Abstract
Based on the in situ and temperature-quench X-ray measurements, the back transformation in the (Mg, Fe)2SiO4-spinels has been characterized in terms of the transformation temperature (T r ),mechanism and kinetics of the transformation, and of the end product(s), with specific emphasis on the effect of oxygen on this transformation. The in situ measurements were conducted to 900° C in vacuum (10-4 to 10-5 torr) and to 600° C in air using synchrotron radiation (SR) at Stanford Synchrotron Radiation Laboratory (SSRL). In the quench-type measurements, samples were heated in air to 1100° C, quenched and examined at ambient conditions using the conventional X-ray diffraction facilities. Important results are (1) in vacuum, all the spinels convert back into the olivine phase, with their T r decreasing with increasing iron content; (2) the spinel → olivine back transformation is a nucleation and growth type of transformation and can be described quantitatively using the Avrami equation; (3) in air, the (Mg, Fe)2SiO4-spinels with 0.2 mole fraction Fe or more are all oxidized, and the composition and phase of the end products depend upon the temperature and the starting composition; and (4) the oxidation of the iron-rich (Mg, Fe)2SiO4-spinels in air occurs at ∼350–400° C, which is significantly lower than its T r (∼ 300° C) in vacuum.
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Ming, L.C., Kim, Y.H., Manghnani, M.H. et al. Back transformation and oxidation of (Mg, Fe)2SiO4 spinels at high temperatures. Phys Chem Minerals 18, 171–179 (1991). https://doi.org/10.1007/BF00234000
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DOI: https://doi.org/10.1007/BF00234000