Ionic mobility in stuffed-silica minerals
Using a combination of dielectric spectroscopy and atomistic computer simulation techniques, the dynamical behaviour of loosely bound cations in the stuffed-silica minerals nepheline, yoshiokaite and ß-eucryptite has been investigated. The investigation has been extended to include the feldspar minerals albite, K-feldspar and anorthite. The low-frequency dielectric properties of all of the minerals have been investigated from room temperature to 1100 K. At each temperature, the dielectric constant, conductivity and dielectric loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures distinct Debye-type relaxation processes were observed, from which activation energies were determined for each system studied. In order to rationalise these data, in the context of actual ionic mobility mechanisms, atomistic simulation.- techniques were used to elucidate the mechanisms and energetics of cation migration. Good correlation between experimentally determined and calculated energy barriers has been demonstrated. The results obtained from computer modelling confirm the nature of the processes responsible for the observed dielectric behaviour. Furthermore, they reveal the importance of framework relaxation effects in the facilitation of ion migration within a structure. This study demonstrates that short-range ionic mobility in framework silicates can be described using conventional (Debye-type) activated "hopping" models. A detailed interpretation of the mechanism of these processes has been possible by a combination of dielectric spectroscopy and computer modelling techniques. By applying the methods used here to a variety of mineral systems, it should be possible to develop a comprehensive body of transferable data relating structural aspects of framework minerals to the systematic prediction of ion migration mechanisms and energetics.