Kenyaite-layered silicates

Figure 6 A conq)arison betweeen the activities of two different layered silicates, magadiite and kenyaite, as a fimction of their interlayer species. Alumina pillared kenyaite appears to have significantly more activity than a comparable pillared magadiite. This indicates a sensitivity towards the layered structure.

In order to systematically change the properties of layered silicate materials, we have investigated the possibility of isomorphous substitution of silicon by aluminum or boron. It is demonstrated that addition of horon and/or aluminum compounds to the reaction mixture leads directly to boron and aluminum containing layered materials in a hydrothermal crystallization process without further treatment. The layered materials obtained were identified as hectorite types, magadiite and kenyaite-like types. The isomorphous substitutions were proven by "B, Al, and Si solid state NMR spectroscopy. 

The hydrous layer silicates are a large family of materials containing two-dimensional tetrahedral silica layers where the individual tetrahedra are linked by the sharing of three comers to form a hexagonal sheet. This family includes such natural minerals as magadiite, kenyaite, makatite, and kanemite, as well as synthetic... 

TABLE 11-2 Phase components in dependence on the concentration of PDDA-Cl (P40) and on the crystallisation time. The crystallization temperature is 175 C. The composition of the reaction mixture is 4 Na O 1 AljOj 40 SiOj 1200 H2O x PDDA-Cl. The phase component were denoted as a amorphous, MFl zeolite MFI, Mo mordenite, SH-P40 novel layer silicate, Ke kenyaite-like silicate, Cr cristobalite, and Q quartz. 

Finally, the acid- or base-catalyzed reaction of hydrolysis and condensation polymerization of TEOS into a layered silicate gallery could affect the physical properties of silica-pillared magadiite and kenyaite. The samples that were silica-pillared by acid- and base-catalyzed reactions show a large increase in basal spacing. Also, they exhibit relatively narrow pore size distributions and show high surface areas, depending on the type of catalyst and layered silicate. These results indicate that variations in the conditions of gelation contribute to the improvement in the physical properties of silica-pillared molecular sieves. 

Layered silicic acids Kanemite, makatite, oaosilicate, magadiite, kenyaite, and layered organo-silicates... 

Figure 9.8 (A) Clayey and calcareous diatomite from northern Lake Chad. Various sedimentary layers can be seen they correspond to transition from a lacustrine environment (bottom) to a palustrine environment (top), from sub-arid to arid conditions. (Photograph courtesy of Professor A. Durand.) (B) Spherule-like crystals of kenyaite (hydrous sodium silicate) precipitated in apolyhaline interdunal ponds, Lake Chad.

The dimension of cross-linking (e.g., 1-, 2-, or 3-dimensional), the extension (as applies to, e.g., the inosilicates), and the number of cross-linked elements (e.g., 2 for double layers or double chains) are identified by the anion complex notation as proposed by Liebau [16]. The symbol 2/< of the anion complex of the metal silicate hydrates describes the two-dimensional cross-linking of the silicon oxygen tetrahedra to tetrahedral sheets. Following an older model, one, two, three, and five tetrahedral sheets are connected to form the bulk layer of kanemite or makatite, ilerite, magadiite, and kenyaite. 

Due to the small size of crystallites and the small number of sharp diffraction lines in the X-ray diffraction pattern, only a few crystal structure determinations have been published. The number of distinct reflections decreases from kanemite to kenyaite, i.e., with increasing thickness of the bulk layer and increasing SiOi/ NaiO ratio (Fig. 4). Only the crystal structures of kanemite [2], makatite [19], RUB-18, an ilerite-type silicate [20], the lithium sodium silicate silinaite [21], and the boron-containing mineral searlesite [22] thus far have been solved. 

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