Illite layers potassium mineral

Figure 17. Proposed phase relations where K is a mobile component and Al, Fe are immobile components at about 20°C and several atmosphere water pressure for aluminous and ferric-ferrous mica-smectite minerals. Symbols are as follows I illite G = non-expanding glauconite Ox = iron oxide Kaol = kaolinlte Mo montmorillonite smectite N nontronitic smectite MLAL aluminous illite-smectite interlayered minerals Mlpe = iron-rich glauconite mica-smectite interlayered mineral. Dashed lines 1, 2, and 3 indicate the path three different starting materials might take during the process of glauconitization. The process involves increase of potassium content and the attainment of an iron-rich octahedral layer in a mica structure.

The CEC of clay minerals is partly the result of adsorption in the interlayer space between repeating layer units. This effect is greatest in the three-layer clays. In the case of montmorillonite, the interlayer space can expand to accommodate a variety of cations and water. This causes montmorillonite to have a very high CEC and to swell when wetted. This process is reversible the removal of the water molecules causes these clays to contract. In illite, some exchangeable potassium is present in the interlayer space. Because the interlayer potassium ions are rather tightly held, the CEC of this illite is similar to that of kaolinite, which has no interlayer space. Chlorite s CEC is similar to that of kaolinite and illite because the brucite layer restricts adsorption between the three-layer sandwiches. 

In each of the different parageneses outlined here, the instability of a mineral can be denoted by its replacement with one or usually several minerals. The rocks in these facies are typified by multi-phase assemblages which can be placed in the K-Na-Al-Si system. This is typical of systems where the major chemical components are inert and where their masses determine the phases formed. The assumptions made in the analysis up to this point have been that all phases are stable under the variation of intensive variables of the system. This means that at constant P-T the minerals are stable over the range of pH s encountered in the various environments. This is probably true for most sedimentary basins, deep-sea deposits and buried sedimentary sequences. The assemblage albite-potassium feldspar-mixed layered-illite montmorillonite and albite-mixed layered illite montmorillonite-kaolinite represent the end of zeolite facies as found in carbonates and sedimentary rocks (Bates and Strahl,... 

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. 

Most of the chlorite-like material formed in soils is dioctahedral rather than trioctahedral. In the process of weathering, illite and muscovite are stripped of their potassium and water enters between the layers. In these minerals and in montmoril-lonites and vermiculites, hydroxides are precipitated in the interlayer positions to form a chlorite-like mineral (Rich and Obenshain, 1955 Klages and White, 1957 Brydon et al., 1961 Jackson, 1963 Quigley and Martin, 1963 Rich, 1968). Al(OH)3 and Fe(OH)3 are likely to be precipitated in an acid to mildly basic environments and Mg(OH)2 in a basic environment. The gibbsite sheets in the soil chlorites are seldom complete and the material resembles a mixed-layer chlorite-vermiculite. The gibbsite may occur between some layers and not between others or may occur as islands separated by water molecules. 

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. 

Fig. 8.1 a. Proportion of illite-smectite layers in crystal structure of mixed-layer minerals (<2 pm fraction) as a function of depth (Oued el-Mya and Ahnet-Mouydir-Gourara Basins) unordered crystal structure, b Criteria for iliite crystallinity potassium content vs. half-width of 10-A Peak (Illizi Basin) Coef. of correlation = 0.96... 

The amount of NH4 that can be fixed by a clay mineral of the 2 1 type is dependent on the extent to which the exchangeable sites between the mica layers are already occupied by ions or substances that cannot be displaced readily. Pure mica, or unweathered illite, for example, has negligible NH4 -fixing capacity because the sites are occupied by K. As the mineral loses much of its potassium over the years, and the active spots are occupied by readily exchangeable Ca, H, Na, etc., then the mineral can fix considerable NH4. ... 

Illite [(K,H30)(AlMg,Fe) (Si,Al)p ((0H), (Hp))] is a non-expanding micaceous mineral. It occurs as aggregates of small monoclinic grey to white crystals. It is layered alumina-silicate consisting of poorly hydrated potassium Cation, which is responsible for the poor swelling behavior. Its structure consists of repetition of tetrahedron- octahedron - tetrahedron (TOT) layers. The cation exchange capacity (CEC) of illite is comparatively better than kaolinite (20-30 meq/100 g). 

---