LAIHUNITIZATION AS A MECHANISM OF OLIVINE OXIDATION

The kinetics of oxidation of fine-grained olivine Fall (mean grain siz e 70 mu m) in air was studied al 600 degrees C. The degree of oxidatio n was measured by Mossbauer spectroscopy. The overall oxidation proces s proceeds in two stages, first via formation of laihunite planar defe cts in the inremnl oxidation layer accompanied by magnesioferrite grow th in the external oxidation laver as summarized by reaction (1) Mg1.7 8Fe0.22SiO4 + a O-2 --> (1-a) Mg2SiO4 + a Mg(0.5)v(0.5)(Fe3+)(1.0)SiO4 + a/2 MgxFe(1-x)up arrow, then by formation of magnesioferrite in the internal oxidation layer as summarized by reaction (2)Mg1.78Fe0.22SiO 4 + O-2 --> Mg2SiO4 + MgFe2O4 + SiO2. Magnesioferrite was identified b y thermomagnetic measurements. Rare laws were extracted from the time- dependent data: linear for (1) indicating this is an interface-control led reaction, and parabolic for (2), indicating this is a diffusion-co ntrolled reaction. Using the present 600 degrees C data and previous h igher-temperature data, activation enthalpies were obtained: Delta H-a = 73(3) kJ/mol for (1) and Delta H-a = 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. Indica tions were obtained that point to different diffusing species for proc esses (1) and (2). Laihunitization is expected to be important under c ertain, in particular low-temperature high-pressure conditions.