Dynamics of Oxidation of a Fe ²⁺ -Bearing Aluminosilicate (Basaltic) Melt

Abstract

Rutherford backscattering spectroscopy (RBS) and microscopy demonstrate that the approximately 1400°C oxidation of levitated droplets of a natural Fe2+-bearing aluminosilicate (basalt) melt occurs by chemical diffusion of Fe2+ and Ca2+ to the free surface of the droplet; internal oxidation of the melt results from the required counterflux of electron holes. Diffusion of an oxygen species is not required. Oxidation causes the droplets to go subsolidus; magnetite (Fe3O4) forms at the oxidation-solidification front with a morphology suggestive of a Liesegang-band nucleation process.

Time-dependent changes of the electrical conductivity of basaltic melts with redox state

“…This implies that apparent divalent cation migration and Fe 2+ /Fe 3+ changes are kinetically decoupled. This was not observed in previous studies that proposed redox mechanisms controlled by divalent cations mobility, since the time-evolution of the Fe 2+ /Fe 3+ ratio was not monitored in these studies (e.g., Cooper et al, 1996b). The rate laws of the evolution of ξ '' observed in our study are comparable to the kinetics of iron redox reactions in silicate melts (e.g.…”

“…These mechanisms were observed for experimental timescales of a few seconds or minutes (Cooper et al, 1996b;Everman and Cooper, 2003) or in the glass region (Cooper et al, 1996a;Cook and Cooper, 2000). As discussed before, at the timescale of our experiments (i.e.…”

“…Another argument for O and Si mobility in the interface zone is that our Ea D values are comparable with Ea values from viscosity measurements (Giordano et al, 2008), which are based on the Si-O bounds (and oxygen mobility). The mobility of Si as Si-O has been previously suggested by Terai and Oishi, 1977. This suggests that Si-O mobility defines the progression of ξ ', which is not considered in the model of Cooper et al (1996b). Si-O species are less mobile than sodium and they are not charged species.…”

“…Such discrepancies between O diffusion coefficients have been interpreted in terms of different transport mechanisms, oxygen self-or tracer diffusion involving mobility of discrete cation-oxygen species within the melt while oxygen chemical diffusion is dominated by the migration of cationic species. Cooper et al (1996b) have demonstrated that the diffusion of an oxygen species is not required during redox reactions in melts and that the change in the oxygen/cations ratio is best accommodated by cations rather than oxygen diffusivity due to their greater mobility.…”

Properties of high-temperature melts using levitation

“…The AAL method has been applied primarily in studies of oxides, with the apparatus open to the atmosphere. Undercooling of molten ceramic superconductors, Al 2 O 3 , aluminates, and geological materials [14], vaporization of impurities from Al 2 O 3 , glass formation, and crystal nucleation studies are reported [15][16][17][18][19].…”