Shale, illitic

Because they are the dominant mineral in shales, illites, and illite-smectites (see below) are the most abundant of all the clays. Illites are defined as micalike materials less than 2 yttm in size, which, like the micas, have a basal spacing of 10 A (Drever 1988). Most illites are dioctahedral and structurally similar to muscovite, although some are trioctahedral like biotite. Illites contain less and Al and more Si than muscovite. They also usually contain some Mg + and Fe, The irregularity of occurrence of interlayer K+ makes bonding between the layers weaker than in muscovite. Illitic clays... 

The diagenetic effects are related to the alteration of rock mineral, shales in particular. Under certain conditions, montmorillonite clays change to illites, chlorites and kaolinites. The water of hydration that desorbs in the form of free water occupies a larger volume. This volume increase will cause abnormal pressures if the water cannot escape. 

Illite Very small platelets, often found in shale and mud stone... 

Kaolin - Kaolinite 4, 5, 6, Dickite 16. 27 Mica - Biotite, Phologopite, Muscovite Illite - Illite 36, Illite-Bearing Shale Mixed-Layer Clays - Metabentonite 37, 42 Montmorillonite - 21. 22A, 22B, 24, 25, 26. 31 Feldspars - Albite, Anorthite, Orthoclase Chlorite - Chlorite... 

Frequency plots of the 001/002 ratio of 249 Paleozoic shale (Weaver,1965) and 149 soil illites (White, 1962) indicate that well-ordered 10A 2M illites have a K20 content on the order of 9—10%. Mehra and Jackson (1959) have presented data which suggest that the completely contracted illite layers in illites have 10% K20. It seems likely that well-organized 10A illite layers contain 9—10% K20 and values less than this indicate the presence of non-illite layers or interlayer cations other than potassium. 

Some of the lMd material (either illite or mixed-layer illite-montmorillonite) presumably formed authigenically on the sea bottom or on land from the weathering of K-feldspars however, much of it was formed after burial. Studies of Tertiary, Cretaceous, and Pennsylvanian thick shale sections (Weaver, 1961b) indicate that little lMd illite was formed at the time of deposition. These shales and many others contain an abundance of expanded 2 1 dioctahedral clays with a lMd structure, some of which is detrital and some of which formed by the alteration of volcanic material on the sea floor. With burial the percentage of contracted 10A layers systematically increases. 

A number of Al chlorites in which both octahedral sheets are dioctahedral have recently been described. Dioctahedral Al chlorites have been reported in bauxite deposits (Bardossy, 1959 Caillere, 1962). These chlorites appear to have been formed by the precipitation-fixation of Al hydroxide in the interlayer position of stripped illite or montmorillonite. A similar type of chlorite, along with dioctahedral chlorite-vermiculite, occurs in the arkosic sands and shales of the Pennsylvanian Minturn Formation of Colorado (Raup, 1966). Bailey and Tyler (1960) have described the occurrence of dioctahedral chlorite and mixed-layer chlorite-montmorillonite in the Lake Superior iron ores. Hydrothermal occurrences have been described by Sudo and Sato (1966). 

Much of the derived expanded clay, even that which resembles montmorillonite (holds two layers of ethylene glycol), will contract to 10 A when exposed to a potassium solution. Weaver (1958) has shown that these clays can obtain sufficient potassium from sea water and readily contract to 10 A. Vermiculite and mixed-layer biotite-vermiculites are rare in marine sedimentary rocks. Weaver (1958) was unable to find any expandable clays in marine sediments that would contract to 10 A when treated with potassium. A few continental shales contained expanded clays that would contract to 10A when saturated with potassium. Most vermiculites derived from micas and illites have high enough charge so that when deposited in sea water they extract potassium and eventually revert to micas and illites. Some layers may be weathered to such an extent that they do not have sufficient charge to afford contraction and mixed-layer illite-montmorillonites form. 

Clay mineral diagenesis also may play a role in dolomite formation during burial. The commonly observed conversion of smectites to illite can result in the release of the magnesium necessary for dolomite formation (e.g., McHargue and Price, 1982). Dolomite formation is observed near and within shale beds however, this process again appears to be a localized mechanism and probably is incapable of producing large quantities of dolomite. 

