PREMELTING AND HIGH-TEMPERATURE DIFFUSION OF CA IN SYNTHETIC DIOPSIDE- AN INCREASE OF THE CATION MOBILITY

A study of Ca self-diffusion along the b axis in synthetic (iron free) diopside single crystal was performed at temperatures ranging from 12 73 K to 1653 K. Diffusion profiles of Ca-44 were measured using alpha- particles Rutherford Backscattering (alpha-RBS) micro analysis. We una mbiguously find two distinct diffusional regimes, characterized by act ivation enthalpies H = 280 +/- 26 kJ/mol and H = 951 +/- 87 kJ/mol at temperatures lower and upper than 1515 K, respectively. This change of diffusion regime takes place near the onset of premelting as detected in calorimetric measurements and can be interpreted in terms of enhan ced formation of Frenkel point defects with an activation enthalpy of formation of 1524 +/- 266 kJ/mol (H-f/2 = 762 kJ/mol), in accordance w ith our high-temperature diffusion data. If premelting of diopside is actually related to Ca-Frenkel point defect concentration, this concen tration could reach up to few mole percents close to the melting tempe rature.