Anisotropy of oxygen diffusion in diopside

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