LAIHUNITIZATION AS A MECHANISM OF OLIVINE OXIDATION

Abstract

The kinetics of oxidation of fine-grained olivine Fa11 (mean grain size 70 μm) in air was studied at 600°C. The degree of oxidation was measured by Mössbauer spectroscopy. The overall oxidation process proceeds in two stages, first via formation of laihunite planar defects in the internal oxidation layer accompanied by magnesioferrite growth in the external oxidation layer as summarized by reaction (1) Mg1.78Fe0.22SiO4 + a O2 → (1-a) Mg2SiO4 + a Mg0.5v0.5(Fe3+)1.0SiO4 + a/2 MgxFe(1-x) ↑, then by formation of magnesioferrite in the internal oxidation layer as summarized by reaction (2) Mg1.78Fe0.22SiO4 + O2 → Mg2SiO4 + MgFe2O4 + SiO2. Magnesioferrite was identified by thermomagnetic measurements. Rate laws were extracted from the time-dependent data: linear for (1) indicating this is an interface-controlled reaction, and parabolic for (2), indicating this is a diffusion-controlled reaction. Using the present 600°C data and previous higher-temperature data, activation enthalpies were obtained: ΔHa = 73(3) kJ/mol for (1) and ΔHa = 118(13) or 121(2) kJ/mol for (2), using the Arrhenius or Arvani formalism, respectively. The latter value is close to literature values of Mg-diffusion in olivine. Indications were obtained that point to different diffusing species for processes (1) and (2). Laihunitization is expected to be important under certain, in particular low-temperature high-pressure conditions.