Montmorillonite structure

Ion exchange reactions between cations present in groundwater and in the interlayer space of the montmorillonite structure. These are mainly the replacement of 2Na+ by Ca2+ and Mg2+ present in ground-waters. This triggers the dissolution of carbonates according to equilibria like the one exemplified by equation (1). The ion-exchange reactions can be described by ... 

X-ray diffraction could also be used to distinguish the two species. We have assumed thus far that only type (a) and possibly (b) exist at low temperatures for montmorillonite structures. This is largely done for lack of contrary evidence. 

If we consider the substitutions in the trioctahedral montmorillonite structure which give rise to a charge imbalance on the basic 2 1 structure, they can be considered as being two in kind ... 

The apparent discrepancy could reside in the fact that if potassium ions are available at all, they will form a mica at temperatures near 100°C. Montmorillonite structures below these conditions (pressure and temperature) need not contain potassium at all. However, at the correct physical conditions the 2 1 portion of the montmorillonite must change greatly (increase of total charge on the 2 1 unit) in order to form a mica unit in a mixed layered mineral phase. Since neither Na nor Ca ions will form mica at this temperature, potassium will be selectively taken from solution. Obviously this does not occur below 100°C since cation exchange on montmorillonites shows the reverse effect, i.e., concentration of calcium ions in the interlayer sites. If potassium is not available either In coexisting solids or in solutions, the sodi-calcic montmorillonite will undoubtedly persist well above 100°C. 

Infrared spectra also show some changes in the montmorillonite structure (Figure 2.22) during treatment with FeCl3 in acetone. 

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. 

In the second study, Kittrick (20) reacted the 0.2 to 5 pm fractions of three montmorillonite clays with low pH (<3.47) solutions for 3 to 4 years. Under these conditions, montmorillonite is unstable with respect to kaolinite. It is uncertain whether the kaolinte formed through precipitation, in which case the nucleation process may have produced numerous small particles, or it formed through the alteration of the pre-existing montmorillonite structure, which could have maintained the existing particle size or even increased it, with growth of the new phase. 

An idealized three-dimensional representation of calcium montmorillonite structure at the atomic level is shown in Figure 1. The 2 1 layered structure is composed of upper and lower layers of silicon oxide tetrahedra linked in hexagonal arrays to form two-dimensional silicate sheets extending in the a,b-directions. Sandwiched between these sheets are partially filled two-dimensional sheets composed of... 

With fresh activated-clay catalyst, endothermic peaks are observed at temperatures of about 300, 1200, and 1600°F. These three peaks are attributed to loss of physically adsorbed water, loss of chemically bound (hydroxyl) water, and collapse of the montmorillonite structure, respectively. The hydroxyl water originally present amounts to 3 or 4%. The magnitude of the peak at 1200°F. decreases if the sample is heated above 800°F. prior to thermal analysis, and disappears completely if the sample is calcined at 1100°F. The thermal-analysis curve for the dehydrated catalyst is flat up to the point at which the montmorillonite structure begins to disappear. If the catalyst has not been heated above 1450°F., it becomes rehydrated upon exposure to moisture and a new endothermic peak appears in the curve between 800 and 1000°F. The size of the new peak increases as that of the original hydroxyl-water peak decreases it corresponds to 1.5 to 2.0% sorbed water with catalyst that has been rehydrated after calcination at 1100°F. The rehydration capacity of the catalyst decreases as the catalyst becomes partially deactivated with use. 

The montmorillonite structure of the starting material was converted by the alkaline treatment into a mixture of zeolitic products and crystalline minerals. There was a remarkable influence of the nature of the medium (sea or distilled water) on the crystallization process. The zeolitic products synthesized in sea water showed higher crystallinity and less heterogeneity than their counterparts synthesized in distilled water. 

Figure 4.8 Montmorillonite structure. (From http //www.scielo.br/. With permission.)...

