch experiments. To this end, the equilibrium concentrations of a given compound in the solid phase (Cis) and in the aqueous phase (CIW) are determined at various compound concentrations and/or solid-water ratios. Consider now the sorption of 1,4-dinitrobenzene (1,4-DNB) to the homoionic clay mineral, K+-illite, at pH 7.0 and 20°C. 1,4-DNB forms electron donor-acceptor (EDA) complexes with clay minerals (see Chapter 11). In a series of batch experiments, Haderlein et al. (1996) measured the data at 20°C given in the margin. 

Fig. 2. Logarithmic activity diagram depicting equilibrium phase relations among aluminosilicates and sea water in an idealized nine-component model of tire ocean system at the noted temperatures, one atmosphere total pressure, and unit activity of H20. The shaded area represents (lie composition range of sea water at the specified temperature, and the dot-dash lines indicate the composition of sea water saturated with quartz, amotphous silica, and sepiolite, respectively. The scale to the left of the diagram refers to calcite saturation foi different fugacities of CO2. The dashed contours designate the composition (in % illite) of a mixed-layer illitcmontmorillonitc solid solution phase in equilibrium with sea water (from Helgesun, H, C. and Mackenzie, F T.. 1970. Silicate-sea water equilibria in the ocean system Deep Sea Res.).

From HemleyJs work on the potassium system (11) one may infer that kaolinite, quartz, and K-mica ( illite) may be stable together, and the equilibrium constant [K+]/[H+] may be extrapolated, (from 200°C.) to 106 at 25°C.—e.g., Hollands (15) value of 10,50 O5. Hem-ley s work on the sodium system (12) in the same way indicates that quartz, Na-montmorillonite, and kaolinite can form a stable assemblage, and a somewhat risky extrapolation of the equilibrium ratio [Na+]/[H+] from 300° to 25°C. gives 107° (15). These ratios are not far from the corresponding ratios in sea water. One could not expect them to be exactly the same since the hydromica and montmorillonite phases in sea water are solid solutions, containing more components than the phases in Hemley s experiments. His experiments surely do not contradict the idea that the previously mentioned phases could exist together at equilibrium. 

Figure 2. Projection on planes Na—K—Mg and Ca—K—Mg of the composition of possible equilibrium phases and of marine sediment minus carbonate. CH = chlorite, IL — illite (hydromica), = montmorillonite, PH = phillipsite, SED = sediments [according to Goldschmidt (7)], CA = average calcareous, SI = average siliceous, AR = average argillaceous sediments (from Ref. 22), OW = ocean water, GL = glauconite,...

Rates of ion exchange on kaolinite, smectite, and illite are usually quite rapid. Sawhney (1966) found that sorption of cesium on illite and smectite was rapid, while on vermiculite, sorption had not reached an equilibrium even after 500 h (Fig. 5.5). Sparks and Jardine (1984) found that potassium adsorption rates on kaolinite and montmorillonite were rapid, with an apparent equilibrium being reached in 40 and 120 min, respectively. However, the rate of potassium adsorption on vermiculite was very slow. Malcom and Kennedy (1969) studied Ba-K exchange rates on kaolinite, illite, and montmorillonite using a potassium ion-specific electrode to monitor the kinetics. They found >75% of the exchange occurred in 3 s, which represented the response time of the electrode. The rate of Ba-K exchange on vermiculite was characterized by a rapid and slow rate of exchange. 

Illite -H2O-H2 System. Vaporization of potassium from the highly acidic illite system, in neutral atmospheres, is expected to provide a relatively insignificant source of alkali in most coal combustion systems. However, in the presence of reactive combustion gases, such as H2O and H2, thermodynamic considerations predict a significant KOH partial pressure. In addition, an increase in the K-pressure should result from a reduction in the O2 pressure, in the presence of H2. However, KMS experiments did not indicate formation of KOH or additional K in the presence of H2 gas. Thus, thermodynamic equilibrium does not appear to have been established in this heterogeneous system, even though the temperatures were sufficiently high to have normally ensured a rapid approach to equilibrium. 

Further evidence of this lack of thermodynamic equilibrium was provided by monitoring formation of SiO by H2 reduction of Si02, present either as the silicate in illite or as pure silica. Figure 16 shows that SiO production from both forms of Si02 is... 

Figure 17. Equilibrium constant jor HiO dissociation (KMS data). Key O, data obtained during illite + Hi experiment , H2O rather than Hi addition , Pm assumed equal to 2Poi for Kp calculation and------------------, Ref. 36 data.

Figure 5.11 A log plot of the buffer capacity due to carbonic acid species for = 10 M (see Fig. 5.10) at saturation with respect to calcite for = I0 M and for equilibrium between the clays illite and kaolinite. The lower curve is...

This equation is plotted in Fig. 5.11 which shows that the reaction at equilibrium has about 10 times more buffer capacity than calcite. Because this reaction will not often be at equilibrium, however, the buffer capacity is a maximum possible value. Further, the reaction is usually irreversible with kaolinite more often stable than illite in weathering environments. For this reason the reaction resists a pH decrease, but not an increase. 

Figure 9.11 Log([K+]/[H" ]) versus log[H4Si04] diagram at 25°C with plotted chemical analyses of waters in contact with clays, as reported by Aagaard and Helgeson (1983). Phase boundaries are drawn consistent with the analyses and with the illite and montmorillonite compositions proposed in the text. Agreement of the data and boundaries suggest equilibrium between the phases and support the idea that illite and montmorillonite behave as two discrete phases.AfterR.M.Garrels in C/oystfe Clay Minerals, 32 161-66, Copyright 1984.

Sears (1976) observed that kaolinite and illite coexist in deep Pennsylvania soils under conditions such that the soil moisture is practically isolated from dilution by fresh recharge. His chemical analyses of soil moisture are plotted on the log([K+]/[H+l) versus log[H4SiOJ] diagram for two different sampling sites. Assuming that the highest silica and K+ concentrations are those most likely to approach equilibrium with both... 

Comans, R. N. J., Haller, M., and de Preter, P. (1991). Sorption of cesium on illite non-equilibrium behaviour and reversibility. Geochim. Cosmochim. Acta 55, 433-440. 

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