Layer silicates, dehydroxylations

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... 

During the ablation experiment, temperature within the char layer exceeds 1000°C and approach 2000-2500°C at the surface. At these temperatures, any carbonaceous residue from the pol3oner will contain graphite. Additionally, mica-type layered silicates, such as montmorillonite, irreversibly transform into other aluminosilicate phases. Between 600 and 1000 C, montmorillonite dehydroxylates and has been observed to initially transform into spinel, cristobolite, mullite and/or pyroxenes (enstatite) (24). At temperatures greater than 1300 C, mullite, cristobolite and cordierite form and subsequently melt at temperatures in excess of 1500 C (mullite 1850 C, pure cristobolite 1728°C and cordierite --ISSO C) (25). The presence of an inorganic that transforms into a high viscosity melt on the surface of the char will improve ablation resistance by flowing to self-heal surface flaws. This is known to occur in silica-filled ablatives (26). 

Fig. 14. Compensation plot for dehydroxylation of kaolinite ( ) and other layer-type silicates (X = montmorillonite, illite and muscovite) data and sources given in Table 11. (Redrawn, with permission, from Advances in Catalysis, ref. 36).

The thermal stability of these materials has been previously studied by Miyata (2) and Reichle ( 3). Upon heating, below 200°C, the Interlayer water is lost. Between 250°C and 500°C the materials dehydroxylate accompanied by the decomposition of the anion. Reichle (3) has shown that this process Is reversible up to 600°C, and takes place without exfoliation of the layers while maintaining the morphological structure of the hydroxide. In their layered form, these hydroxides have found wide use as anion exchangers (4 ), and sorbents for various hydrocarbon molecules (5) and water. Intercalation of heteropoly anions ( 6) and polymerized bidimensional silicate anions into the interlayer has also been reported ( 7). 

Thermogravimetric curves for leuchtenbergite, clinochlore, and ripidolite are reproduced in Figure 30. The first two show the two-step system corresponding to dehydroxylation of the hydroxide and silicate layers, respectively. For leuchtenbergite, these commence at 500 and 725°C and for clinochlore at 530 and 800°C. Ripidolite, however, shows only one step covering the range 500 to 900°C, because dehydroxylation of one layer is not complete before that of the other commences. 

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