Compositional Ranges and Stability

In the general formula of the alumino-silicates, MAlSia,023 +2, both the type of cations M and the parameter x can vary considerably. Furthermore, the Si02 content can be replaced by AIPO4 to some extent. Finally, limited substitution of Al in the aluminate component MAIO2 by other small trivalent or even bivalent atoms has also been reported. 

Because of their relatively poor space filhng, the high-quartz-type and keatite-type alumino-silicates have ranges of thermodjmamic stability only at low pressure and elevated temperatme. Large compositional areas are not 

Further components, such as Li2[Zn02], Li2[Be02], or even Ali/3[A102], have been reported as possible substitutions for Li[A102] in metastable h-quartz s.s., in concentrations between 10 and 25mol% [2.14]. 

In fact, the keatite structure appears to be much more flexible than the high-quartz structure. This has been shown by low-temperature ion exchange 

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