Phase Equilibria in the System MgO‐FeO‐Fe₂O₃: The 1300°C Isothermal Section and Extrapolations to Other Temperatures

Abstract

The 13OO0C isotherm of the system MgO-FeOFe203 was studied thermogravimetrically under conditions of controlled fugacity of oxygen. The magnesioferrite solid solutions that coexist in equilibrium with magnesiowiistite solid solutions have a cation to anion ratio experimentally indistinguishable, in the present study, from 0.750. Curves are presented expressing the change in composition of coexisting magnesioferrite and magnesiowiistite phases as a function of temperature and oxygen fugacity. These curves for the system Mg0-Fe0-Fez03 can b e used (1) to predict compositional relations under isothermal conditions for temperatures where reaction rates are sluggish, (2) as additional oxygen buffer curves for experimental control of oxygen fugacity, and (3) in certain instances to distinguish equilibrium from nonequilibrium assemblages.

Reactions of dense MgO with calcium ferrite-based slags at 1573 K

“…In the MgO-FeO-Fe 2 O 3 system, Speidel has shown that magnesiowustite or magnesioferrite spinel may exist depending on the oxygen potential. [12] The SEM results of this study showed that under p o 2 ϭ 10 Ϫ6 and 3.73 ϫ 10 -4 atm, a magnesioferrite spinel formed at the slag-refractory interface and behind this spinel layer was a magnesiowustite solid solution inside the refractory. Under p o 2 ϭ 10 Ϫ8 atm, only a magnesiowustite solid solution layer was observed at the interface.…”

Oxygen fugacity and valence state of chromium in ferropericlase: Can Cr2+ be a redox indicator for the deep mantle?

“…The phase diagram of the MgO -FeO -Fe 2 O 3 system at 1300 ºC (Speidel, 1967) indicates that ferropericlase is the unique stable phase under the experimental conditions in the present study. However, our samples contained a minor amount of magnesioferrite, even in synthesized samples.…”

“…Moreover, as the Mn ions in the present experimental condition are permanently divalent because f O 2 at the MnO -Mn 3 O 4 buffer is much higher (Huebner, 1971), the redox reaction of Mn would not have affected the experimental results. A 100 mg pellet was made and placed into a Pt -cup and heated at 1300 °C within an atmosphere -controlled furnace for 48 h. The justification for choosing this temperature condition is that the phase diagram of the MgO -FeO -Fe 2 O 3 system has previously only been established at this temperature condition (Speidel, 1967). The oxygen fugacity (log f O 2 ) was controlled at −7, −9, −11, using a mixed gas of H 2 and CO 2 .…”

“…The relative intensities of the doublet and singlet peaks are derived from the ferrous and ferric in ferroperi- Speidel (1967) in the MgO -FeO -Fe 2 O 3 system; the Fe 3+ /ΣFe ratio decreases from ~ 0.1 to ~ 0.01 as it is reduced from near the QMF buffer to near the IW buffer for ferropericlase with ~ 75 wt% MgO. Figure 3 shows the Cr -K edge XANES spectra of the synthesized samples.…”

Activity of Iron Oxide in Magnesiowüstite in Equilibrium with Solid Metallic Iron

“…In this study, however, the compositions of Fe x O-MgO solid solutions were determined only after preparations at 1573 K, and are superimposed on the isothermal section of the ternary system of stoichiometric FeO, Fe 2 O 3 and MgO at 1573 K, 14) shown in Fig. 4.…”