O-18 DIFFUSION THROUGH AMORPHOUS SIO2 AND CRISTOBALITE

Secondary ion mass spectrometry was used to profile the diffusion of o xygen in polycrystalline beta-cristobalite and vitreous SiO2. The trac er concentration profiles of cristobalite are consistent with a model based on two mechanisms: bulk and short-circuit diffusion. The profile s of partially crystallized samples containing vitreous SiO2 and beta- cristobalite were fitted using the sum of two complementary error func tions and taking account of some interstitial-network exchange. The bu lk oxygen diffusivity, in the temperature range 1240-1500-degrees-C, i s about five times greater for vitreous silica than for beta-cristobal ite.