Computer Science
Scientific paper
Apr 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.1595h&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 61, Issue 8, p. 1595-1611.
Computer Science
6
Scientific paper
Measured H and Na concentration depth profiles in albite samples dissolved at 300°C at various pH conditions (Hellmann et al., 1997-Part III) are indicative of the complex nature of diffusion within leached/H-enriched layers. A qualitative comparison between the measured profiles and profiles based on various diffusion models reveals that the inward diffusion of H species and the outward diffusion of Na are not independent, but rather are interrelated by- an interdiffusion process that can be modeled with a single interdiffusion coefficient ~D. The coefficient ~D varies as a function of the concentration of either H or Na and is thus dependent on depth. The proposed interdiffusion model is based on rates of Na diffusion that are up to several orders of magnitude faster than H diffusion (DNa/DH ≫ 1), this being in accord with direct diffusion measurements from the glass dissolution literature. Modeling results reveal that the rate of H diffusion is one of the most important parameters in determining the depths of leached/ H-enriched layers. Based on a qualitative comparison between the measured profiles and the interdiffusion model, a lower limit of DH ≥ 10-13cm2s-1 can be estimated for leached/H-enriched layers created at acid pH (3.3-3.4) and 300°C. Depending on the estimated value of DNa/DH, this corresponds to DNa ≫ 10-13cm2s-1. The use of a structural factor with ~D imparts an even greater concentration dependence on the interdiffusion coefficient. Increasing the value of the structural factor has the effect of greatly increasing the depth of leaching/H enrichment for any given set of constant DH and DNa/DH values. Irreversible chemical reactions which result in the uptake of H species, such as framework bond hydrolysis reactions, are also potentially important in correctly modeling diffusion of leached/ H-enriched layers. Increasing the rate of reaction acts as a damping factor on steady-state diffusion profiles. Chemical reactions within leached/ H-enriched layers potentially necessitate the addition of a chemical reaction term to the applied diffusion model in order to avoid an underestimation of diffusion rates.
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