Mixed-layer illite/smectite

Figures 10a and 10b show experimental images of a mixed-layer illite/smectite obtained with the c -axis perpendicular to the electron beam Figure 10a was obtained with the objective lens near Scherzer focus, and Figure 10b was obtained from the same area with...

Clay minerals consist primarily of a mixed layer illite/smectite. 

One might imagine a soil having this sequence of phases (gibbsite, kaolinite, K-mica, and K-feldspar) appearing in order from the surface down to bedrock. However, in a real soil illite or mixed-layer illite-smectite are usual soil phases rather than K-mica. 

Mixed-layered illite/smectites from cores in geologic areas where there has been substantial erosion of the overlying stratigraphic section give no evidence of the conversion of illite interlayers back to smectite. 

Illite is a minor diagenetic constituent occurring as fibres closely associated with chloritized pseudomatrix and infiltrated coatings. The presence of honeycombed mixed-layer illite/smectite (I/S) might indicate that illitization occurred via this intermediate stage. 

Mudrocks from both types of basin are grey and calcitic, dominated by clay minerals and calcite, and 5-15% quartz, feldspar (plagioclase K-feldspar) and mica. They are largely hemipelagic and pelagic beds, but include some Bouma E intervals. Clay minerals are dominated by illite and chlorite, but mixed-layer illite/smectite, serpentine, corrensite and possibly kaolinite are present in some samples. Pyrite is ubiquitous. 

Ramseyer, K. Boles, J.R. (1986) Mixed-layer illite/ smectite minerals in Tertiary sandstones and shales, San Joaquin basin, California. Clays Clay Miner., 34, 115-124. 

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. 

Abid, I.A. (1996) Mixed-layer illite/smectite diagenesis in the rift and post-rift sediments of the Jeanne d Arc Basin, offshore Newfoundland, Canada. PhD thesis. Memorial University, St John s, Newfoundland, 243 pp. 

Inoue (1983) Experiment 25-300X, Pure clays KOH Mixed layer illite/smectite Ion exchange... 

The mixed-layer structure of illite and smectite was obtained by alternating layers of illite and smectite. To allow comparison to other 1.0-nm structures, a layer spacing of exactly 1.0 nm was used for muscovite and biotite, and a layer spacing of exactly 2.0 nm was used for the 2-layer illite/smectite structure. 

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

In a survey of U.S. stream sediments, Kennedy (1965) concluded that the makeup and properties of the stream sediments essentially equaled that of local soils. In the eastern states (50 to 150 cm precipitation), dominant clays in the <4 jum (0.004 mm) fraction were illite, kaolinite, ver-miculite, and interlayered clays, with a CEC of 14 to 28 meq/lOOg. In central and west-central states (25 to 100 cm precipitation) Kennedy found dominant smectite, vermiculite, mixed-layer illite, kaolinite, quartz, and feldspar in the <4 /zm fraction, with a CEC range of 25 to 65 meq/100 g. In California and Oregon, because of the wide range of wet and dry conditions (<25 to >200 cm precipitation), clays were highly variable, and had a range of CEC s from 18 to 65 meq/l(X) g for the <4 /um fraction. 

Ransom B, Helgeson HC (1993) Compositional end members and thermodynamic components of illite and dioctahedral aluminous smectite solid solutions. Clays Clay Minerals 41 537-550 Reynolds RC, Hower J (1970) The nature of interlayering in mixed-layer illite-montmorillonites. Clays Clay Minerals 18 25-36... 

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. 

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. 

Mineralogy. The most important mineral constituents were determined by X-ray diffraction for five samples (four of them from NR-10). Quartz and opal (mainly CT) are important components in all five samples. In a marl (NR-10, 81.5 m), calcite, kaolinite, pyrite, gypsum and illite/smectite mixed-layer minerals occur in addition to those. 

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

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 ... 

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. 

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