Illite structure

Tables IX and X contain correlation coefficients for the oxides and for the ions in their various structural positions. Correlations are relatively limited. There is a negative correlation between MgO and A1203 and between octahedral Mg2+ and octahedral Al3+ as would be expected. The same relation exists between ferric iron and aluminum and potassium and ferric iron. This latter correlation may reflect the fact that Fe3+ is the only major ion that can substitute in the illite structure and not increase the layer charge in fact, the layer charge may decrease (assuming tetrahedral Fe3+ is relatively uncommon). It is also possible that some of this Fe, can occur in the interlayer position. Fig.3 shows the graphical relation of K20 and Fe203.

Fig.2. Histograms showing the distribution of the cations of twenty-nine illite structural formulas.

Figure 3.9. Schematic of the illite structure showing one gibbsite sheet between two silicate sheets. Potassium holds the layers together in a 12-fold coordination, preventing expansion. Isomorphous substitution can occur in the layers marked with an asterisk (from Taylor and Ashcroft, 1972, with permission).

K20(7.4), Na20(0.2), Mg0(2.1), and S(O.l). A water analysis, carried out by heating to 1300 K in air, indicated a water content of 7.3 percent. It should be noted that water is present in the illite structure as OH groups and can be expelled only by heating to relatively high temperatures. 

Smectite clay catalysts are potential alternative adsorbents, although some modifications of the natural mineral are necessary. Interlayer sites in smectite dehydrate at temperatures above 200°C, collapsing to an illitic structure. Since the ion-exchange capacity of smectite centres on the interlayer site, collapse must be prevented if clay catalysts are to be used in thermal treatments of chemical organic toxins. The intercalation of thermally stable cations, which act as molecular props or pillars, is one... 

Forty years have passed since the first petrogenetic application of the illite structural changes for characterizing diagenetic processes in sedimentary basins (Weaver 1960). Weaver s sharpness ratio as well as Kibler s (1964, 1968) empirical illite crystallinity index, have been easy-to-use X-ray powder diffraction (XRD) measures of the manifold, inter-related changes that the hydrous, mica-like phyllosilicates experience during increasing burial. 

Cs NMR results for Cs on the surfaces of illite, kaolinite, boehmite and silica gel (Figure 3) show that for this large, low charge cation the surface behavior is quite similar to the interlayer behavior. They also illustrate the capabilities of NMR methods to probe surface species and the effects of RH on the structural environments and dynamical behavior of the Cs. The samples were prepared by immersing 0.5 gm of powdered solid in 50 ml of O.IM CsCl solution at 2 5°C for 5 days. Final pHs were between 4.60 and 7.77, greater than the zero point of charge, except for boehmite, which has a ZPC... 

The problem with limited selectivity includes some of the minerals which are problems for XRD illite, muscovite, smectites and mixed-layer clays. Poor crystallinity creates problems with both XRD and FTIR. The IR spectrum of an amorphous material lacks sharp distinguishing features but retains spectral intensity in the regions typical of its composition. The X-ray diffraction pattern shows low intensity relative to well-defined crystalline structures. The major problem for IR is selectivity for XRD it is sensitivity. In an interlaboratory FTIR comparison (7), two laboratories gave similar results for kaolinite, calcite, and illite, but substantially different results for montmorillonite and quartz. 

Initially, there are several types of micas which have similar properties but which have different physical and chemical origins. Illite, the low potassium aluminous mica-like mineral ( 10 X, non-expandable structure upon glycollation) can form diagenetically (Velde and Hower,... 

If then illite, or a potassic, mica-like mineral, is present in most of the geologic environments, the variations of its structure and chemistry must be examined with care in order to establish its chemical stability relative to the system in which it is found. 

The main method used to distinguish the relative quantities of neoformed illite is by the polymorph or structure of the material. Using the criteria that 2M and 3T polymorphs of dioctahedral potassic mica are high temperature forms (Velde, 1965a), the determination of the relative quantities of lMd, and 1M vs. 2M, 3T polymorphs permits a semi-quantitative estimation of the proportion of neo-formed or low temperature illite present in a specimen. A method commonly used is a determination of relative intensities of X-ray diffraction peaks of non-oriented mica (Velde and Hower, 1963 Maxwell and Hower, 1967). Usually only 2M and lMd polymorphs are present in illite specimens which simplifies the problem. The 1M polymorph is typical of ferric illites and celadonite-glauconites, the more tetrasilicic types. 

It is interesting to note that the 1M polymorph represents an ordered form while lMd structures are disordered (Guven and Burnham, 1967) and that the typical sequence in the process of glauconitization is lMd to 1M (Burst, 1958). Illite remains, for the most part, disordered even in Paleozoic sedimentary rocks (Velde and Hower, 1963). This would suggest that the glauconite structure, being more symmetric, might be more stable than illite, a point which will be discussed when experimental studies are considered. 

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