Natural Illite Compositions

The AG between the assemblage of muscovite + chlorite at composition y and illite of this is likely to be relatively small and the tendency to recrystallize the muscovite from x to y compositions will be small at sedimentary conditions. However, as more thermal energy is added to the rock system, under conditions of deeper burial, the recrystallization will proceed more rapidly as temperature is increased. Evidence for such an effect can be found in Millot (1964) where sedimentary rocks coming from deeply buried or slightly metamorphosed series show the chloritization or kaolinitization of detrital mica grains in splendid photographs. 

These grains have an exterior of mica (illite) which is invaded by a new material at the ends and in the center due to the response of mica to the new chemical and physical conditions of sedimentation and burial. 

The effect is most marked in more mature sedimentary rocks where temperatures have been relatively high. The phenomenon represents not only the chloritization or kaolinitization of muscovite but also its illitization. 

In hydrothermal alteration the transformation of muscovite to an aluminous illite has been noted (Kelley and Kerr, 1957). 

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Figure 10. Natural illite compositions plotted in 3R - 2R - MR coordinates.

Figure 5. Typical X-ray diffraction traces of the (001) reflection for mica and mica-illite minerals. Assymmetry is shown towards large values. A - natural 2M muscovite B = natural illite (IMd) C - synthetic illite (lMd), 75% mica, 25% prophyllite composition D = synthetic 1M muscovite E - natural 1M glauconite F = synthetic 1M celadonite mica.

The bulk compositions of natural illite, celadonite and glauconite ... 

Figure 25. Chemical compositions of natural mixed layered expandable phases (natural) in the MR- -2R -3r2 coordinates. ML = mixed layered beidellite series MLm = mixed layered montmorillonite series I = illite compositional field.

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... 

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... 

Illite layers form relatively quickly by WD (most in less than 20 WD cycles), and the reaction rate is not affected greatly by changes in solution compositions or temperatures that are typical of near-surface environments. Thus, that which has been studied in the laboratory also may occur abundantly in nature. 

The author has performed a number of experiments on natural materials which have various types of interlayering and various non-expandable phases present such as illite, kaolinite and quartz (Velde, in press). The starting materials were chosen in order to determine the effect of composition and physical conditions upon expandable minerals. This expands the information from the synthetic system described above by adding the chemical variables Mg and Fe. 

The major aluminous clay minerals, alkali zeolites and feldspars which are most commonly associated in nature can be considered as the phases present in a simplified chemical system. Zeolites can be chemiographically aligned between natrolite (Na) and phillipsite (K) at the silica-poor, and mordenite-clinoptilolite at the silica-rich end of the compositional series. Potassium mica (illite), montmorillonite, kaolinite, gibbsite and opal or amorphous silica are the other phases which can be expected in... 

The differences in cation compositions are probably due to the fact that phases containing these ions (illite, smectite, etc.) have sufficient time to form in natural systems but did not form in the experimental system. The high HCO3 content of the experimental system is due to contact with an infinite reservoir of CO2 having a partial pressure of 0.1 atmosphere. 

This suggests that 2M illites should have a muscovite composition, which does not seem to be the case in nature. 

Beckett described inductively coupled plasma mass spectrometry (ICP-MS) as an off-line detector for FFF which could be applied to collected fractions [ 149]. This detector is so sensitive that even trace elements can be detected making it very useful for the analysis of environmental samples where the particle size distribution can be determined together with the amount of different ele-ments/pollutants, etc. in the various fractions. In case of copolymers, ICP-MS detection coupled to Th-FFF was suggested to yield the ratio of the different monomers as a function of the molar mass. In several works, the ICP-MS detector was coupled on-line to FFF [150,151]. This on-line coupling proved very useful for detecting changes in the chemical composition of mixtures, in the described case of the clay minerals kaolinite and illite as natural suspended colloidal matter. 

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 features of the adsorbed complex of the Cambrian argillites were intensely transformed by diagenetic reactions, making them unsuitable for any interpretations. The complex composition of the clays which consist of kaolinite, illite, chlorite and mixed-layer minerals, together with the absence of organic matter leads to the conclusion that the areas of denudation must have been of a local nature. At the same time, we may reconstruct a temperate to humid climate interrupted at times by periods of relative aridity. This explains the lack of laterites on the eruptive and metamorphic rocks. 

In the central part of the Illizi Basin and over the Ghadames Depression (WT-i, HD-i, RYB-i, AKF-i) we observe sediments characteristic of deeper marine environments, i.e. fine-grained sandstones with intercalations of clays and silts. The characteristic feature of these Siegenian sandstones is the chloritic composition of the clay fraction of their cements and the chloritic-illitic nature of the argillaceous intercalations. Because of this situation, these deposits could have been derived from a hard substrate (effusive or metamorphic rocks) inundated by the Siegenian sea and open into the direction of the present Libyan coast. 

Judging from the mineral composition of the shales in which such secondary transformations would be reduced to a minimum there could have been only 15-30% detrital illite as a primary constituent in the sandstones of the Hassi Messaoud field whereas the amount of illite detected in the Cambrian siltstones is 30-50%. Consequently, only 1% of secondary quartz could have resulted from the transformation of illite to kaolinite. Thus, widely developed kaolinization of illite and feldspars is accompanied by the liberation of silica in the form of silicic acid which is mostly consumed in the formation of overgrowth on quartz grains. The extent of this silicification, however, is controlled by other factors the chemical nature of the environment created and especially the permeability of the rocks. The lower the permeability, the more intense silicification will be. 

The analyses of natural mordenites, revealed two distinct coirqiositional types [98K1] (1) typical mordenites, where Na and Ca were the main cations and (2) a K-rich mordenite. These two types probably reflect the nature and the chemical composition of the materials, which acted as precursors for the mordenite formatiou The Na-Ca mordenite has as precursors mainly any (Na,Ca)-iich heulandite phases initially present, and secondly some illite/smectite and a gel-like material. The K-rich mordenite has as precursor clinoptilolite. 

Red clays used for the manufacture of terra cotta products are actually natural mixtures with a complex composition. They generally contain kaolinite, illite and/or other clays rich in alkaline, sand, mica (formula Si3Al3Oi0(OH)2), goethite (FeO(OH)) and/or hematite (Fe203), organic matter and, very often, calcium compounds. The latter, just like the micas and the other alkaline-rich compounds, help lower the firing temperature of the shard. 

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