Trioctahedral minerals kaolinites

The least compHcated clay minerals are the 1 1 clay minerals composed of one tetrahedral (T) layer and one octahedral (O) layer (see Fig. 1). These 1 1 clay minerals are also referred to as TO minerals. The TO package has a basal spacing (nominal thickness) of 0.7 nm (7 E) and they are commonly referred to as 7 E minerals. Kaolinite, the dioctahedral 1 1 mineral, has filling two of three octahedral sites, and serpentine [12168-92-2J, (Mg)3Si205(0H)4, the trioctahedral 1 1 mineral has filling all three octahedral sites. The kaolin minerals have limited substitution in the octahedral... 

Considering the compositions of the mixed layered minerals found in sedimentary rocks (Figure 25) it is obvious that magnesian-iron expandable dioctahedral minerals will be in equilibrium not uniquely with kaolinite but also in many instances with a magnesian-iron phase—either chlorite or an expanding trioctahedral mineral. In such a situation the slope in... 

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 two layer silicates are divided into the kaolinite (dioctahedral) and serpentine (trioctahedral) subgroups. The dioctahedral minerals are hydrous aluminum silicates containing minor amounts of other constituents. The trioctahedral minerals vary widely in composition and isomorphous substitution is common however, these minerals are relatively rare and chemical data are limited. 

Dioctahedral micas and vermiculites of clay size can be concentrated by differentially dissolving kaolinite and trioctahedral minerals with a fluoride solution proposed by Rich... 

Probably the most ubiquitous silicate minerals in soils throughout the world are the layer silicates known as the kaolin minerals. The group includes the dioctahedral minerals kaolinite, halloysite, dickite, and nacrite, and the trioctahedral minerals chrysotile, antigorite, chamosite, and cronstedite. Halloysite and disordered forms of kaolinite seem to be the only members of... 

In this chapter, the dioctahedral and trioctahedral minerals are treated in separate sections. Greater emphasis is given to the crystal structures of the triclinic form of kaolinite, dickite, and cronstedite than their prevalence in soils would warrant, because their structures are best known. They are used, therefore, to illustrate the principles that determine the crystallographic nature of all minerals of the group, including those such as halloysite with structures yet to be determined in detail. 

If we look back to the experimental studies on natural expandable minerals at high pressures, it can be recalled that the production of a chlorite-phase occurred when interlayering in the natural dioctahedral mineral had reached about 30% interlayering. It is possible that below this transition only expandable phases are present for most magnesium-iron compositions one is dioctahedral, the other would be trioctahedral. Thus, at temperatures below the transition to an ordered allevardite-type phase, dioctahedral mixed layered minerals will coexist with expandable chlorites or vermiculites as well as kaolinite. The distinction between these two phases is very difficult because both respond in about the same manner when glycollated. There can also be interlayering in both di- and... 

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).

The 1 1 clay-mineral type consists of one tetrahedral sheet and one octahedral sheet. These two sheets are approximately 7 A thick. This two-sheet type is divided into kaolinite (dioctahedral) and serpentine (trioctahedral) groups. The kaolinite minerals are all pure hydrous aluminum silicates. The different members are characterized by the manner of stacking of the basic 7 A layers (Brindley, 1961b). 

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.

The definition of the verdine facies is largely due to the work of Odin (1988). Bailey (1988) defined the mineral odinite. Verdines and odinite span much of the range of octahedral site occupation from 2.5 to 2.0 ions, containing much ferric iron and alumina. Assuming that all of the minerals are 7 A structures, the difference in observed composition of the different phases can be illustrated by their octahedral cation occupancy AI2 kaolinite (dioctahedral) (Mg, Al)3 2.5 7 A trioctahedral chlorite and (Mg, Al)2 2.5 odinite (di-, trioctahedral). 

Figure 11 Representation of the evolution of clay pellets in shallow shelf sediment areas according to the oxido-reduction conditions locally present. Lower arrow shows berthierine formation through reduction of iron, shifting the pellet composition from the ferric (R = Fe ) pole to the ferrous pole (R = Fe ). This reaction passes through a chemical evolution by the formation of a berthierine/smectite mixed layer mineral (chi in the figure). The arrow towards glauconite indicates the change in composition with increase in potassium and some reduction of ferric iron. The diagram represents feldspar, dioctahedral clays, and trioctahedral clays, respectively. R + = Fe +, R = Al, Fe. The compositional positions of the minerals Mu (muscovite) kaol (kaolinite) and end-member celadonite (Ce) are indicated.

Because of their alternating tetrahedral (T) and octahedral (O) layers, the two-layer phyllosil-icates are said to have T 0 structures. Listed in Table 9.1 are other T 0 minerals, including the kaolinite polymorphs nacrite, dickite, and halloysite, and the trioctahedral serpentine minerals lizardite, antigorite, and chrysotile, in which brucite layers alternate with layers of silica tetrahedra. 

In the same structural category, three-layer minerals are found in which the octahedral Al or Mg layer is sandwiched between two Si04 tetrahedral layers, the free apices of which point towards each other (Figure 2.4). Talc, Mg3[(OH)2/Si40,o] is one common example of a three-layer trioctahedral sheet silicate, whereas pyro-phyllite, Al2[(OH)2/Si40io] is the three-layer equivalent of kaolinite with dioctahedral nature (Table 2.2). 

It shows pronounced thermal effects on heating and generally has a more ordered structure than other clay minerals. Figures I and 2 illustrate typical DTA data for kaolinite, halloysite, and montmorillonite. Kaolinite and halloysite lose their hydroxyls between 450 to 600°C. Variations within this range are attributed to differences in entrapped water vapor that is dependent on sample size and shape factors. The loss of hydroxyls from montmo-rillonites in the range of 450 to 650°C is t5q)ical for dioctahedral forms of these minerals. Dehydroxylation is more gradual for trioctahedral forms and can continue to temperatures up to 850°C. 

The suggestion that kaolinite could, theoretically, have monoclinic symmetry was pointed out first by Gruner [1932]. One of the octahedral cation sites in trioctahedral kaolin minerals... 

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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