Hydrogen substitution in lithium-aluminosilicates

Abstract

Quartz and keatite are SiO₂ polymorphs with relatively dense frameworks containing small voids. Aluminosilicates with small charge compensating cations such as lithium, magnesium and zinc can adopt the same types of structures with the cations occupying the voids. It 1s shown that the lithium content in both quartz- and keatite-type (Li, Mg_0.5, Zn_0.5)AlSi₂O₆ solid solutions can be ion exchanged against hydrogen from hot concentrated H₂SO₄. This ion exchange can be reversed. At high temperatures water can irreversibly be driven off the hydrogen forms. Neither ion exchange nor thermal dehydration affects space group symmetry or framework linkage, but lattice parameter variations indicate considerable framework distortions. Thermal expansion also varies strongly with cation content. The data indicate that during dehydration aluminium ions leave the framework and enter the voids thereby providing charge compensation for the remaining framework aluminium content.[/p]