Dioctahedral minerals

Clay minerals that are composed of two tetrahedral layers and one octahedral layer are referred to as 2 1 clay minerals or TOT minerals. The apical oxygens of the two tetrahedral sheets project into the octahedral sheet. The 2 1 stmcture has a basal spacing (nominal thickness) of 1.0 nm (10 E). Pyrophjlhte [12269-78-2] Al2Si40 Q(0H)2, is the dioctahedral mineral, ie, AF" in the octahedral sites, and talc [14807-96-6], Mg3Si402Q(0H)2, is the trioctahedral, ie, in the octahedral sites. Both these minerals are essentially free of substitution in the octahedral site and therefore do not have a net... 

The two series of phase relations deduced above result in, at a first approximation, two "facies" for the expandable dioctahedral minerals— that of low temperature where fully expandable minerals exist and where the tie-line or association beidellite-montmorillonite persists. More elevated conditions produce a kaolinite-illite tie-line characteristic of sequences of buried rocks. 

The phase diagrams developed from available chemical data on natural dioctahedral minerals permits the distinction of different parageneses where the chemical components are inert, i.e., where their relative masses determine the phases produced. Other systems are possible which involve dioctahedral montmorillonites. These, or some of these, will be discussed later in conjunction with other clay minerals. 

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... 

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... 

In zone II, that of normal mixed layered dioctahedral minerals, there are few characteristic mineral reactions. However, the change of the interstratified material as it becomes "allevardite-type" mineral, i.e., showing a discrete super-lattice reflection, is undoubtedly complex. 

Zone 111 is defined by the presence of an ordered mixed layered dioctahedral mineral which has an obvious superlattice reflection. Mixed layered proportions vary from 50% to 25% expandable material. The mixed layered phase is called here "allevardite-Iike". Indications from studies on deeply buried and shallow rocks suggest that as pressure increases, the mixed layer superlattice reflection appears at lower temperature. 

V is characterized by kaolinite-illite-chlorite assemblages beyond the stability of an expanding mixed layered potassic dioctahedral mineral and below the thermal stability of pyrophyllite. The establishment of such conditions will be difficult in that the non-appearance of a mineral is a poor diagnostic and, as we have seen, kaolinite is frequently eliminated from sediments before its upper stability limit in the presence... 

The converse is true of the Mg ion. It is more abundant in the octahedral sheets of the low-temperature 2 1 dioctahedral minerals, attaining an average value of 3.55% in the montmorillonites and even higher values in glauconite and celadonite. Mg in the octahedral position increases the size of the octahedral sheet and decreases structural strain. 

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. 

Midler, A., 1961. AFchlorite a new dioctahedral mineral of the chlorite group. Clays Clay Miner. Abstr. 

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). 

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...

Tobelite-like layers are often found in interstratified dioctahedral minerals having non-expandable (mica-like) and expandable (smectite-like and/or vermiculite-like) layers. Drits et al. (1997) demonstrated that, in interstratified illite-smectite minerals from North... 

Principal substitution Trioctahedral minerals Dioctahedral minerals... 

Since the end of the nineteenth century, some micas have been synthesized in a fluoride-solid state phase from fluorinated silicates, aluminates, and fluorinated salts such as K2SiF6 (Doelter 1888 Baur 1911 Yoder and Eugster 1955). Due to their properties, such materials have been produced on an industrial scale and are well-studied (Jackel 1952 Roy 1952). Nevertheless, dioctahedral minerals are easier to synthesize than dioctahedrd ones. In 1972, Baroid Division N.L. Corporation produced Barasym SMM 100, a 2 1 dioctahedral layer silicate described as an interstratified mica-beidellite mineral, previously synthesized by Grandquist (Grandquist and Township 1966) and characteri (Wright, Grandquist and Kennedy 1972). 

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... 

The primary structural unit is composed of one octahedral layer condensed with one tetrahedral layer. In the dioctahedral minerals of the group such as kaolinite, the octahedral cation sites are occupied by aluminum. Only two trivalent aluminium ions are required to balance the six negative valence electrons that occur on the oxygen and hydroxyl ions surrounding each set of three octahedral cation sites. One cation site of the three is always vacant. 

The dioctahedral kaolin minerals have the generali2ed chemical formula Al2Si205 (0H)4- H20. In kaolinite, dickite, and nacrite, n=0 in hydrated halloysite, n=2 and in other halloysites, n varies between 0 and 2. A small amount of isomorphous substitution of A1 and Fe for Si in the tetrahedral sheets and Fe and Mg for A1 in the octahedral sheets might occur in halloysites, but has not been suggested for other dioctahedral minerals. 

Following Smith and Yoder [1956], Zvyagin [1967] has proposed symbols to represent the numbers of layers and symmetry of the kaolin group minerals. In the system proposed by Smith and Yoder, kaolin that contains one layer in a triclinic unit cell is symbolized as IT, and dickite that contains two layers in a monoclinic cell is symbolized as 2M. Bailey [1963], recognizing that the ordering of cations or the vacant octahedral position in the dioctahedral minerals changes the size, shape, and symmetry of the unit cell, has modified the Smith and Yoder system to indicate in one double symbol the basic crystallographic form and its ordered modification. 

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