Vermiculite, dehydration

Vermiculites exist in various stages of dehydration. Because of the similar dimensions of the water-cation layer in vermiculite and the brucitelike layer in chlorite, vermiculites can be confused with the chlorites. The common substitutions of Fe" or Fe for Mg (in either the water or octahedral sheet of vermiculites), and AF for Si (in the tetrahedral sheets), as well as the hydration variations, present enormous potential for structural distortion in these types of minerals. Fibrous vermiculite was described by Weiss and Hofmann (1952). 

There are more complicated structures intermediate between pyrophyllite and talc with variable substitution of A1J and Mg2. Electroneulrality is maintained by hydrated cations between layers. Thus the montmorillonites arc unusual days forming thixotropic aqueous suspensions that arc used as well-drilling muds and in nondrip puints. They are derived from the formulation AU(OH)jSi40 ,-x-H2o with variable amounts of water, Mg3+ (in place of some Al5 ), and compensaUng cations. M"+ (M = Ca in fuller s earth, which is converted to bentonite, M = Na). Vermiculite likewise has variable amounts of water and cations, (t dehydrates to a talc-like structure with much expansion when heated (see page 750). 

Walker, G.F., 1956. The mechanism of dehydration of Mg-vermiculite. Proc. Natl. Conf. Clays Clay Miner. 4th-Natl. Acad. ScL Natl. Res. Counc., Publ, 456 101-115. 

Clays are aluminosilicates with a two-dimensional or layered structure including the common sheet 2 1 alumino- and magnesium- silicates (montmorillonite, hectorite, micas, vermiculites) (figure 7.4) and 1 1 minerals (kaolinites, chlorites). These materials swell in water and polar solvents, up to the point where there remains no mutual interaction between the clay sheets. After dehydration below 393 K, the clay can be restored in its original state, however dehydration at higher temperatures causes irreversible collapse of the structure in the sense that the clay platelets are electrostatically bonded by dehydrated cations and exhibit no adsorption. 

A large attraction force between interlayer cations and adjacent siloxane cavities allows some cations with certain hydration energy to dehydrate. If the dehydrated cation radius is smaller than the inside diameter of the siloxane cavity, the mineral could collapse and an inner-sphere complex would form (e.g., K-vermiculite) (Fig, 4.3). When vermiculite contains a relatively strongly hydrated cation such as Ca2+ or... 

The dehydration of vermiculite crystals (267 to 290 K) was studied [156] at low pressure using a quartz crystal microbalance. A detailed theoretical discussion of reactions controlled by diffusion processes, within a layer-type reactant structure that does not reciystallize during reaction, is provided. Water removal is treated as gas diffusion within the semi-infinite crystal medium. A quasi-linear expression q)plied during the early stages of reaction of large crystals (f, = 27 kJ mol ) and the later decrease of the rate of mass loss was proportional to attributable to control... 

Walker (1956,1957) and van Olphen (1963) discussed the conditions of temperature and humidity at which partly-hydrated Mg-vermiculites exist in air. Walker interpreted differential thermal analyses (DTA) charts as indicating that interlayer water not associated with the hydration of adsorbed Mg ions is released at a lower temperature than is that around the Mg. Most of both of these types of interlayer water is released by 275°C. Vermiculite held under 63 MPa (10,000 p.s.i.) water vapor pressure shows its first dehydration at 550°C (Roy and Romo, 1957). 

Page et al. (1967), working with vermiculitic soils, observed that in the wet state, fixation of NH4 and K can be attributed almost exclusively to ion-exchange. Under dehydrated conditions, if the amount of NH4 and K added does not exceed the... 

The adsorption of crude oil was performed in dehydrated (expanded) and hydrophobized vermicuHte samples, denoted here as EV (expanded vermicuHte) and HV (hydrophobized vermiculite), respectively. Vermiculite samples with five different grain sizes were used 100-150, 150-200, 200-250, 250-325, and 325-400 mesh. The crude oil had the following properties density, 34.2 °API (at 15.6°C) and viscosity, 8.4 cP (at 37.8°C) ... 

The hydrated vermiculite samples were expanded (dehydration process) by heating them at 800°C for 30 min. The hydrophobization process was performed by heating the EV samples with camauba (Copernicia cerifera) wax to produce a 10%... 

For both, hydrated and dehydrated vermiculite samples, the wetting enthalpy values are the same AH, vett = 5.30 0.20Jg. This shows that at least for the considered reaction time (5.0 min), the dehydrated vermuclite does not behave as a hydrophilic matrix. This hypothesis is reinforced by the X-ray powder dif action patterns for contact times (between dehydrated vermiculite and water) of 24, 48, and 72 h, as shown in Fig. 8.5. 

As can be seen in Fig. 8.1a, the hydrated vermiculite sample exhibits its 001 diffraction peak at 6.12°, corresponding to an interlayer distance of 1.44 nm. On the other hand, the dehydrated sample. Fig. 8.1b exhibits the 001 diffraction peak 9.40°, corresponding to an interlayer distance of 0.92 nm. However, in such a diffractogram, the 001 diffraction peak of the hydrated sample could still be observed, showing that the dehydration process was not complete. 

The diffraction patterns of the dehydrated samples dispersed in water for 24, 48, and 72 h are shown in Figs. 8.5c—8.5e, respectively. As can be verified, the intensity of the 001 diffraction peak of the hydrated samples enhances in intensity for largest contact times. However, even after 72 h, less than half of the sample was rehydrated. So, both data sets, calorimetric and XRD, show that the dehydrated vermiculite does not behave as a hydrophilic compound, with its total surface, that is, the external and internal space of the lamella, exhibiting a very low affinity toward water molecules. This indicates that besides being unfavorable from a thermodynamic point of view (endothermic), the rehydration process, that is, adsorption of water molecules on the surfiice, is kinetically slow. 

As can be seen by comparing the SEM micrographs shown in Fig. 8.6, the dehydration process affects the microstructure of vermiculite. After dehydration, the... 

Figure 8.5 X-ray diffraction patterns for hydrated vermiculite (a), dehydrated vermiculite (b), and dehydrated vermiculite in 24 h (c), 48 h (d), and 72 h (e). The symbols ( ) and ( ) are associated with the 001 diffraction plane of vermiculite in the hydrated and dehydrated samples, respectively.

Figure 8.6 SEM micrographs for hydrated vermiculite,1500x (a) and dehydrated vermiculite, 750x (b).

Micas are also aluminosilicates. For example, in muscovite, KAl2[AlSi30io] (0H)2, the extra cations are located between the aluminosilicate anionic sheets. This material can be split up into sheets so thin that 1,000 are needed to make a pile only 1 inch high. Vermiculite is a hydrated mica that splits off into soft flakes, scales, and layers when dehydrated. It makes an excellent packing material or soil conditioner. A variety of materials such as clays, feldspars, and talc are also aluminosilicates, as is the absorbent used as cat litter. 

Laboratory studies of vermiculites under hydrothermal conditions were made by Roy and Romo [1955,1957]. Under 10,000 Ib/in. water pressure, they observed partial dehydration at 550°C and only nonexpanding structures above 650°C. At 300°C, a migration of Mg from octahedral sites in the silicate layers to interlayer positions occurs, the product being a chloritelike phase. They conclude that no primary vermiculite could have crystallized under even mild hydrothermal conditions and that the mechanism of its formation is by the low-temperature alteration of mica and chlorite. 

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