Kaolinite is the dominant clay mineral which occurs both in allogenic and authigenic forms.
The X-ray powder diffractogram profiles with 7.098 Å, 7.076 Å, 3.539 Å reflections (fig. 5) are characterized by kaolinite that does not show any transformation upon glycol saturation.
However samples become amorphous upon heating at 500°C which confirms the presence of kaolinite (Carroll, 1979). Scanning electron microscope (SEM) images of kaolinite show different morphologies. Allogenic kaolinite associated with channel and bar facies enclosing silt-sized quartz grains and thus forms component of the matrix, whereas in floodplain facies associations it occur as a mixture with illite and smectite and authigenic kaolinite occur as books in channel and bar facies associations (fig. 6). The poorly crystalline kaolinite with minor amounts of illite, quartz, and organic matter occur at the top of the depositional cycles. The sharp X-ray diffraction pattern and resistance to heat treatment are the diagnostic features of the crystalline nature of kaolinite.
Kaolinite forms under humid climatic conditions in continental sediments by the action of low-pH ground waters on detrital aluminosilicate minerals such as feldspars, mica, rock fragments, mud intraclasts and heavy minerals (Emery et al., 1990). The annual precipitation, hydraulic conductivity, rate of fluid flow and the amount of unstable detrital silicates in the sand body are the key factors that control the amount and distribution pattern of kaolinite. In permeable channel sand deposits eogenetic grain dissolution is most common. Calcic plagioclase and albite tend to be more susceptible to kaolinitization than K-feldspar (Worden and Morad, 2003).
In humid conditions greater amounts of meteoric waters are available that promote eogenetic kaolinite. Warm, wet, typically verdant, eogenetic environments also have an abundance of organic matter that undergoes bacterially mediated decay (Berner, 1980). Fe-bearing minerals in the sediment are readily reduced to aqueous Fe2+ by redox processes, which typically is available for siderite growth because very low concentrations of SO42- leads to the absence of Fe-sulphides. The clay minerals in this environment are typified by kaolinite because its formation requires low ionic concentrations in pore waters (Worden and Morad, 2003).4.2.
2. SmectiteMontmorillonite as major constituent and saponite as subordinate are two main smectite group minerals identified on X-ray powder diffractograms of by 14.573 Å reflections which show transformation to 17.541 Å on glycol saturation and collapse to 10 Å on heating at 450°C (fig.
5). The smectite crystallites in smectite aggregates show a wide range of variations in the morphological features. Distinctions among smectite aggregates can be made on the basis of the habit of individual crystallites and on their arrangements in the aggregate.
In this regard, a distinction should be made between internal (structural) and external factors. The internal factors are related to the crystal structure and to its imperfections such as dislocations. The major external factors are temperature, pressure (hydrostatic), degree of disequilibrium (supersaturation, concentration, and diffusion gradients), viscosity of the medium, and the presence of impurities. During the crystallization of layer silicates the internal factors seem to be strongly effective, leading to the prominence of lamellar forms (Güven and Pease 1975).Scanning electron microscope images reflect different morphological features, modes of aggregations, and crystal structures of smectite minerals which are significantly related to their rheological behavior and other physical properties.
Different habits of smectite crystallites are depicted in fig. 6 (c, d & e) with their predominant forms as: a) Lath-shaped particles: these are genuine laths or laths formed by folding of thin lamellae, b) Cellular dendrites: these dendrites resemble a honeycomb in which the branches have strictly defined orientations, c) Spherulites: these consist of a bundle of radiating needles or lamellae according to the classification of dendrites Saratovkin (1959).In some cases the morphological features of the aggregate may be inherited from the parent volcanic glass, with the devitrification process altering the mineralogy but not the texture. The smectite minerals are seem to be developed under the effect of seasonal climate changes or transported with the sediments to the basin from the stream catchment areas. It is also evident from the absence of any glass shreds in the surface and sub-surface samples of sandstone and mudstone facies studied presently from different horizons of Dhok Pathan Formation. 4.2.
3. IlliteThe X-ray powder diffractogram profiles with 10.048 Å, 10.018 Å, 4.996 Å, and 3.567 Å reflections (fig.
5) are characterized by illite which do not show any change upon glycol saturation. However, samples profiles slightly collapsed upon heating at 400°C. Illite is present as detrital grain as an authigenic phase and as alteration product of kaolinite, micas, and feldspars. Scanning electron microscope images show irregular flakes with lath-like morphologies (fig.
6, h). The lath morphology of illite depends on its development mechanisms, majorly depending on whether they are attached to the surface of sand grains or develop as sheets that bend from the point of attachment. Illite is stable in the presence of kaolinite at increasing temperatures and under alkaline conditions. The degradation of smectite at temperatures around 100°C may form Illite. During diagenesis, potassium feldspar was likely the main source of the potassium utilized in the illitization (Bertier et al.
, 2008). Illite can be formed by the conversion of microcline at 25°C and 1 bar pressure in H2CO3 .In shale-rich sequences where smectite is converted to I/S (Illite/Smectite) with the proportion of illite layers increasing with depth and temperature a similar reaction commonly occurs in the interbedded sandstones (Weaver 1989). 4.
2.4. VermiculiteThe X-ray powder diffractogram profiles with 14.24 Å reflections that collapse to 10 Å on heating at 400°C but do not show any change upon glycol saturation (fig. 5) are characterized by vermiculite.
SEM images show lath like morphology (fig. 6, g). Clay-sized vermiculite and vermiculite layers interstratified with mica or chlorite layers are quite common in soils where weathering is not overly aggressive (Barshad and Kishk 1969).
The removal of potassium from biotite, muscovite, and illite or the brucite sheet from chlorite generally gives rise to the formation of these clays which is accompanied by the oxidation of iron