This study has shown that the sample of Devonian shale is composed primarily of silicates with much lower amounts of carbonate minerals. Here, as with the Green River shale sample, the silicate minerals were associated primarily with the organic-rich areas of the shale, and when present, the carbonate minerals were found mainly in the organic-poor areas. The siliceous minerals of this Devonian shale sample were found to be quartz, illite, and muscovite, with trace amounts of kaolinite. Calcite, dolomite, Fe-rich dolomite or magnesite, and siderite were observed in the carbonate regions of the Devonian shale. However, the... 

It is generally agreed on the basis of TEM evidence (e.g., Rask et ai, 1997) and on the basis of chemical substitution in the several structural sites of illite (e.g., Lanson and Champion, 1991 Awwiller, 1993 Lynch et ai, 1997) that the illitization of smectite, like other replacement reactions in late diagenesis, proceeds through a dissolution/precipitation mechanism. Similar to other replacement minerals, illite also occurs as cements in sandstones as well as in shales, where it forms overgrowths on detrital illite particles and discrete crystals (cements) (e.g., Lanson and Champion, 1991 Rask et at, 1997). 

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. 

Diffusional transfers of potassium and silicon between sandstones and shales may be sufficient to accomplish feldspar dissolution, illitization, and quartz cementation (Thyne, 2001 Thyne et al, 2001). Losses of the magnitude observed for detrital carbonates in shales exceed the capacity of diffusion-mediated transfer. Large-scale advection seems required, although our understanding of shale permeabilities seems to preclude this (Bjprlykke, 1989, 1993 and Lynch, 1997). The possibility of convection driven by salinity heterogeneity within thick shale sequences has been demonstrated by Sharp et al (2001), who note that more information for rock properties and fluid compositions within deep basinal shales is needed before the generality of their results can be assessed. 

Berger G., Velde B., and Aigouy T. (1999) Potassium sources and illitization in Texas Gulf Coast shale diagenesis. J. Sedim. Res. 69, 151-157. 

Lindgreen H. (1994) Ammonium fixation during illite-smectite diagenesis in upper Jurassic shale. North Sea. Clay Min. 29, 527-537. 

Lynch F. L. (1997) Frio shale mineralogy and the stoichiometry of the smectite-to-illite reaction— the most important reaction in clastic sedimentary diagenesis. Clays Clay Mire 45, 618-631. 

Lynch F. L., Mack L. E., and Land L. S. (1997) Burial diagenesis of illite/smectite in shales and the origins of authigenic quartz and secondary porosity in sandstones. Geochim. Cosmochim. Acta 66, 439—446. 

Pearson M. J. and Small J. S. (1988) Illite-smectite diagenesis and paleotemperature in northern North Sea Quaternary to Mesozoic shale sequence. Clay Min. 23, 109-132. 

Fine clastic sediments, mostly mudrocks, in contrast to their coarser counterparts, are either derived by first cycle weathering of silicate minerals or glass, or from recycling of older mudrocks. Physical comminution plays only a secondary role. The average shale is composed of 40-60% clay minerals, 20-30% quartz, 5-10% feldspar and minor iron oxide, carbonate, organic matter, and other components (Yaalon, 1962 Shaw and Weaver, 1965). Granitic source rocks produce shales richer in kaolinite and illite, the... 

Mineral composition, as determined by X-ray diffraction, shows a dominance of clay minerals, although quartz and opaline silica are persistent as sub-dominant and locally dominant or co-dominant (Table II). Of the clays, expandable lattice clay minerals, predominantly montmorillonite, occur in all the deposits with kaolinite or illite appearing as accessory or subdominant components. A marked contrast in the dominant clay species occurs between the brown oil shale unit and the two units below it at Condor. In these lower units, kaolinite is in greater abundance than other clays as well as quartz, an aspect already alluded to in the variations in Table I. (Loughnan (8) also noted that the structure of the kaolinite changes from ordered in the lower units to disordered in the brown oil shale unit). 

The Cleveland Member consists primarily of black to brownish-black laminated siliceous shale. It contains minor amounts of calcareous laminae and cone-in-cone limestone. Pyrite is present as nodules, framboids, and irregular forms. Other primary minerals include illite clay and clay- and silt-size quartz. 

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