Stuffed silica structures

Silica polymorph Stuffed silica structure Reference 

The earth s crust is composed almost entirely of silicates and silica, which constitute the bulk of all rocks and of soils, clays, and sands, the breakdown 

Element % weight) % atomic) Radius (A)(0) % volume) Element % weight) 

All other elements are present in smaller amounts All others less  

Since there are a number of ions of similar size which can occupy, for example, dctahedral holes (i.e. coordination number 6), and since atoms of many kinds were present in the melts or solutions from which the minerals crystallized, isomorphous substitution is very common in the silicates, which rarely have the ideal  

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Silica polymorph Stuffed silica structure Reference... 

The substitution of aluminum for silicon in a silica covalent network leads to a charge unbalance, which must be compensated by extra-framework cations, mostly alkaline. This occurs in the cases of the so-called stuffed silicas these materials have structures strictly related to the crystalline forms of silica, but with cations in the interstices to counterbalance the presence of A1 ions substituting for Si. This is the case, for example, of Eucriptite (LaAlSi04, a stuffed /3-quartz) or nepheline (NaAlSiOa, a stuffed tridymite). A similar mechanism also occurs in the amorphous networks of glasses [175]. 

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