Illite kaolinite, 186 reaction

Ca (aq), Mg (aq), and HCOjCaq). Silicate weathering is an incongruent process. The most important of these reactions involves the weathering of the feldspar minerals, ortho-clase, albite, and anorthite. The dissolved products are K (aq), Na (aq), and Ca (aq), and the solid products are the clay minerals, illite, kaolinite, and montmorillonite. The weathering of kaolinite to gibbsite and the partial dissolution of quartz and chert also produces some DSi,... 

Fig. 8. Compensation plot for reactions of n-dodecanol and of stearic acid on illite, kaolinite, and montmorillonite ( ). Points have also been included for the reaction of the alcohol in the absence of volatile material ( x) and the reactions of cyclohexanol in presence (O) and absence ( ) of volatile products (31, 291, 292) on montmorillonite. Reproduced with permission from J. Chim. Phys. (291).

Table 2.5. Rate constants for the kaolinite to illite conversion reaction...

Figure 2.3. Arrhenius plot used to extrapolate rate constants for the kaolinite to illite transformation reaction from high temperature measurements to find the rate constant at 100°C.

In a final application of kinetic reaction modeling, we consider how sodium feldspar (albite, NaAlSisOs) might dissolve into a subsurface fluid at 70 °C. We consider a Na-Ca-Cl fluid initially in equilibrium with kaolinite [Al2Si20s (OF )/ ], quartz, muscovite [KAl3Si30io(OH)2, a proxy for illite], and calcite (CaC03), and in contact with a small amount of albite. Feldspar cannot be in equilibrium with quartz and kaolinite, since the minerals will react to form a mica or a mica-like... 

There are no unequivocal weathering reactions for the silicate minerals. Depending on the nature of parent rocks and hydraulic regimes, various secondary minerals like gibbsite, kaolinite, smectites, and illites are formed as reaction products. Some important dissolution processes of silicates are given, for example, by the following reactions ... 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

Pyrophyllite is probably not stable below some 300°C at 1 Kb pressure. This temperature will be reduced at lower total water pressure but probably will remain at a substantial value (Velde and Kornprobst, 1969). Its existence in sedimentary rocks should be indicative of relatively high temperatures if it is stable. It is typically found with illite-chlorite or occasionally with allevardite (Dunoyer de Segonzac, 1969 Ehlmann and Sand, 1959). The reaction Kaolinite + quartz = pyrophyllite is an important marker in phyllosilicates parageneses when it can be observed. 

Interpretation of the mechanisms of the hydrocarbon desorption reactions mentioned above was considered (31,291) with due regard for the possible role of clay dehydration. While this water evolution process is not regarded as a heterogeneous catalytic reaction, it is at least possible that water loss occurs at an interface (293) so that estimations of preexponential factors per unit area can be made. On this assumption, Arrhenius parameters (in the units used throughout the present review) were calculated from the available observations in the literature and it was found (Fig. 9, Table V, S) that compensation trends were present in the kinetic data for the dehydration reactions of illite (+) (294), kaolinite ( ) (293,295 298), montmorillonite (x) (294) and muscovite (O) (299). If these surface reactions are at least partially reversible,... 

Hig. 9. Compensation behavior for the dehydration reactions of kaolinite ( ), illite ( + ), mommorillonite ( x), and muscovite (o), estimated from published kinetic data (293-299). 

Oligocene Gulf Coast shales at 2 km depth at 4.5 km depth the kaolinite in these rocks is reduced to —10% whereas the chlorite has increased to 4%. This suggests that the same illitization reaction that affects kaolinite in sandstones may be active in shales as well. Alternatively, the kaolinite may be converted into chlorite. 

Ehrenberg and Nadeau (1989) concluded that extensive illitization occurs in the Gam Formation mainly through reaction between potassium feldspar and early diagenetically formed kaolinite ... 

As hydrolysis of the silicates and aluminosilicates continued (equation 1), dissolved sodium ions were continually produced, and acid was consumed. The pH of the water gradually increased. Formation of kaolinite was replaced by formation of montmoril-lonite and finally by production of illite. Silica was more soluble than alumina and as aluminosilicates were attacked by the water a protective coating of A1(0H) formed around the particles. This coating helped transport mineral particulates to the sediment. The reactions indicated by equation 1 were replaced by the family of reactions indicated in equation 2, written using the hydrolysis of albite as an example. 

As a second example of mineral-controlled buffer capacity, consider the reaction in pure water between the clays kaolinite and illite (here assumed the same as muscovite), which may be written... 

This equation is plotted in Fig. 5.11 which shows that the reaction at equilibrium has about 10 times more buffer capacity than calcite. Because this reaction will not often be at equilibrium, however, the buffer capacity is a maximum possible value. Further, the reaction is usually irreversible with kaolinite more often stable than illite in weathering environments. For this reason the reaction resists a pH decrease, but not an increase. 

Minerals known to be present in the low temperature ash extracted from coals were heated in a microscope heating stage from 25 to about 1400 C. Mineral types were arranged in homogeneous fields where two fields shared a linear boundary or three fields were in contact at a point. Because of the known reactivity of calcite and pyrite, all specimens contained this pair. Clay minerals, kaolinite, illite and montmorillonite were used as the third component. Reaction temperature between calcite and pyrite is lowered by the presence of clays. Iron was observed to migrate into the clay domain after the formation of pyrrhotite from pyrite and oldhamite was observed forming between the domain of lime formed from calcite and the pyrrhotite. 

Because the most reactive phases found in the experiments with pairs of minerals were clays, calcite, and pyrite, these were prepared in triplet mounts. In trials using either montmorillonite or illite with calcite and pyrite, a liquid formed at the mutual boundary of the latter pair at 600-650°C. Pyrite and calcite had, of course, previously reacted and this liquid therefore occurred between the product phases pyrrhotite and lime. Subsequent x-ray analysis showed the presence of pyrrhotite, lime, and oldhamite. In both instances, the temperature of this reaction was lower than that obtained in the pair mount of calcite and pyrite, 1140 C. When kaolinite was in the mount with calcite and pyrite, the same reaction occurred at 750-760 C. Though the mechanism by which the clays reduce the reaction temperature is not yet understood, the differences in reaction temperature with and without clay is considered significant. 

Illite, a mineral that forms in alkaline solutions (15), is more stable than kaolinite in 30% NaOH. The experiments (Table II) in which illite was reacted either at room temperature or at 105 C with 30% NaOH showed that no discernible reaction took place. In the microwave irradiated experiments, however, considerable changes occurred. Illite strongly absorbed the microwave energy, causing considerable heating of the sample. When illite was irradiated for... 

The reactions between clay minerals, NaOH solutions, and micro-wave irradiation showed that the clay mineral structures began to break down, and the released Al and Si (and some K) combined with Na from the solution to form new minerals. The exact reaction path could not be determined from the present experiments. Since the new minerals formed had approximately the same Al Si ratio as the clay minerals, no excess Al (as AI2OJ or Al(OH)g) or quartz was expected, nor was any found in the XRD pattern. The following equations, arranged with increasing time, seem most reasonable to describe the reactions observed for kaolinite and illite ... 

The molal K/Mg ratio in pore waters where the transition is taking place would be 0.51. The second reaction consists of the weathering of illite to kaolinite ... 

---