Effects of the exchangeable alkali metal ions on the thermal behavior of magadiite and kenyaite

Abstract

The cation exchanged magadiite and kenyaite were prepared by exchanging the interlayer Na ion of synthesized samples with a series of alkali metal cations, and the effects of the exchangeble cations on the thermal behavior of magadiite and kenyaite were investigated in the temperature range of 200 to 1100°C.
Dehydration of the constitution water occurred in a stepwise fashion. On the first stage the adjacent silanol groups on the same layer surface condensed to form siloxane bonds and the resulting water was evolved. The dehydration temperature did not affect any cations. On the second stage the dehydration took place with the condensation reaction between the two silanol groups on the surfaces located opposite of each other. A three-dimensional network structure was consequently formed, and their dehydration temperatures increased with the increasing radii of the cations. After the dehydration, Li and Na ions resulted in the formation of quartz, and K, Rb andCs ions resulted in the formation of cristobalite. On the process of heating, these crystal phases were finally transformed into tridymite, and the rate of the polymorphic transition was decreased in the order Li>Na>K>Rb> Cs.
The principal results are summarized as follows. The interlayer cation species influenced (1) the dehydration temperature of the constitution water: 300^-700°C, (2) the thermostability of the layered structure (temperature of destruction of layered structure): 600-850°C, (3) the recrystallization temperature and the types of resulting crystal phase: 600-900°C, (4) the polymorphic transition temperature: 600-1100°C, and (5) the kinetics for polymorphic transition: 800°C. It was postulated that the fluctuations for such thermal behavior were cased by the difference in ionic radius, electric charge density, diffusion coefficients, and breaking of=Si-O-Si=bond of the cations.