Illite composition

R coordinates (Figure 10). The main features of their formulas are traditionally compared to muscovite, the mineral closest to illite compositions. Where muscovite has the ideal formula KAl Si A10 (0H) , illite... 

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

Figure XI. Representation of the phase relations near the illite compositions in the MR - 2R - 3R coordinates. M = muscovite y = illite solid...

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... 

Figure 32. Results of experiments on natrual minerals are schematically shown in Mr3-2R -3R coordinates. Kaol = kaolinite ML j = mixed layered beidellitlc mineral MLj, = mixed layered montmorillonitic mineral I = illite compositional field chi = chlorite Exp3 trioctahedral expandable-chlorite mixed layered mineral (expanding chlorite and corrensite).

The compositions of the Sumikawa discharge fluids are controlled by K-feldspar, illite, calcite and Ca-aluminosilicates (epidote, prehnite, wairakite). 

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. 

Formation has compositionally distinct upper and lower members. Both can be explained by illite-smectite mixtures, but Lower Cunard slates also contain quartz. Like the Lower Cunard rocks, the Feltzen Formation has compositions explained by mixtures of illite, smectite and quartz. 

In the Goldenville Group, the Moshers Island Formation also has two compositional members an upper one explained by mixtures of chlorite, kaolinite illite, and smectite, and a lower one explained by only illite and smectite. 

In contrast, metawackes in the Government Point, Green Harbour, and Moses Lake formations (Goldenville Group) have compositions explained by mixtures of illite, albite, and quartz, but each of the these formations differ slightly in terms of the amounts of quartz and feldspar they contain, relative to illite. In addition, the Moses Lake Formation contains significant amounts of chlorite. 

Alteration assemblages may include primary chlorite, illite, smectites, and/or kaolinite, and various primary and secondary iron oxides, carbonates, and sulfides (Fig.1), any one of which may serve as indicators of fluid composition. Lithologic geochemical surveys rely on an understanding of these patterns to vector towards uranium deposits. The interpretation of hydromorphic geochemical surveys, including lake and stream sediment, and soil, depends on the mobility of uranium and associated elements in the surface and near surface environment. 

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. 

Figure 2. Illustration of the possible displacement of a sedimentary bulk composition upon reduction of Fe + to Fe + during diagenesis (after Velde, 1968). I = illite Chi = chlorite Kaol = Kaolinite M03 = expanding trioctahedral phases 1 = initial bulk composition 2 = reduced bulk composition. Upper diagram shows initial assemblage and the lower diagram those after reduction of Fe +.

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.

Studies of hydrothermal alteration products associated with ore mineralization in acidic rocks have established the general propensity for the original minerals to be replaced by illite, sericite or hydromica in the innermost zone near the source of hydrothermal fluids and by kaolinite or expandable minerals further from the vein or center of fluid emanation. The newly-formed "mica" can be 2M, 1M, or lMd in polymorph and range compositionally from muscovite to a low potassium, silicic species which can be assimilated in the term illite (Lowell and Guilbert, 1970 Schoen and White, 1966, 1965 Kelly and Kerr, 1957 Bonorino, 1959 Tomita, e al., 1969 Yoder and Eugster, 1955 Meyer and Hemley, 1959, among many authors). 

Figure 9. Phases found between the compositions muscovite (Mu)— pyrophyllite (Py) at 2Kb pressure (after Velde, 1969). M = mica (tending to an illite-like phase) ML = random mixed layered phase All = allevardite-like phase Mo = fully expandable phase Kaol = kaolinite ...

Figure 11 indicates the necessary change in composition which a muscovite would need to become stable under conditions in a sedimentary rock where chlorite is present (x to y). The solid solution for mica-illites is delimited by the shaded area which represents a much larger variation than is possible under metamorphic or igneous conditions. The detrital muscovite (composition x) is in itself stable if the bulk composition of the sediment as projected into the coordinates is found at x. 

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. 

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