Trioctahedral minerals illites

Pelitic rocks investigated in the same areas where corrensites are formed during alpine metamorphism (Kiibler, 1970) revealed the absence of both montmorillonite and kaolinite but the illite or mica fraction was well crystallized as evidenced by measurement of the "sharpness" of the (001) mica reflection (Kiibler, 1968). This observation places the upper thermal stability of the expandable and mixed layered trioctahedral mineral assemblages at least 50°C. above their dioctahedral correlevants. This is valid for rocks of decidedly basic compositions where no dioctahedral clay minerals are present. 

The results obtained on the association of the clay minerals as well as on the crystal-lochemical pecularities of the Triassic deposits in the Triassic Province lead us to conclude that the association Mg-chlorite + swelling trioctahedral mineral + Fe-illite may be interpreted as an indication of the dolomite-sulfate stage of the salinization of a sedimentary basin of the terrigenous-chemical type. 

Figure 32. Results of experiments on natrual minerals are schematically shown in Mr3-2R -3R coordinates. Kaol = kaolinite ML j = mixed layered beidellitlc mineral MLj, = mixed layered montmorillonitic mineral I = illite compositional field chi = chlorite Exp3 trioctahedral expandable-chlorite mixed layered mineral (expanding chlorite and corrensite).

With increasing temperature ( 200°C), either due to deeper burial or increase in heat-flow rates, upward migrating K, Mg, and Fe, derived from the underlying sediments, become sufficiently abundant that the remaining expanded layers are lost and some discrete 10A illite (2M) and trioctahedral chlorite are formed however, much of the illite at this stage still contains an appreciable proportion of dioctahedral chlorite and the chlorite contains some 10A layers. This is the typical clay-mineral suite... 

The type phengite suggested by Foster (1956) has a composition almost identical to that of the Belt illite (No.8). When the H20 content is appreciably higher and the K20 content lower than muscovite, the minerals have been called hydromicas or hydromuscovites. The excess H20 in some instances is present as interlayer water, particularly in the trioctahedral hydrobiotites. Table XII contains a selection of sericite and hydromuscovite analyses and Table XIII the structural formulas. The H20 and K20 values of these minerals are similar to those reported for the illite minerals however, the MgO content of the sericites and hydromuscovites is lower and the NazO contents higher than for the illites (Table XIV). 

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. 

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.

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... 

Throughout the areas studied we observe in rocks of different granulometry, from sandstones to rocks of the terrigenous-chemical complex, primary clay mineral associations. The main components of the associations are a number of varieties of Mg-chlorites, primary hydromicas grouped together under the term ferruginous illites, trioctahedral swelling minerals and mixed-layer clays. 

The clay mineral spectrum is notably less differentiated than in the other facies, the dominant minerals being trioctahedral chlorites and dioctahedral illites. In the chlorite structure, non-swelling layers predominate whereas the alternation of layers of different types exhibits a trend towards ordering. The proportion of mixed-layer minerals of the iUite-montmorillonite type decreases especially as one approaches the massive layers of rock salt. 

Trioctahedral lllite. This is observed in all argillaceous fractions. The characteristic feature of this mineral in the terrigenous-halitic complex is its heterogeneity. In diffractograms of oriented samples the peak occurs in the range 10-10.3 A. Based on spectral analyses the hydromicas may be classed as Fe-illites. 

Smectite group minerals, when present in the petroleum reservoirs, play an important role in the migration of hydrocarbons. At the shallow level of reservoir rocks smectite can exist, but at deeper level the increase of temperatiue transforms it to other minerals. Generally dioctahedral smectites are transformed to illite and trioctahedral smectites are transformed to chlorite, releasing the interlayer water molecules in both cases. That released water increases the pore fluid pressure that may lead to migration of the hydrocarbons. 

The existence of vermiculitelike minerals in soils was first demonstrated in 1947. Yellow-brown crystals in the sand fractions of certain Scottish podzols were found to be derived from biotite by a process of natural weathering and to have some of the characteristics of vermiculites (Walker [1947,1949a]). In the clay fractions of the same soils, expanding lattice minerals with swelling characteristics reminiscent of, but noticeably different from, those of montmorillonite were encountered. The source of these clay minerals is not the weathered mica of the sand fractions, but a trioctahedral illite, which occurs in unaltered form in the C horizons of the soils and gradually alters to a trioctahedral vermiculitelike mineral with decreasing depth of profile (Walker [1947, 1950]). 

The development of vermiculite minerals in soils at the expense of micas is now well established as a common phenomenon, more particularly by the work of Jackson and his collaborators e.g., Jackson et al. [1952], Schmehl and Jackson [1956], Jackson [1959,1963], Brown and Jackson [1958]) as well as by others e.g., Fieldes and Swindale [1954], Rich [1958], Cook and Rich [1962], Millot and Camez [1963], Nelson [1963]). In spite of the frequent occurrence of dioctahedral clay vermiculites in soils, dioctahedral clay micas, in general, appear to resist decomposition better than their trioctahedral counterparts and, where direct comparison is possible, the dioctahedral type may remain unaffected, whereas the trioctahedral mica in the same profile is almost completely altered (Mitchell [1955]). Vermiculitelike minerals, however, may also develop in soils by other routes, for example, from montmorillonite (Bundy and Murray [1959], Jackson [1963]) or from chlorite (Droste and Tharin [1958], Brown and Jackson [1958], Droste et al. [1962], Millot and Camez [1963]). Such alterations are reversible, and they depend on a chemical equilibrium between the mineral and the soil solution. Hence clay chlorites, illites, and montmorillonites may develop from clay vermiculites in an appropriate environment, and intermediate types are common. The alteration of clay vermiculites to kaolinite in podzols has also been proposed (Walker [1950], Brown [1953], Jackson et al. [1954], McAleese and Mitchell [1958a]). 

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