Beyond the obvious fact that in many of the silicate subtypes they play an ionic 'glue' like role by bonding the tetrahedra units together; how do cations control the structure of a silicate and how does it vary between different cations and silicates?
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1$\begingroup$ An answer to this question would be a book with chapters about mineralogy, chemistry, physics and crystallography. I think it's better to edit and focus the question a bit more, also to clarify it a bit. $\endgroup$– GimelistNov 17, 2015 at 5:07
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I would say that cations would control the structure in several ways; in the case of aluminum octahedra in aluminosilicates, for example, they might completely influence it by causing the presence of completely separate alternating layers of bonded octahedra. The octahedra may even dominate the unit-cell, appearing in a larger numerical and volumetric capacity than the tetrahedra.
Additionally, the different bonded cations have different polarities, and by extension, exert different forces on the relevant bonds; this plays a huge role in determining how the structure responds under conditions of high pressure relevant to the Earth's interior, e.g determining where shearing takes place, etc.
