Abstract

The cation distribution in synthetic and natural (Fe,Mn) olivines has been investigated by use of Mössbauer spectroscopy. A partially ordered distribution with Mn2+ entering the M2 site and Fe2+ entering the M1 site was observed which, however, becomes more disordered at higher temperature of quenching. Gibbs free energy of exchange for the reaction  
Mn2+(M1)+Fe2+(M2)=Mn2+(M2)+Fe2+(M1)

at 1000°C is −3.5 kcal/mole. The observed variations in the a and b parameters of the solid solution series are in general agreement with the expected order–disorder behavior (Lumpkin et al., 1983) but would suggest a higher degree of Fe order into M1 than the results we obtained from Mössbauer spectra.

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