DIFFUSION IN SINGLE CRYSTALS OF MELILITE: III. DIVALENT CATIONS IN GEHLENITE

Abstract

Diffusion coefficients of cations, Ni, Co, Mn, Ca, Sr, and Ba, which are possibly in divalent state, and Fe in synthetic single crystals of gehlenite (Ca2Al2SiO7), which is one of the endmembers of melilite, were determined using 63Ni, 60Co, 54Mn, 45Ca, 85Sr, 133Ba, and 59Fe radioactive tracers in the temperature range from 1000 to 1300°C in the conventional nitrogen atmosphere. The tracer-diffusion coefficients are lower in gehlenite than in åkermanite for Fe, Mn, Ca, Sr, and Ba, as expected from the melting relations of these minerals, being consistent with O self-diffusion coefficients. The tracer-diffusion coefficients of Ni, on the other hand, are higher in gehlenite by about an order of magnitude, and those of Co are about the same in both minerals. The activation energy for diffusion of Ni is larger for gehlenite than for Akermanite, possibly indicating these cations, at least in some part, diffuse through the T1 site in the gehlenite structure. The mechanism of diffusion of the intermediate-sized cations in concentrations at the tracer level is apparently different from that at the larger concentrations in which Al3+ + Al3+ − Co2+ + Si4+ type paired interdiffusion is required. The overall trend of diffusion coefficient against ionic radius is similar for gehlenite and åkermanite.