Structure and energetics of vitreous and crystalline tectosilicates

The energetics of complex framework silicates is related to their structure and composition in a systematic fashion. This is illustrated by thermochemical data for crystalline silicas including zeolites and the silica polymorphs, and for amorphous silicas, including glasses, CVD and sol-gel materials. Relative to quartz as the stable crystalline polymorph and bulk glass as the most stable amorphous form, enthalpy increases with both increasing density (coesite, stishovite and pressure or radiation densified glasses) and with decreasing density (tridymite, cristobalite, zeolitic silicas, sol-gel and CVD amorphous structures). Preliminary data suggest the most open structures (e.g. faujasite approaching SiO₂ in composition) are not as destabilized as their large molar volume would imply, suggesting that large pores, with normal framework density between them, are more stable than a relatively uniform expansion of structure. Coupled substitution: Si⁴⁺ = (l/n) Mⁿ⁺ + T³⁺ stabilizes tectosilicates. Stabilization increases in the order M = Mg, Ca, Sr, Ba, Li, Na, K, Rb, Cs and in the order T = B, Fe³⁺, Ga, Al, that is, with increasing basicity of the nonframework cation and with increasing similarity to Si of the cation substituting in the framework. These trends are discussed in terms of structure and bonding.