FORMATION OF FERRIOLIVINE AND MAGNESIOFERRITE FROM MG-FE-OLIVINE - REACTIONS AND KINETICS OF OXIDATION

Olivine samples (Fa 11) have been oxidized in air (f(O2)=0.2 atm) at t emperatures ranging from 350-700 degrees C and examined by Mossbauer s pectroscopy, transmission electron microscopy, X-ray powder diffractio n and thermomagnetic analysis. Oxidation of olivine was found to resul t in ferriolivine, magnesioferrite (major oxide phase) and magnetite ( minor oxide phase) formation. Ferriolivine forms planar (001) precipit ates, 0.6 nm in thickness, in the olivine host; the composition is lik ely to be Mg(0.5)nu(0.5)(Fe3+)(1.0)SiO4. Magnesioferrite MgFe2O4 exsol ves as fine-grained precipitates (5-6 nm in size) filling interstices between the ferriolivine planar precipitates. Oxidation kinetic data a t 700 degrees C show two stages of oxidation corresponding to formatio n of ferriolivine in the first stage and magnesioferrite in the second stage. The linear rate law with a rate constant k(Fol) = 1.23 . 10(-3 ) s(-1) was found for the first stage whereas a parabolic rate-law wit h a constant of k(oxi) = 3.28 . 10(-3) s(-1) was determined for the se cond stage of oxidation. It was found that ferriolivine is not an inte rmediate metastable phase in the oxidation process, terminated by magn esioferrite formation. The ferriolivine and magnesioferrite are consid ered to have formed by independent reactions which do not necessarily proceed simultaneously.