Layer silicates, dehydrations

When these minerals are heated they dehydrate in a remarkable way by extruding little wormlike structures as indicated by their name (Latin vermiculus, little worm) the resulting porous light-weight mass is much used for packing and insulation. The relationship between the various layer silicates is summarized with idealized formulae in Table 9.10 (on page 357). 

In the case of 2 1 layer silicates, Davidtz showed that for butyl alcohol dehydration, the active sites were the tetrahedral and not the octahedral sites, and catalyst activity was observed to be a hnear function of tetrahedral-surface charge-density. 

The dehydration of hydrous layer silicates is often deceleratory throughout because water release is diffrision controlled and may be reversible. Arrhenius parameters are often sensitive to prevailing conditions, notably the availability of water vapour. 

Muller F, Drits VA, Plangon A, Robert J-L (2000a) Structural transformations of 2 1 dioctahedral layer silicates dnring dehydroxylation-rehydroxylation reactions. Clays Clay Minerals 48 572-585 Muller F, Drits VA, Tsipursky SI, Plangon A (2000c) Dehydration of Fe, Mg-rich dioctahedral micas. (II) cation migration. Clay Mineral 35 505-514... 

The mechanism of dehydration of layer silicates [51] is believed to differ from the mechanism of dehydration of crystalline hydrates and this has been used to explain the observed differences in parameters E (in vacuum) and in decomposition temperatures. This view is reflected, in particular, in the fact that the special term dehydroxylation is used in the literature to describe clay dehydration. However, this belief is not justified. As can be... 

Many of the layered silicates retain their structures on dehydration. Thus, pyrophyllite Al2(0H)2(Si205)2 forms a well-ordered relict structure Al2(OD)-(Si20s)2 [1030b]. 

Recent applications of the dynamic diffraction experimental techniques were mostly in the areas of temperature-induced dehydration processes in natural zeolites, hydrothermal crystallization of aluminosilicate and aluminophosphate microporous materials, thermal decomposition of layer silicate minerals, high-temperature synthesis of advanced ceramics, and hydrothermal ion-exchange and conversion processes in synthetic molecular sieves. The time-resolved powder diffraction patterns relative to the isothermal nucleation and growth process of zeoUte-A by the hydrothermal treatment of activated metakaollnite is shown in Figure 6. 

Ikeda and coworkers synthesized a novel silicate zeolite CDS-1 (CDO) with a cylindrically double saw-edged framework in 2004 [114]. CDS-1 was converted by dehydration-condensation between the layers of pentagonal cylinder-layered silicate (PLS-1). Then in 2008, Okubo and coworkers synthesized novel silicate sodalite (SOD) by topotactic conversion [119]. Another example is shown by Gies and coworkers with the preparation of silicate zeolite RUB-41 (RRO) [109]. By studying the synthesis of hydrous-layered silicates carefully, they used dimethyldipropylammonium hydroxide as the organic SDA to synthesize the layered silicate precursor (RUB-39), and RUB-41 was synthesized after the calcination of RUB-39. 

The ice-ocean model Europa s core consists of dehydrated silicates, since heat production made dehydration possible. Around the core, there is a thick layer of liquid water (about 100 km), and above that a thin layer (about 10 km) of water ice. 

The thick ice model enough heat was generated in the interior of the moon to dehydrate the silicates. The water set free froze to give an ice layer about 100 km thick. 

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. 

Magnesium silicate is used against juvenile and adult store product pests, exerting its lethal activity predominantly on juvenile and adult forms by sorption of the cuticular lipid layer, thus causing dehydration of the insects. 

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