Diffusion in single crystals of melilite: II. Cations

Abstract

Diffusion coefficients of divalent cations, Ni, Co, Fe, Mn, Ca, Sr, and Ba, in synthetic åkermanite were determined by tracer-diffusion experiments. Diffusion coefficients of cation pairs, Al + Al-Mg + Si, Al + Al-Co + Si, and Mg-Co in melilite solid-solution systems, were determined by coupled annealing of synthetic melilites of endmember compositions. Cobalt tracer diffuses slower along the c- crystallographic axis than along the a- axis by a factor of about three, possibly due to the anisotropic sheet-like arrangement of oxygen ions parallel to the (001) plane which is perpendicular to the c- axis in the crystal structure of melilite. In the coupled diffusion experiments, the cation pairs interdiffuse complementary, as expected for conservation of material and ionic charge balance in the substitution. The tracer-diffusion coefficient of Co in åkermanite agrees well with that obtained by extrapolation of the inter-diffusion coefficient of Mg-Co cation pair to the composition of åkermanite 100-Co · åkermanite 0. This suggests the mechanism of tracer-diffusion of Co in åkermanite is essentially the same as in Mg-Co inter-diffusion. The inter-diffusion coefficients of cation pairs show a strong dependence on the endmember compositions, and can be correlated with the melting temperature or solidus curves in the phase diagrams. The observed trend in the inter-diffusion coefficient vs. melting temperature relations is similar to those observed for alloys and consistent with an empirical rule for self-diffusion coefficient-melting temperature relation. In the diffusion coefficient vs. ionic radius diagram for divalent cation diffusion in åkermanite, smooth curves were observed as in the case of olivine systems. However, the shapes of the curves were significantly different from those observed in olivine systems. The observed trend of diffusion coefficients with ionic radius can be explained on the basis of crystal structure-controlled diffusion in åkermanite. The observed results suggest that Mg diffusivity strongly depends on the chemical compositions of melilite solid-solution series: the estimated diffusivity of Mg in åkermanite is considerably larger than that of O, but is smaller than O in gehlenite.