Tracer diffusion of Mg, Ca, Sr, and Ba in Na-aluminosilicate melts

We employed the thin source technique to investigate tracer diffusion of Mg, Ca, Sr, and Ba in glasses and supercooled melts of albite (NaAlSi₃O₈) and jadeite (NaAlSi₂O₆) compositions. The experiments were conducted at 1 bar and at temperatures between 645 and 1025°C. Typical run durations ranged between 30 min and 35 days. The analysis of the diffusion profiles was performed with the electron microprobe. Diffusivities of Ca, Sr, and Ba were found to be independent of either duration t of the experiment or tracer concentration M, initially introduced into the sample. Mg exhibits a diffusivity depending on run time and concentration and tracer diffusivity is derived by extrapolation to M/√t = 0. Temperature dependence of the diffusivity D can be represented by an Arrhenius equation D = D_o exp(−E_a/RT), yielding the following least-squares fit parameters (with D in m²/s and E_a in kJ/mol): D_Mg = 1.8 · 10⁻⁵ exp(−234 ± 20/RT), D_Ca = 3.5 · 10⁻⁶ exp(−159 ± 6/RT), D_Sr = 3.6 · 10⁻⁶ exp(−160 ± 6/RT), and D_Ba = 6.0 · 10⁻⁶ exp(−188 ± 12/RT) for albite; and D_Mg = 8.3 · 10⁻⁶ exp(−207 ± 18/RT), D_Ca = 3.8 · 10⁻⁶ exp(−153 ± 4/RT), D_Sr = 2.3 · 10⁻⁶ exp(−150 ± 4/RT), and D_Ba = 3.7 · 10⁻⁵ exp(−198 ± 4/RT) for jadeite composition. Ca and Sr diffusivities agree within error in both compositions and exhibit the fastest diffusivities, whereas Mg reveals the lowest diffusivity. The relationship between activation energy and radius shows a minimum at Ca and Sr for albite and jadeite compositions extending the relationship already observed elsewhere for alkalies. With increasing substitution of Si by (Na + Al), diffusivities increase, whereas activation energies decrease. Furthermore, a simple model modified from that of Anderson and Stuart (Anderson O. L. and Stuart D. A., “Calculation of activation energy of ionic conductivity in silica glasses by classical methods,” J. Am. Ceram. Soc.37, 573-580, 1954) is discussed for calculating the activation energies.