Montmorillonite solid solution

As we have seen in the previous section, the bulk chemical compositions of montmorillonites taken from the literature are dispersed over the field of fully expandable, mixed layered and even extreme illite compositions. Just what the limits of true montmorillonite composition are cannot be established at present. We can, nevertheless, as a basis for discussion, assume that the ideal composition of beidellite with 0.25 charge per 10 oxygens and of montmorillonite with the same structural charge do exist in nature and that they form the end-members of montmorillonite solid solutions. Using this assumption one can suppose either solid solution between these two points or intimate mixtures of these two theoretical end-member fully expandable minerals. In either case the observable phase relations will be similar, since it is very difficult if not impossible to distinguish between the two species by physical or chemical methods should they be mixed together. As the bulk chemistry of the expandable phases suggests a mixture of two phases, we will use this hypothesis and it will be assumed here that the two montmorillonite... 

This equation shows that activity of Ca + is related to pH, concentration of H2CO3 and temperature. Because pH is related to the concentration of Cl for the equilibrium curves 1 and 2 in Fig. 2.14, the relationship between the concentrations of Ca " " and Cl" can be derived for calcite-albite-sericite-K-feldspar-quartz equilibrium (curves 4 and 7 in Fig. 2.14) and calcite-albite-sericite-Na-montmorillonite-quartz equilibrium (curves 5 and 8 in Fig. 2.14) with constant w2h2C03- The range of zh2C03 in the solution in equilibrium with calcite is assumed to be 10 to 10 . The other equilibrium curves for the assemblage including Ca minerals are also drawn (Fig. 2.14). These assemblages are wairakite-albite-sericite-K-feldspar-quartz (curve 3), Ca-montmotillonite-albite-sericite-Na-montmorillonite-quartz (curve 6), Ca-montmorillonite-albite-sericite-K-feldspar-quartz (curve 9) and anhydrite (curve 10). The effect of solid solution on the equilibrium curves is not considered because of the lack of thermochemical data of solid solution. 

The kinetics of PAA, synthesized from 4,4 -oxydianiline and pyromellitic dianhydride, solid-state imidization both in filler absence and with addition of 2 phr Na+-montmorillonite was studied [1], The nanofiller was treated by solution of P-phenylenediamine in HC1 and then washed by de-ionized water to ensure a complete removal of chloride ions. The conversion (imidization) degree Q was determined as a function of reaction duration t with the aid of Fourier transformation of IR-spectra bands 726 and 1014 cm 1. The samples for FTIR study were obtained by spin-coating of PAA/Na+-montmorillonite mixture solution in N,N-dimethylacetamide on KBr disks, which then were dried in vacuum for 48 h at 303 K. It was shown, that the used in paper [1] method gives exfoliated nanocomposites. The other details of nanocomposites polyimid/Na+-montmorillonite synthesis and study in paper [1] were adduced. The solid-state imidization process was made at four temperatures 7) 423, 473, 503 and 523 K. 

I would like to propose now, as a working hypothesis, two concepts of solid solution in montmorillonites which are somewhat unusual. In continuing through the discussions, the major supporting evidence for these concepts will be pointed out and used to develop the idea of phase relations between clay minerals. It is then the reader who will judge the validity of the ideas both as the evidence is presented and as the concept is used in each instance. 

Figure 18. Schematic representation of several possible types of solid solution. Shaded and blank layers represent expanding and mica-like units (2 1 structures). Solid and unfilled circles represent two species of interlayer ions, a totally random in all aspects b = interlayer ion ordering, single phase montmorillonite c = ordered interlayer ions which result in a two-phase mica structure, two phases present d = randomly interstratified mineral, one phase e = regular interstratification of the 2 1 layers giving an ordered mixed layered mineral, one phase present f = ordered mixed layered mineral in both the interlayer ion sites and the 2 1 interlayering. This would probably be called a single phase mineral.

The solid solution (interlayering in this case) between mica and montmorillonite is gradually decreased from the montmorillonite end member and, as temperature increases, only the 30% expandable layer form persists near 400°C. 

If we now consider the bulk compositions of the mixed-layered minerals which contain both expandable and non-expandable layers, two series are apparent, one between theoretical beidellite and illite and one between theoretical montmorillonite and illite (Figure 25). The intersection of the lines joining muscovite-montmorillonite and beidellite-celadonite (i.e., expandable mineral to mica), is a point which delimits, roughly, the apparent compositional fields of the two montmorillonite-illite compositional trends for the natural mixed layered minerals (Figure 26). That is, the natural minerals appear to show a compositional distribution due to solid solutions between each one of the two montmorillonite types and the two mica types—muscovite and celadonite. There is no apparent solid solution between the two highly expandable (80% montmorillonite) beidellitic and montmorillonitic end members. The point of intersection of the theoretical substitutional series beidellite = celadonite and muscovite-montmorillonite is located at about 30-40% expandable layers— 70-60% illite. This interlayering is similar to the "mineral" allevardite as defined previously. It appears that as the expandability of the mixed... 

Figure 46c. Both potassium and silica are intensive variables, represented as the activities of the ions. It should be noted that the solid solution between mica and montmorillonite (mixed layering) is not considered as a separate phase but is shown here as a shaded area.

If we consider three components, the phases will be arranged as in Figure 48a at conditions of initial burial. The solid solution series are somewhat abbreviated for simplicity. The phase relations are dominated by fully expanding and mixed layered minerals which cover a large portion of the compositional surface. Notably two dioctahedral expandable minerals exist as does a large undefined series of trioctahedral phases designated as expanding chlorite, vermiculite and trioctahedral montmorillonite. 

Once the illite-chlorite zone is entered, i.e., the facies where dioctahedral mica-montmorillonite mineral solid-solutions are no longer stable, how does the assemblage change into muscovite-chlorite The major... 

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. 

Equations 2.25-2.34 represent the reactions between solid and liquid (Boxes 3 and 4). Among them, Equations 2.25-2.28 represent the dynamic heterogeneous exchange of ions between montmorillonite and solution if the ions are labeled by radioactive isotopes, they also include heterogeneous isotope exchange ... 

Ransom B, Helgeson HC (1993) Compositional end members and thermodynamic components of illite and dioctahedral aluminous smectite solid solutions. Clays Clay Minerals 41 537-550 Reynolds RC, Hower J (1970) The nature of interlayering in mixed-layer illite-montmorillonites. Clays Clay Minerals 18 25-36... 

The structure is based on groups of three layers Single sheet of (Al, Mg) (O, OH)g octahedra are sandwiched between two sheets of SiO tetrahedra. Montmorillonite is a member of the smectite group and forms solid solution with beidellite. 

The structural integrity of M2D-C3-0-(E0)ra-CH3 was maintained for all solid media investigated, as confirmed by qualitative analysis of the flow injection analysis (FIA) API-MS spectra (example shown in Fig. 2.8.10a). Intercalation of the surfactant solution between the clay layers was considered as the likely cause of the reduced recovery on montmorillonite (74%), according to experimental observations and... 

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