Clay minerals mixed-layer clays

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. 

Mixed-layer clays, particularly lUite—smectite, are very common minerals and illustrate the transitional nature of the 2 1 layered siHcates. The transition from smectite to iUite occurs when smectite, in the presence of potassium from another mineral such as potassium feldspar, or from thermal fluids, is heated and/or buried. With increasing temperature smectite plus potassium is converted to iUite (37,39). 

An alternative description of iUite—smectite mixed-layer clays begins with megacrystals of smectite that incorporate smaller packets of iUite (163). These constituents are observed as mixed-layer minerals in x-ray analysis. Diagenesis increases the percentage of iUite layer and with increasing alteration the mixed-layer mineral takes on the characteristics of an iUite dominated iUite—smectite. 

Chlorite is another mineral that is commonly associated with mixed-layered clays. Complete soHd solutions of chlorite mixed-layer minerals have not been identified. In contrast to iUite—smectite mixed-layer minerals, chlorite mixed-layer minerals occur either as nearly equal proportions of end-member minerals (Rl) or dominated by one end member (RO) (142). Mixed-layer chlorite may consist of any of the di—tri combinations of chlorite and chlorite mixed-layering occurs with serpentine, kaolinite, talc, vermicuhte, smectite, and mica. References of specific chlorite mixed-layer minerals of varied chemical compositions are available (142,156). 

Mixed layer clay mineral (sericite/smectite) is found in Kuroko ore bodies and altered dacitic rocks underlying the ore. This mineral is thought to have formed by the... 

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 eomparison, the Te-type deposits contain fine-grained quartz, chalcedonic quartz, sericite, barite, adularia, ehlorite/smectite interstratified mixed layer clay mineral and rarely anatase. Carbonates and Mn-minerals are very poor in the Te-type deposits and they do not coexist with Te-minerals. Carbonates are abundant and barite is absent in the Se-type deposits. The grain size of quartz in the Te-type deposits is very fine, while large quartz crystals are common in the Se-type deposits although they formed in a late stage and do not coexist with Au-Ag minerals. 

Principal gangue minerals in base-metal vein-type deposits are quartz, chlorite, Mn-carbonates, calcite, siderite and sericite (Shikazono, 1985b). Barite is sometimes found. K-feldspar, Mn-silicates, interstratified mixed layer clay minerals (chlorite/smectite, sericite/smectite) are absent. Vuggy, comb, cockade, banding and brecciated textures are commonly observed in these veins. 

Zeolite minerals (wairakite, laumontite etc.), mixed-layer clay minerals and sme-cite occur in the upper part of the propylitically altered rocks (e.g., Seigoshi, Fuke, Kushikino), but they are sometimes poor in amounts. Generally carbonates are more abundant in the mine area as in the Toyoha district. Temporal relationship between the formation of high temperature propylitic alteration minerals (epidote, actinolite, prehnite) and low temperature propylitic alteration minerals) (wairakite, laumontite, chlorite/smectite, smectite) in these areas (Seigoshi, Fuke, Kushikino) is uncertain. 

Seigoshi argentite electrum, pearceite, polybasite, pyrargyrite, stephanite, chalcopyrite, fahore, galena, sphalerite quartz, adularia, inesite, xonotlite, chlorite, mixed layer clay mineral, sericite. calcite, rhodochrosite... 

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 layered clays, most often ordered, are present up to temperatures near 200°C at depths of 500 to 1500 meters. The minerals form in two distinct zones. At shallow depths (between 100 and 200°C) mixed layering is between 90 and 0% montmorillonite. Above 200°C or so no expandable minerals are present. In the second zone (1.5Km. depth) one finds ordered interlayering showing the superstructure reflection... 

Those chlorites associated with mixed layered clay minerals are most silica-rich and have the greatest compositional variations for grains in a single thin section they tend to be iron-rich and aluminous. One chlorite vein was found to transect a glauconite pellet. This chlorite was quite iron-poor indicating attainment of a local chemical equilibrium between chlorite and iron mica upon its crystallization. 

IIYAKA (J.T.) and ROY (R.), 1963. Controlled synthesis of heteropolytypic (mixed-layered) clay minerals. Clays and Clay Min. 10, 4-22. 

There is a large number of clays which are not pure mineral types but consist of interstratified units of different chemical composition. (In detail, this may include nearly all the 2 1 layer minerals.) These are called mixed-layer clays. The two or possibly three different units can be regularly interstratified ABABAB or more commonly randomly interstratified AABABBABA. The most common regularly interstratified clay mineral, corrensite (Lippman, 1954), consists of alternate layers of chlorite and vermiculite or chlorite and montmorillonite. 

Fig.30. Generalized classification of dioctahedral 2 1 clay minerals. Mixed refers to mixed-layering.

Grim. R.E., Droste, J.B. and Bradley, W.F., 1961. A mixed-layer clay mineral associated with an evaporite. Clays Clay Miner., Proc, Natl. Conf. Clays Clay Miner., 8<1959) 228—236. 

Heckroodt, R.O. and Roering, C., 1965. A high-alluminous chlorite-swelling chlorite regular mixed-layer clay mineral. Clay Miner., 6 83-89. 

Two sediments from the bituminous subunit (NR-10, 151 and 170 m) are almost free of carbonate. Opal and quartz are dominant in these samples and are accompanied by mixed-layer clay minerals, kaolinite and illite. In the most organic-carbon-rich sediment (NR-10, 170.5 m 25.5% C ) opal is present as amorphous opal A and not as opal CT like in the other sample. Furthermore, the carbonate signals in the X-ray diffractogram are extremely broad. This indicates that the minerals in this sediment are diagenetically less altered than those in the other samples studied. 

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. 

The most widespread fill material is reddish brown (2.5 YR 4/4, 5 YR 4/4) loam with a minor admixture of relatively large oolitic bauxite pebbles (derived from the Late Triassic - Camian - beds) and coarse clasts of black chert. Pilot X-ray diffraction analysis revealed mostly muscovite/illite, plus mixed-layer clay minerals of illite/montmorillonite type, chlorite plus mixed-layer clay minerals of chlorite/montmorillonite type, calcium montmorillonite, and diaspore plus gibbsite, or just traces of bauxite minerals (Misic, 2000). The mineral composition is not as uniform as might be expected, and further research, intended for application of factorial analysis, is in progress. A potential sediment source area in the present Cerkniscica River basin (Fig. 1) appears obvious at first glance, but similar outcrops of bauxite and chert do also appear at other sites that are not much more remote. 

To a clay mineralogist, the term illite is synonomous with variability in both composition and crystallinity ( 8). The situation is even further complicated by the fact that much of the material in coal referred to as illite is actually an illite dominated mixed layered clay wherein the illite lattices are randomly interstratified with I4X clay lattices usually chlorite. This mixing of clay mineral lattices further adds to the inherent variability in both composition and crystallinity of the illitic material. The constitution of illite can therefore be expected to vary significantly from sample to sample. It should be quite apparent from the above discussion that no standard illite exists that could be used to represent illite in standard samples. 

Eberl, D. and Hower, J., 1977. The hydrothermal transformation of sodium and potassium smectite into mixed-layer clay. Clays Clay Miner., 25 215—227. 

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