High-temperature order-disorder in (Fe0.5Mn0.5)2SiO4 and (Mg0.5Mn0.5)2SiO4 olivines: An in situ neutron diffraction study

Simon A T Redfern*, C. Michael B Henderson, Kevin S. Knight, Bernard J. Wood

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    49 Citations (Scopus)


    Time-of-flight neutron powder diffraction has been used to determine the time-temperature dependence of M-site occupancies and crystal structures of MgMnSiO4 and FeMnSiO4 olivines between room temperature and 1000°C. In each sample Mn shows a preference for the M2 site at low temperature, but becomes increasingly disordered between M1 and M2 on increasing temperature. The equilibrium non-convergent disordering behaviour is described by a Landau expression for the free-energy change due to ordering. The kinetics of non-convergent ordering of the M-site cations in these samples have been analysed using a Ginzburg-Landau model, giving an activation energy for Fe-Mn exchange between M1 and M2 of 193 ± 3 kJ/mol and for Mg-Mn exchange of 172 ± 3 kJ/mol. The M-site occupancy at room temperature is shown to be a function of the cooling rate of the crystal, indicating the possibility that (Mg,Fe)2SiO4 olivine might be useful as a geospeedometer for relatively rapid cooling events, such as the cooling of small high-level basaltic intrusions. The relationship between M-site ordering in these Mn-bearing samples and structural parameters such as <M-O> bond lengths, octahedral bond-angle variance, and cell parameters is described.

    Original languageEnglish
    Pages (from-to)287-300
    Number of pages14
    JournalEuropean Journal of Mineralogy
    Issue number2
    Publication statusPublished - Mar 1997


    • Crystal structure
    • Kinetics
    • Neutron diffraction
    • Olivine
    • Order-disorder


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