Computer Science
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.3253c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 60, Issue 17, pp.3253-3265
Computer Science
13
Scientific paper
Rutherford backscattering spectroscopy, in conjunction with optical and scanning electron microscopy, has been used to characterize the oxidation process in a homogeneous, well-annealed glass prepared from a nepheline-normative olivine basalt. Initially melted and annealed at an oxygen fugacity substantially below the quartz-fayalite-magnetite (QFM) buffer, the glasses were oxidized in air under the time and temperature ranges 1-100 h and 550-600°C, respectively. Oxidation causes (1) formation of crystalline CaO and MgO that partially covers the free surface of the glass and (2) an internal reaction zone that is depleted of Ca 2+ and Mg 2+ but enriched in Na + The reaction morphology is uniquely consistent with a model in which oxidation occurs by the outward diffusion (to the free surface) of Ca 2+ and Mg 2+ that is charge compensated by an inward flux of electron holes (polarons): oxidation of the glass occurs as the oxygen/cation ratio increases, not by addition of oxygen, but rather by removal of cations. The flux of Na + from depth in the glass to the oxidizing region, which is also charge compensated by a counterflux of electron holes, is a response to the thermodynamic driving force seeking to stabilize Fe 3+ as a network former, consistent with equilibrium thermodynamic and spectroscopic studies. Growth of the oxidized/transformed glass follows parabolic (chemical-diffusion-limited) kinetics. Using a first-order, Wagnerian approach, the diffusion coefficient and driving force terms of the parabolic reaction-rate constant are separated, giving an average divalent cation diffusion coefficient of . The oxidation mechanism seen for the glass, that is, one dominated by diffusion of network modifying cations and not an oxygen species, is anticipated to also occur in iron-bearing aluminosilicate melts: the discrepancy between the kinetics of redox reactions and of oxygen tracer diffusion noted in the literature for melts is most likely explained in this way.
Cooper Reid F.
Fanselow John B.
Poker David B.
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