Illite/smectite mixed-layer clays

The similar structure of illite and smectite allows mixing or interstratification of 2 1 units to form mixed-layer clays. Most illites and smectites are interstrati-fied to a small degree, but they are not classified as such until detectable by X-ray diffraction. As one might expect, illite-smectite mixed-layer clays have intermediate cation exchange capacity between the end-member compositions. 

The suite of minerals occurring as impurities in U.S. coals of commercial quality is moderately consistent this suite includes quartz, calcite, pyrite, various clay minerals including kaolinite, illite, and varieties of illite-smectite mixed-layer clay minerals. The weathering of pyrite produces some sulfate minerals in many coals. Several other minerals are present in most coals in trace amounts. 

Clays are volumetric ally the most abundant mineral group in coal. They can be authigenic or detrital in origin. Kaolinite is the most common clay and the most common authigenic mineral in coals. The silicon and aluminum in kaolinite are, perhaps, residual from the dissolution of ferromagnesian minerals and feldspars. Illite and mixed layer clays in coal are almost exclusively detrital in origin. Chlorites, smectites, and other clay minerals may be abundant locally. 

Clay minerals Kaolinite, illite-sericite, mixed-layer clays, smectite nUte-sericite, chlorite... 

The mineralogical composition of Callovo-Oxfordian argillites and of its clay minerals has been analysed. The high content of the illite/smectite mixed layer confirms the strong coupling potential between the water content and the mechanical behaviour of the formation under investigation. 

Main gangue minerals of the Se-type deposits comprise quartz, adularia, illite/ smectite interstratified mixed layer clay mineral, chlorite/smectite interstratified mixed layer clay mineral, smectite, calcite, Mn-carbonates, manganoan caleite, rhodoehrosite, Mn-silicates (inesite, johannsenite) and Ca-silicates (xonotlite, truscottite). 

In the Se-type gangue minerals comprise quartz, adularia, illite/smectite inter-stratified mixed layer clay mineral, smectite, calcite, Mn carbonates (manganoan calcite, rhodochrosite), Mn silicates (inesite, johansenite) and Ca silicates (xonotlite, truscottite). In comparison, the Te-type contains fine-grained, chalcedonic quartz, sericite, barite, adularia and chlorite/smectite interstratified mixed layer clay mineral. Carbonates and Mn minerals are very poor in the Te-type and they do not coexist with Te minerals. Carbonates are abundant and barite is absent in the Se-type. Grain size of quartz in the Te-type is very fine, while large quartz crystals are common in the Se-type. 

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. 

Mixed-layer clays, particularly illile-smeclite. are very common minerals and illustrate the transitional nature of the 2 1 layered silicates. The transition from smectite to illite occurs when smectite, in the presence of potassium front another mineral such as potassium feldspar, or from thermal fluids, is heated and/or buried. With increasing temperature smectite plus potassium is convened to illite. 

Clay minerals Illite, kaolinite, halloysite, smectites, chlorites, Vermiculites, palygorskite, mixed-layer clays, etc. 

Figure 6 Representation of chemical compositions of potassic, low-temperature micas in space. The poles represent feldspar, dioctahedral clays, and trioctahedral clays, respectively. M = Na, Ca, and especially K ions, R = Al, Fe R = Fe Mg. The compositional positions of the minerals Mu (muscovite) kaol (kaolinite), smectite, and mixed layer mica/smec-tites are indicated. Initial materials are kaolinite (kaol) and iron oxides. A second step is the production of an iron-aluminous smectite and then the formation of either illite via an iUite/smectite mixed layer mineral or glauconite via a glauconite mica/iron-smectite mixed layer phase.

The three-layer phyllosilicates include talc and pyrophyllite, illite and the smectite group clays, various mixed-layer clays, vermiculite, and the micas (e.g., muscovite, phlogopite, and biotite). We will limit ourselves to a discussion of the more environmentally important of these minerals, which include the micas, the smectites and illites, interlayered (mixed-layer) smectite-illites and vermiculite. 

Authigenic clay minerals identified by SEM/EDS include minor amounts of kaolinite, chlorite, illite and mixed-layer illite/smectite, which form grain coatings, platelets or fibrous cements. 

Rectorite is one of an almost infinite number of randomly mixed layer clays (411. collectively called illites, which include several other ordered interstratified varieties, including chlorite, corrensite and allevardite. Rectorite has the advantages of a mica and a smectite, in that alternate interlayers are expanding and non-expanding. It may be viewed as an ordered synthetic mica-montmorillonite (SMM) in the nomenclature familiar to catalytic... 

Tailings Mineralogy. Tailings minerals consist of sand, clays, amorphous oxides, and trace metals. The sand is 97.5-99% Si02, 0.5-0.9% Al203, and 0.1-0.9% Fe (4, 27, 28). The oil sands, and hence the clay minerals found in the fine tails suspension, come from the McMurray Formation. The majority of clays in this formation are kaolinite and illite with traces of smectites, chlorite, vermiculite, and mixed-layer clays (5, 29). The upper McMurray Formation has a larger amount of smectites, whereas the lower McMurray Formation has larger amounts of vermiculite and mixed-layer clays. However, in both areas, kaolinite and illite are still the predominant clay minerals (5). 

The clay mineral spectrum is made up essentially of illite, kaolinite, chlorite and mixed-layer swelling clays of the illite-smectite type. The qualitative and quantitative composition of the clay minerals depends on their depositional environment. In littoral zones where the influence of the continents is pronounced, the kaolinite content may reach 30% whereas in the deeper parts of the sea it will be only 10%. For the mixed-layer clays the opposite trend may be observed. The widespread presence of these minerals, the genetic conditions of which are similar to those of smectite, suggests an arid climate over the East Saharan synclinorium during the Visd. This also explains the formation of evaporites in isolated highly saline basins in the western Saharan region. 

FIGURE 5 (a) TEM of iUite crystal hexagons and illite/smectite mixed clay laths (b) HRTEM of illite crystaUites prepared perpendicular to the beam, showing 10-A bands of stacked Uhte layers. (Reproduced with permission of Kluwer Academic Publishers from Ref. 8.)... 

So-called mixed-layer chlorite-vermiculites are common in marine sedimentary rocks, but it appears that in most, if not all, instances the vermiculite layers will not contract when saturated with potassium and the expanded layers are probably some form of smectite. These clays probably formed from volcanic material, montmorillonite or chlorite, rather than from the degradation of micas and illites. 

IHite/Smectite. Another common intergrowth of sheet silicates is the mixed-layering of illite and smectite. As discussed above, illite and smectite are clay minerals whose basic structures resemble the mica muscovite. Their compositions may differ significantly from muscovite, but they generally have a lower occupancy of the interlayer sites than mica. Numerous other compositional differences are possible for smectite however, this discussion will be restricted to a dioctahedral illite and a dioctahedral smectite containing potassium and vacancies in the interlayer sites as given above. 

The transformation of smectite to mixed layer smectite-illite, and ultimately to illite, with increasing temperature is an extremely important reaction in many sedimentary basins, including the northern Gulf of Mexico Basin (Hower et al., 1976 Boles and Franks, 1979 Kharaka and Thordsen, 1992). The water and solutes released and consumed by this transformation are major factors in the hydrogeochemistry of these basins, because of the enormous quantities of clays involved. Several reactions conserving aluminum or maintaining a constant volume have been proposed for this transformation (Hower et al., 1976 Boles and Franks, 1979). The reaction proposed below (Equation (4)) conserves aluminum and magnesium, and is probably a closer approximation based on the composition of formation waters in these systems ... 

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