SILICON SELF-DIFFUSION IN QUARTZ AND DIOPSIDE MEASURED BY NUCLEAR MICROANALYSIS METHODS

Diffusion coefficients of silicon have been measured for synthetic qua rtz and natural diopside, using two micro-analysis techniques, Rutherf ord backscattering spectrometry (RES) and nuclear reaction analysis (N RA). Diffusion experiments on quartz have been performed between 1350 degrees C and 1600 degrees C for durations of a few days to a few minu tes, at high pressure (2GPa) in the beta-quartz field, as well as at a tmospheric pressure on metastable beta-quartz. Both experiments give a pproximately the same results and no pressure effect on silicon diffus ion is observed. We find the following diffusion coefficient for silic on in synthetic quartz: D-(cm2 s-1) = 2.9 x 10(7)exp(-746 KJ mol(-1)/R T) In the case of diopside, we have used a natural gem-quality single crystal. Annealings were performed between 1040 degrees C and 1250 deg rees C for durations ranging from 1 month to 6 days under controlled a tmosphere. 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 eventu al pO(2) dependence. On the basis of our results, we propose an inters titial diffusion mechanism for silicon atoms in quartz based on Frenke l pairs of defects, and we suggest also the possibility of such an int erstitial mechanism occurring in diopside.