Abstract. Olivine samples (Fa 11) have been oxidized in air 0Co2=0.2 atm) at temperatures ranging from 350-700 ~ and examined by M6ssbauer spectroscopy, transmission electron microscopy, X-ray powder diffraction and thermomagnetic analysis. Oxidation of olivine was found to result in ferriolivine, magnesioferrite (major oxide phase) and magnetite (minor oxide phase) formation. Ferriolivine forms planar (001) precipitates, 0.6 nm in thickness, in the olivine host; the composition is likely to be Mgo.sVo.5(Fe3+)l.0SiO 4. Magnesioferrite MgFe20 4 exsolves as fine-grained precipitates (5-6 nm in size) filling interstices between the ferriolivine planar precipitates. Oxidation kinetic data at 700 ~ show two stages of oxidation corresponding to formation of ferriolivine in the first stage and magnesioferrite in the second stage. The linear rate law with a rate constant kvol = 1.23-10 -3 S-1 was found for the first stage whereas a parabolic rate-law with a constant of koxi=3.28 9 10 -3 s -1 was determined for the second stage of oxidation. It was found that ferriolivine is not an intermediate metastable phase in the oxidation process, terminated by magnesioferrite formation. The ferriolivine and magnesioferrite are considered to have formed by independent reactions which do not necessarily proceed simultaneously.
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