Trioctahedral minerals

Despite the small number of analyses, it is apparent that the range of solid solution for expandable trioctahedral minerals is very great. 

If we consider the three types of substitutions dioctahedral, trioctahedral and interlayer ion, we see that Na and trioctahedral minerals are stable to higher temperatures than others. This is logical when one considers that sodic trioctahedral magnesian minerals are unknown, paragon-ite is unstable below about 340°C (Chatterjee, 1968) and phlogopite is unstable below 250°C (Velde, unpublished). By contrast muscovite appears stable at low temperatures under high K O concentration (Velde, 1969) or appropriate H+, K+ and SiO activities (Garrels and Howard, 1959). 

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... 

This regularly interstratified trioctahedral mineral (see Lippmann,... 

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. 

Walker, G.F., 1950. Trioctahedral minerals in the soil clays of northeast Scotland. Mineral. Mag., 29 72-84. 

What can also be seen in Table 1.2 is that each group is usually further divided into two series dioctahedral (D) and trioctahedral (T) according to the number of central positions of the octahedrons occupied by cations. In the trioctahedral minerals, every central position is filled, usually with Mg2+, while in the dioctahedral minerals, two-thirds of the central position is filled with Al3+ ions (e.g., montmorillonite Chapter 2, Figure 2.1). 

A mixture of dioctahedral smectite and berthierine gives the mineral spacings for the 060 spacings observed by Bailey and Odin, which are between those of clearly aluminous dioctahedral and trioctahedral minerals. 

Berthierine, as shown by Brindley (1982) is essentially a trioctahedral mineral, following the line of trioctahedral chlorites in Figure 7. In our simulations of the XRD spectra of odinite, we use a ferrous serpentine and a ferric dioctahedral smectite component. Translated into constituent ions of a mineral structure, this mineral combination will give a bulk average composition between nontronite (ferric, dioctahedral smectite) and berthierine (trioctahedral chlorite). 

If we look back to the singular chemical features of odinite as having large amounts of ferric iron present and having an overall low occupation of the octahedral site for a trioctahedral mineral, a mixed layered mineral of ferric, dioctahedral smectite (nontronite), and berthierine (ferrous 7 A chlorite) would give the overall chemical characteristics of odinite. Thus, one can fit the XRD data by using a ferric... 

Figure 10.19. A. Relationship between the 8.45 T Cs MAS NMR room temperature chemical shifts of fully hydrated Cs-exchanged clay minerals and their degree of tetrahedral Al substitution. Open squares denote the dioctahedral minerals, open circles denote the trioctahedral minerals. Note that due to motional averaging in these samples, only one caesium resonance is observed. B. The same relationship for samples fully dehydrated at 450°C. The 2 lines correspond to the 2 Cs resonances observed in these samples. Note the similar behaviour of the dioctahedral and trioctahedral minerals when dehydrated. From Weiss et al. (1990a) by permission of the Mineralogical Society...

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. 

Principal substitution Trioctahedral minerals Dioctahedral minerals... 

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

This solution partially dissolves the dioctahedral minerals, but at a much slower rate than the trioctahedral minerals. 

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 the trioctahedral minerals of the group such as chamosite, the octahedral cation sites are occupied mainly by magnesium and ferrous ion. Three divalent cations are required to balance the six negative valence electrons, and all three octahedral cations sites are occupied. 

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. 

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