Fe--Al Oxides: Phase Relationships below 1,000 C

The subsolidus phase relationships of magnetite, hercynite, hematite, corundum, wostite, and iron are described. The phases were synthesized from chemical mixtures. Reactions and solid solution between them were induced under controlled conditions of composition, temperature, total vapor pressure, and partial pressure of oxygen. Reaction rates are slow, so that the experiments lasted from 1 to 40 days, and quenching is completely successful. A solvus was determined which limits solid solution along the magnetitc-hercynite join at temperatures below 860o±15oC. Compositions of the spinel solid solutions were determined by measuring the shift of the (440) reflection, using a powder X-ray diffractometer. The calibration curve, 20 vs. composition, was made from measurements of spinel solid solutions synthesized in the one-phase region. The cell edge ao changes from 8–391±0.002 A (magnetic, Fe+2Fe2+2O4OJ to 8.150±0.004 (hercynite, Fe+2Al2O4)by ao°8.391–0.00190x - 0.5X210-5 (X is mol per cent FeAl2O4 in solid solution). In the system Fe-Al2-O3-O there are five univariant assemblages: 1. Hematite-corundum+magnetite +V (vapor) 2. Corundum+magnetite+hercynite+V 3. Magnetite+hercynite+wüstite+V 4. Hercynite+wilstite+iron+V 5. Hercynite+iron+corundum+V The lines were located by determining the composition of the magnetite, hercynite, hematite, and corundum solid solutions for each assemblage. The diagrams provide a basis for the discussion of the paragenesis of the oxide minerals. The progressive metamorphism of laterite deposits can be represented by (1) laterites and bauxites: hematiteH+hydrated aluminum oxides; (2) diasporites: hematite+diaspore+corundum, with magnetite as a rare accessory; (3) emery: corundum+magnetite, with hematite as an accessory. The path of these mineral changes on the diagrams shows the decrease in oxygen content of the solids with decrease in the partial pressure of oxygen and relates the aluminum content of the magnetite to temperature. The occurrences of hercynite are discussed. It is a rare mineral because it requires unusual conditions to grow, i.e. relatively low oxygen pressure and an extremely Fe-Al-rich environment.