ANISOTROPY OF OXYGEN DIFFUSION IN DIOPSIDE

Abstract

18O diffusion coefficients have been measured by nuclear reaction analysis (NRA) in Fe-free synthetic diopside single crystals along the three crystallographic directions and in Fe-bearing natural diopside single crystals along the c direction at room pressure in the range 1050-1370°C and under controlled oxygen partial pressure (10-3-10-12 atm). Diffusion along a and c crystallographic directions is one order of magnitude faster than along b direction. Diffusion along c in natural diopside is about two times faster than in the synthetic sample. The activation energy along b is 323+/-27 kJ mol-1 and diffusion is insensitive to oxygen fugacity. For a and c directions activation energies are around 250 kJ mol-1 and the diffusion coefficients are slightly dependent on fO2 (~fO20.04). We suggest that the observed diffusion anisotropy is related to the oxygen diffusion paths within the crystallographic structure that prefer the under-bonded O2 oxygen sites. We propose a single law to describe diffusion along the two fast crystallographic directions a and c in diopside for natural conditions close to the QFM buffer: log Da,c (m2 s-1)=-10.0+/-0.6-(259+/-15 kJ mol-1)/2.303 RT, and Db#Da,c