Tridymite derivatives

In addition to the three principal polymorphs of siUca, three high pressure phases have been prepared keatite [17679-64-0] coesite, and stishovite. The pressure—temperature diagram in Figure 5 shows the approximate stabiUty relationships of coesite, quart2, tridymite, and cristobaUte. A number of other phases, eg, siUca O, siUca X, sihcaUte, and a cubic form derived from the mineral melanophlogite, have been identified (9), along with a stmcturaHy unique fibrous form, siUca W. 

Triehlorotriethylamine-hydroehloride see Trichlormethine) Tj-Trichothecene see Toxins derived from Fusarlum sporotrichioides) Tridymite see Crystalline siliea)... 

Others have examined the geometry of the chromate and dichromate ions and compared then to models of the silica surface (12). Again, the uncertainty is high because we have very little idea what the silica surface really looks like. Hydroxyl spacings derived from a particular face of cristobalite or tridymite may have little relevance to amorphous, high surface area silica gel. 

The fact that silsesquioxane molecules like 2 7 contain covalently bonded reactive functionalities make them promising monomers for polymerization reactions or for grafting these monomers to polymer chains. In recent years this has been the basis for the development of novel hybrid materials, which offer a variety of useful properties.3,4 This area of applied silsesquioxane chemistry has been largely developed by Lichtenhan et al.4 With respect to catalysis research, the chemistry of metallasilsesquioxanes also receives considerable current interest.1,2,10,21 As mentioned above, incompletely condensed silsesquioxanes of the type R7Si7Og(OH)3 (2-7, Scheme 4) share astonishing structural similarities with p-tridymite and p-cristobalite and are thus quite realistic models for the silanol sites on silica surfaces.1,2,21 26 Metal complexes derived from 2-7 are therefore commonly regarded as realistic models for industrially important metal catalysts immobilized on silica surfaces.1,2,22 It is... 

Derived 4 2 structures Si02 (tridymite), ice-I Si02 (keatite), ice-III... 

In contrast to the other types of tetrahedron linkage already described, the three-dimensional frameworks commonly exist in a number of entirely different forms which give rise to several quite distinct families of petrologically important minerals. A few silicates have structures based on the frameworks found in the various forms of silica. Thus the structures of kalsilite, K(AlSi)04, and nepheline, Na3K(Al4Si4)016, can be regarded as derived from that of tridymite by the replacement of one-half of the silicon ions by aluminium and the introduction of the appropriate number of alkali ions to restore neutrality. The structures of carnegeite, Na(AlSi)04, and eukryptite, Li(AlSi)04, are similarly related to those of cristobalite and quartz, respectively. It will be noted that in all these minerals aluminium is, of course, always present and that the 0 (Si, Al) ratio is always 2 1. 

The data for the NaP03-Si02 system are given in Table IX. Crystalline phases of all the samples are composed of a-cristobalite and trisodium trimetaphosphate. Some of them contain small amounts of or-tridymite. Ortho-, di-, and triphosphate were found by paper chromatography in the aqueous solutions of the samples. These ortho- and chain phosphates may be derived from amorphous substances, because the x-ray diffraction patterns of these samples do not exhibit any peaks attributable to phosphates other than trisodium trimetaphosphate. However, the structures of these amorphous substances cannot be determined from the data presented here. It has been shown that the crystals of neither of the systems contain P-O-Si linkages. 

When the divalent cation becomes too large for the spinel structure to be adopted (e.g. Ca +, Ba +, and Pb +), a tridymite (SiOg) structure is obtained that has the divalent cation in the interstices (see Silicon Inorganic Chemistry). These are synthesized by reaction of the oxides at temperatures over 800 °C. The barium and calcium derivatives are used in ceramics and refractories. In particular, BaAl2 04 is used... 

Cristobalite (SiO ) / tridymite (8102) derived, respectively, from a network of silicon atoms, zinc-blende, or wurtzite, with one atom of oxygen situated in the middle of each Si-Si segment BeF2 of cristobalite structure. 

Interstitial (or stuffed) derivatives, examples are Re03 - CaTIOs, Si02 (tridymite) KNa3(AISi04)4 (nepheline), and... 

The most important stuffed derivatives of tridymite fall in the binary system NaAlSiO -KAlSiO. The sodium end-member is nepheline (NaAlSiO ), and the potassium end-member, kalsilite. Natural nepheline occurs at the Na KAl Si O g point, 25% of the way from soda-nepheline to kalsilite. Solid solution has been observed between natural nepheline and Na3(AlSi04)4. Experimental solid solutions in the nepheline area are shown in Fig. 1-16, which also illustrates the framework silicate feldspar and leucite solid solutions. The effect of temperature on nepheline and kalsilite solid solutions is shown in Fig. 1-17,... 

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