Montmorillonite, crystal structure

Montmorillonite-Calcite (me) Mixture. Heated mixtures of montmorillonite and calcite yielded the phases given in Table I. Although the montmorillonite structure persisted through 400 °C, it underwent dehydroxy lation between 400 and 500 °C. Grim and Bradley (16) have shown that the general layered structure is able to survive the elimination of the (OH) water with moderate readjustments. This structure produces an X-ray diffraction pattern like that given in Table III. Table III represents data close to those observed in this study. This phase is called dehydroxylated montmorillonite in Table I. This phase disappeared between 700 and 800 °C as a result of the complete destruction of the montmorillonite crystal structure. Calcite decomposed between 500 and 600 °C to form lime that was present through 900 °C. 

Of the naturally existing smectites, montmorillonite is a clay of major interest in the subsurface environment. Figure 1.5 shows the crystal structure of montmorillonite,... 

Fig. 1.5 The crystal structure of smectite, illustrating beidelite, montmorillonite and nontronite (Borchard 1989 after Brindley and MacEwan 1953)...

Montmorillonite, one of the most commonly encountered smectites, is similar to pyrophyllite (2 1) but has some interlayer cations and extra water. In pyrophyllite the layers are neutral because Si " in the tetrahedral sheet is not replaced by Al. In the smectites there is substitution of Al for Si " in the tetrahedral sheets, and occasionally Al appears in octahedral locations as well (for the names assigned to the end members, see Brindley and Brown, 1980, pp. 169-170.) The charge imbalances of the substitutions are compensated by interlayer cations, usually Na or Ca. These cations are easily exchangeable. The hydration level of the smectites is also variable. These minerals are very responsive to changes in water content as well as to the salt contents of the water. Other liquids that might be associated with the minerals and temperature can also effect changes in the chemical and crystal structure. 

FIGURE 14.1 Crystal structure of 2 1 type aluminosilicate (montmorillonite). 

The binuclear iron(III) complex [Fe2(HPTP)(p-OH)(N03)2](C104)2 (1) with HPTP = A W,7W,-tetrakis(2-pyridyl methyl)-2-hydroxy-1,3-diamino-propane [5], reacts with H2O2 to yield a blue species [6], The crystal structure of (1) and its interaction with montmorillonites (MMT) has been reported [7]. The occlusion of [Fe2(HPTP)(p-OH)] complexes in new families of hexagonal mesoporous materials such as MCM-41 and HMS will be described elsewhere [8], Related HPTB complexes, with benzimidazole instead of pyridine, have their tripodal N atoms tmns to strong basic peroxo-ligands, the benzimidazole ligands perpendicular to the Fc203 plane and tmns to each other on every iron [9]. 

Aluminium, on the other hand, accumulates in the clay mineral fraction because it forms insoluble aluminosilicates and hydroxyoxides. The AI remains behind in the soil as other ions leach away. Iron also accumulates in soils but this is not apparent from Table 7.3 because the silicate clay minerals, with the exception of hydrous mica, are low in Fe. Iron precipitates in soils only as hydroxyoxides. Hydrous mica is altered parent material and is not reconstituted from the soil solution as are kaolinite, montmorillonite, and allophane. The <105° C water in Table 7.3 is, roughly speaking, adsorbed water the >105° C water is hydroxyl ions and water within crystal structures. 

The most frequently encountered clay minerals found in sedimentary rocks are kaolinite, montmorillonite, and illite. These are all phyllosil-icates with a crystal structure similar to that of micas sheet-layer structures with strong covalent bonding within each sheet and among the two- or three-sheet layers belonging to the unit structure and only weak bonding (van der Waals attraction or hydrogen bonding) between the adjacent layered structures (4). 

Figure 2. Crystal structure of montmorillonite clay. (Reproduced with permission from reference 14. Copyright 1977 John Wiley and Sons.)...

Figure 14. (A) Polyhedral representation of montmorillonite showing the linkage of the tetrahedral sheet with the octahedral sheet. (B) Effective change in the average Fe-0 bondlength upon reduction of Fe(III) to Fe(II) results in the distortion of the local clay crystal structure.

Clays are classified on the basis of their crystal structure and the amount and locations of elelectric charge (defidt or excess) per unit cell. Crystalline days range from kaolins, which are relatively uniform in chemical composition, to smectites, which vary in their composition, cation exchange p>rop>erties, and ability to expand. The most commonly employed smectite clay for the preparation of polymeric nanocomposites is bentonite, whose main mineral component is montmorillonite (Utracki, 2004). 

These minerals are hydrated aluminosilicates which are characterized by a sheetlike structure and can be conveniently divided into three groups (1) the kaolinite group, (2) the mont-morillonite group, and (3) the potash clay (or hydrous mica) group (Table 7.7). In the kaolinite group, all have the same chemical composition and differ only in individual crystal structures. The montmorillonite group can be represented by means of ion substitutions in the general chemical formula. For example, in montmorillonite itself, approximately 16% of the aluminum... 

These varieties of carbonated apatite whose formulae may be represented as Cajo ,(P04)6 (C03) j (F,0H)2, where jc = 1, are often designated as Francolite (F OH) or Dahllite (OH F). Up to 25% replacement of PO4 by CO3 is, however, sometimes found, and replacement of up to 10% Ca by Mg can occur. A wide variety of other metals, including uranium are often incorporated in trace amounts. Common major impurities found with phosphorites are iron, alumina, quartz, montmorillonite and organic matter. Almost every element has been found, at least in trace amounts, in phosphorite minerals. Much of this arises from the remarkable nature of the Apatite crystal structure which allows substitution of the Ca ", and F by alternative cations and anions (Chapter 5.3). 

Fig. 1 Representation for the crystal structure of montmorillonite [14]. Copyright 2011. Reproduced by permission of Elsevier Science Ltd.

The three minerals - quartz, calcite and mica - tested by Von Moons, were ground to a very small particle size and the PI then determined on the fraction Imer than two microns. The activity of these minerals is low, as might be expected frxim their relatively simple crystal structure. Illite is probably the most widespread of all clay minerals but it usually occurs in conjunction with other minerals. The clay known as bentonite consists almost exclusively of the mineral montmorillonite. In its natural state bentonite is usually a sodium clay... 

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