Clay mineral saturation, calculation

Uncertainty in the calculation, however, affects the reliability of values reported for saturation indices. Reaction log Ks for many minerals are determined by extrapolating the results of experiments conducted at high temperature to the conditions of interest. The error in this type of extrapolation shows up directly in the denominator of log Q/K. Error in calculating activity coefficients (see Chapter 8), on the other hand, directly affects the computed activity product Q. The effect is pronounced for reactions with large coefficients, such as those for clay minerals. 

Fig. 23.6. Calculated saturation indices (log Q/K) of aluminum-bearing minerals plotted versus temperature for a hot spring water from Gjogur, Hveravik, Iceland. Lines for most of the minerals are not labeled, due to space limitations. Sampling temperature is 72 °C and predicted equilibrium temperature (arrow) is about 80 °C. Clinoptilolite (zeolite) minerals are the most supersaturated minerals below this temperature and saponite (smectite clay) minerals are the most supersaturated above it.

MC and MD studies of hydrated smectites with monovalent counterions Li+, Na+, K+, Cs+ were also performed [62, 63, 69, 70, 72, 77-80], An increase of the simulation cell size of 2 1 Na-saturated clay or alternation of its shape from rectangular did not have a significant effect on the calculated interlayer properties [70]. It has been revealed that the mechanism of swelling and hydration depends upon the interlayer ion charge. Also the greater role of the clay mineral surface in organizing interlayer water in the case of K-montmorillonite with a weakly solvating counterion was concluded [64, 68]. 

The non-equilibrium condition of most groundwater systems with respect to many primary minerals, or similarly the metastability which exists with respect to many semi-crystalline or amorphous phases are common problems, especially for silicates. Some clear identification is needed for system reaction time, or the rate at which equilibrium is approached, and similarly identification is needed for metastable plateaus of pseudo-equilibrium, especially for compounds such as amorphous silica, cristobalite, quartz, clay minerals, etc. The likely magnitude of saturation indices which could apply to a given mineral could be specified for a variety of conditions. In this volume, Glynn, and elsewhere others, have recently shown that some error occurs in the calculated saturation values for trace elements when pure end member minerals are assumed to be present, when actually the phases are solid solutions. The consensus among modelers appears to be that error is present and significant the challenge is to develop procedures that quantify the error, so models become tools that provide realistic and interpretable results. 

Drits VA, Plangon A, Sakharov BA, Besson G, Tsipursky SI, Tchoubar C (1984) Diffraction effects calculated for stmctural models of K-saturated montmorillonite containing different types of defects. Clay Minerals 19 541-561... 

In this Section we first show that a local variation of viscosity in the pore water of a saturated smectitic clay such as montmorillonite or beidellite, which is a platelet crystal of about one nanometer (=10 m) thickness, can be calculated by a molecular dynamic (MD) simulation. Then, by applying the HA with the locally distributed viscosity, we can calculate the seepage field of the smectitic clay, which consists of stacks of clay minerals. Consequently, we apply a three-scale analysis of homogenization for a bentonite clay with quartz grains of about 10 [xm (1 fxm = 10 m). 

Table 9.1 Surface tension values and parameters for a group of clay minerals, each saturated with a specific cation (Norris, 1993 Norris et al., 1993). The heading Nat refers to the clay with no cation exchange treatment. Amm refers to an exchange with the ammonium cation. All values are in units of mJ/w measured at 2CPC. The values listed under are the calculated...

We now consider a more realistic case. Let the saturated density of pure smectitic clay (for example, beidellite) be about 1.8[Mg/m ]. The crystal density of the beidellite determined from an MD simulation is found to be 2.901 [Mg/m ]. A stack is assumed to consist of nine minerals. The molecular formula of the hydrated beidellite is Nai/3Al2[Sin/3Ali/3]Oio(OH)2 nH20 where n is the number of water molecules in an interlayer space. We assume that n = 1, 3, 5, and the distance between two minerals (i.e., the interlayer distance) can be obtained from Fig. 8.4 from this, we can determine the volume of external water that exists on the outside of the stack. For each case of n = 1, 3, 5 we calculate the characteristic functions as shown in Fig. 8.10 (note that the scale is different in each case). Then we compute the C-permeability as shown in Fig. 8.11. Based on numerous experimental results, Pusch (1994) obtained the permeability characteristics of clays as a function of density as shown in Fig. 8.12. We recall that the permeability of the saturated smectitic clay is not only a function of the density but also of the ratio of interlayer water to the external water, which indicates that there exists a distribution of permeability for the same density. The range of permeability given in Fig. 8.12 with a saturated density of 1.8Mg/m corresponds well to our calculated results, which were obtained using the MD/HA procedure. 

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