Silicon self-diffusion in quartz and diopside measured by nuclear micro-analysis methods

Diffusion coefficients of silicon have been measured for synthetic quartz and natural diopside, using two micro-analysis techniques, Rutherford backscattering spectrometry (RES) and nuclear reaction analysis (NRA). Diffusion experiments on quartz have been performed between 1350 degrees C and 1600 degrees C for durations of a few days to a few minutes, at high pressure (2GPa) in the beta-quartz field, as well as at atmospheric pressure on metastable beta-quartz. Both experiments give approximately the same results and no pressure effect on silicon diffusion is observed. We find the following diffusion coefficient for silicon in synthetic quartz: D-(cm2 s-1) = 2.9 x 10(7)exp(-746 KJ mol(-1)/RT) In the case of diopside, we have used a natural gem-quality single crystal. Annealings were performed between 1040 degrees C and 1250 degrees C for durations ranging from 1 month to 6 days under controlled atmosphere. From these preliminary results, we obtain ((cm2 s-1)) = 2.3 x 10(-10)exp(-211 KJ mol(-1)/RT) which is not corrected for an eventual pO(2) dependence. On the basis of our results, we propose an interstitial diffusion mechanism for silicon atoms in quartz based on Frenkel pairs of defects, and we suggest also the possibility of such an interstitial mechanism occurring in diopside.