DIFFUSION IN SINGLE CRYSTAL OF MELILITE: INTERDIFFUSION OF AL + AL VS. MG + SI

Abstract

Interdiffusion coefficients of Al + Al vs. Mg + Si in the gehlenite–åkermanite system of melilite were determined by coupled annealing of synthesized end-member single crystals. The observed diffusion coefficients for a couple-annealed sample vary for about 2 orders of magnitude, showing strong dependence on the gehlenite–åkermanite composition: diffusion coefficient observed at 1350 °C, for example, is 3 × 10−13 cm2 s−1 at 5 mol% åkermanite composition (Ak5), increases to 2 × 10−11 cm2 s−1 at Ak80, and then decreases to 1 × 10−12 cm2 s−1 at Ak95. The diffusion coefficient–temperature relation indicates high activation energy of diffusion of about 420 kJ mol−1 for gehlenite-rich melilite. The observed diffusion coefficient–composition relation may be explained by a combination of (1) the diffusion coefficient–melting temperature relation (Flynn's rule) and (2) the feasibility of local charge compensation, which can possibly be maintained more easily in the intermediate chemical composition. The high activation energy value for gehlenitic melilite appears to correspond to the complex diffusion mechanism. The observed highly variable diffusion coefficients suggest that gehlenite–åkermanite zoning in the melilite crystals in Ca, Al-rich inclusions in the carbonaceous meteorites may provide a sensitive indicator for the thermal history of the inclusions.