Anisotropy of oxygen diffusion in diopside

O-18 diffusion coefficients have been measured by nuclear reaction analysis (NRA) in Fe-free synthetic diopside single crystals along the three crysta llographic directions and in Fe-bearing natural diopside single crystals al ong the c direction at room pressure in the range 1050-1370 degreesC and un der controlled oxygen partial pressure (10(-3)-10(-12) atm). Diffusion alon g a and c crystallographic directions is one order of magnitude faster than along b direction. Diffusion along c in natural diopside is about two time s 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 a nd c directions activation energies are around 250 kJ mol(-1) and the diffu sion coefficients are slightly dependent on fO(2) (approximate to fO(2)(0.0 4)). We suggest that the observed diffusion anisotropy is related to the ox ygen 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 nat ural conditions close to the QFM buffer: log D-a,D-c (m(2) s(-1)) = -10.0 +/- 0.6-(259 +/- 15 kJ mol(-1))/2.303 RT, and D-b much less than D-a,D-c (C) 2001 Elsevier Science B.V. All rights reserved.