Na AlSi

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. 

Nepheline, Na(AlSi)04, has a structure very similar to that of tridymite, the positions of the Si and O atoms corresponding closely. Nepheline may, in fact, be regarded as two-unit formulae of tridymite, Si204, in which one of the Si atoms is replaced by Al, and the deficiency in valency is made up by the addition of Na. The Na atoms fit into the large holes in the tridymite framework (see Figure 7a). 

Ncphe/ine syenite is a rock containing the mineral nepheline, Na(AlSi)04, the structure of which has already been discussed. This rock, which is found only in the IJ.S.A. and Scandinavia, has usually been purified to some extent before being put on the market. Chemical analyses of some Norwegian nepheline syenites are given in Table 17. Note the high alkali and low iron contents. 

AlSi-MCM41 Diflunisal [48], Naproxen [48], Ibuprofen [48], Ibuprofen Na [48]... 

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