Keatite 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. 

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

As in the case of P-quartz, the Li ion can also be replaced by Mg in P-spodumene, a stuffed derivative of keatite. The degree of substitution, however, is lower than that of P-quartz. 

The high-quartz and keatite-type alumino-silicates are such stuffed derivatives. Both types of framework are relatively dense, compared with, for example, those of tridymite and cristobalite. Therefore, only small univalent or divalent M ions can be accomodated in their cavities, Li ions being typical. Indeed, solid solutions of the composition LiAlSi 02x4-2 exist with both types of framework over wide ranges of x. 

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