Biotite-illite

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... 

Dominant gangue minerals are quartz, muscovite, chlorite, actinolite, hornblende, epidote, and biotite (Table 2.22). Minor minerals are rutile, illite, sphene, and glauco-phane. It is interesting to note that silicate minerals such as chlorite, epidote, pumpellyite, and albite are common and actinolite has been reported from the basalt near the Ainai Kuroko deposits (Shikazono et al., 1995) and they are also common in the basic schist which host the Motoyama Kuno deposits (Yui, 1983). 

The aluminosilicates examined were chosen as end members of those groups of phyl losil icates that commonly occur in soils muscovite and biotite mica, Fithian and Morris illite, Montana vermicul ite, montmoril Ionites from Upton (Wyoming bentonite), Camp Berteau, Redhill and New Mexico, and kaolinites from St. Austell, England, and Georgia, U.S.A. 

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... 

MgOn(OH)j -] units (Fig. 7.5), and the illite type in which the octahedral sheet is sandwiched between two layers of tetrahedra (cf. micas such as muscovite, Fig. 7.4). Many important clay minerals such as vermiculite, biotite, and smectites (notably montmorillonite and beidellite, the princi-... 

Typically illite-rich sediments develop the assemblage chlorite-illite-quartz or chlorite-illite-biotite-quartz upon epi-metamorphism (Maxwell and Hower, 1967 Dunoyer de Segonzac, 1969). 

Mixed-layer illite-montmorillonite is by far the most abundant (in the vicinity 90%) mixed-layer clay. The two layers occur in all possible proportions from 9 1 to 1 9. Many of those with a 9 1 or even 8 2 ratio are called illites or glauconites (according to Hower, 1961, all glauconites have some interlayered montmorillonite) and those which have ratios of 1 9 and 2 8 are usually called montmorillonite. This practice is not desirable and js definitely misleading. Other random mixed-layer clays are chlorite-montmorillonite, biotite-vermiculite, chlorite-vermiculite, illite-chlorite-montmorillonite, talc-saponite, and serpentine-chlorite. Most commonly one of the layers is the expanded type and the other is non-expanded. 

Trioctahedral illites have been reported by Walker (1950) and Weiss et al.(1956). Walker s analysis, which he considers only a rough approximation, is given in Table XI. The clay biotite occurs in a Scottish soil and is believed to be authigenic however, it weathers so easily to vermiculite that unweathered material is difficult to find. Due to its instability, it is not likely that much clay-sized biotite exists although trioctahedral biotite-like layers may occur interlayered with dioctahedral illite layers. Such interlayering has been reported by Bassett (1959). 

Vermiculite and vermiculite layers interstratified with mica and chlorite layers are quite common in soils where weathering is not overly aggressive. (A few references are Walker, 1949 Brown, 1953 Van der Marel, 1954 Hathaway, 1955 Droste, 1956 Rich, 1958 Weaver, 1958 Gjems, 1963 Millot and Camez, 1963 Barshad and Kishk, 1969.) Most of these clays are formed by the removal of K from the biotite, muscovite and illite and the brucite sheet from chlorite. This is accompanied by the oxidation of much of the iron in the 2 1 layer. Walker (1949) has described a trioctahedral soil vermiculite from Scotland formed from biotite however, most of the described samples are dioctahedral. Biotite and chlorite with a relatively high iron content weather more easily than the related iron-poor dioctahedral 2 1 clays and under similar weathering conditions are more apt to alter to a 1 1 clay or possibly assume a dioctahedral structure. 

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. 

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. 

Figure 3. Typical aspect of a thin section of an imported majolica, SA 27, from Spain with characteristic sedimentary temper. The colorless fragments are quartzes, the brown lathes biotites. In the bottom right corner a colorless lath of a muscovite illite can be seen. 1 Nicol. Length of the image area is 0.82 mm.

Illite = similar to biotite mica, but less cation-rich Montmorillonite = (Na,Ca)(Al,Mg)6(Si40io)3(OH)6-nH20). 

True (flexible) mica Biotite Muscovite, illite Trioctahedral Dioctahedral Non-hydrated monovalent cations... 

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. 

The formation and survival of unstable or metastable micas and clays in sediments and soils at low temperatures reflects kinetic as well as thermodynamic factors. First, the rates of reactions involving solid-aqueous and especially solid-solid transformations in dilute solutions are very slow at low temperatures (most natural waters are dilute )- The slow kinetics of clay transformations reflects small differences in free energy between stable and metastable clays. Also, the occurrence of specific clays is related to the chemistry and crystal structure of source minerals. Thus, illite often results from the weathering of muscovite, and vermiculite results from the weathering of biotite (cf. Drever 1988), consistent with the similar chemistries and structures of these pairs of T 0 T minerals. 

Malmstrom M, Banwart S (1997) Biotite dissolution at 25°C the pH dependence of dissolution rate and stoiehiometty. Geoehim Cosmochim Acta 61 2779-2799 Malathi N, Puri SP, Saraswat IP (1969) Mossbauer studies of iron in illite and montmorillonite. J Phys Soe Japan 26 680-683... 

Mineral transformations during diagenesis (e.g., illite smectite transition), metamorphism (e.g., recrystallization of clay minerals to biotites, amphiboles, etc) or alteration (e.g., serpentinization of mafic minerals) are likely to release the radiogenic noble gases that were produced within their lattices. This assumes that ... 

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