Preferred bentonite clays are those whose chief constituent is mont-morillonite, a mineral of the composition corresponding to the empirical formula, 4Si02-Al203 H20. The principal sources of raw clay for the manufacture of the presently most widely used natural catalyst (Filtrol Corporation) are deposits in Arizona and Mississippi. The clay from these deposits contains appreciable amounts of impurities, principally CaO, MgO, and Fe203, which replace part of the A1203 in the ideal montmorillonite structure. The catalyst is prepared by leaching the raw clay with dilute sulfuric acid until about half of the alumina and associated impurities is removed. The resulting product is then washed, partially dried, and extruded into pellets, after which it is activated by calcination. A typical analysis of the finished catalyst is as follows (Mills, 12). 

Corrensite is present in several genetic types as described by Kubler (1973). It has been observed in the Triassic dolomitic limestones associated with evaporites and in volcanic sandstones which experienced some burial. Correlations with other data like i o or vitrinite reflectance and the temperatures measured in the holes show that the transformation of volcanic ferromagnesian minerals into corrensite takes place at a temperature of 90-100 °C at which the haphazardly arranged layers of montmorillo-nite in the chlorite-montmorillonite structure largely disappear. Corrensite is everywhere encountered in complexes, the temperatures of which, as measured in the boreholes, went up to 148 °C. 

Mallakpour S, Dinar M. Surface treated montmorillonite structural and thermal properties of chiral poly(amide-imide)/organoclay bionanocomposites containing natural amino acids. J Inorg Organomet Polym Mater 2012 22(5) 929-37. 

Sodium montmorillonite structure. (Reproduced by permission of Southern Clay.)... 

Nanocomposites are a relatively new class of hybrid materials characterized by an ultra fine dispersion of nanofillers into a polymeric matrix. As the result of this dispersion, these materials possess unique properties, behaving much diflferentiy than conventional composites or microcomposites, and offering new technological and economical opportunities. The first studies on nanocomposites were carried out in 1961, when Blumstein performed the polymerization of vinyl monomer intercalated into montmorillonite structure. Since then, clay-based polymer nanocomposites have emerged as a new class of materials and attracted considerable interest and investment in research and development worldwide (Schaefer and Justice 2007). 

The presence of metal species in the montmorillonite structure may catalytically enable the oxidative cleavage of alkene substituents in alkylammonium compounds to produce aldehydes at elevated temperatures [15, 18]. 

Nuclear Magnetic Resonance (NMR) Spectroscopy This techni-que85-89 utilizes solid-state NMR to analyze nanoscale dispersion for the overall sample. The iron in the montmorillonite structure facilitates the relaxation of nearby protons, which provides information on the dispersion of the clay in the polymer matrix. In the cases reported, a signal in the polymer is identified and its relaxation time (fy) is measured the relaxation time depends on how close the proton is to a paramagnetic iron atom. On average, the protons of the polymers will be closer to the iron in the clay in a well-exfoliated system and... 

FIG U RE 7.2 Schematical representation of the montmorillonite structure. (From Anadao P., 2011, In Advances in Nanocomposite Technology, ed. Hashim, A., 133-46. Rijeka InTech.)... 

Self-Assembly of Alkylammonium Ions on Montmorillonite Structural and Surface Properties at the Molecular Level... 

Smectite clay - Like mica, smectite cl (commonly called bentonite) has either a pyrophyllite or talc structure. Montmorillonite, a common high-aliunimun smectite, can be characterized by die pyrophyllite crystal structure with a small amount of octahedral Al replaced by Mg. The resulting charge imbalance is compensated by exchangeable cations, usually Na or Ca, between the laminae. In addition to diese counterions, oriented water, similar to that in vermiculite, occupies the interlaminar space. When Ca is the exchangeable cation, there are two water l ers, as in vermiculite when Na is the counterion, there is usually just one water layer. Figure 18 shows the montmorillonite structure. 

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