An Experimental Study of Fe–Al Solubility in the System Corundum–Hematite up to 40 kbar and 1300°C

Abstract

The mutual solubility in the system corundum–hematite [a-(Al, Fe)2O3] was investigated experimentally using both synthetic and natural materials. Mixtures of g-Al2O3 and a-Fe2O3 (weight ratios of 8:2 and 10:1) were used as starting materials for synthesis experiments in air at 800–1300 C with run times of 7–34 days. Experiments at 8–40 kbar and 490–1100 C were performed in a piston-cylinder apparatus (run times of 0 8–7 4 days) using a natural diasporite consisting of 60–70 vol. % diaspore and 20–30 vol. % Ti-hematite. During the diasporite–corundite transformation, the FeTiO3 component (12–18 mol %) of Ti-hematite only slightly increased, implying that oxygen fugacity was maintained at high values. Run products were studied by electron microprobe and X-ray diffraction (Rietveld) techniques. An essentially linear volume of mixing exists in the solid solution with a slight positive deviation at the hematite side. Up to 1000 C, corundum contains <4 mol % Fe2O3 and hematite <10 mol % Al2O3; at 1200 C these amounts increase to 9 3 and 17 0 mol %, respectively. At 1300 C hematite was no longer stable and a-(Al0 88Fe0 12 3þ O3) coexists with the orthorhombic phase Fe0 53 3þ Al0 47O3. The present results agree with corundum (solvus) compositions obtained in previous studies but indicate a larger solubility of Al in hematite. The miscibility gap in the solution can be modelled with an asymmetric Margules equation with interaction parameters (2s uncertainties): WCor 1⁄4 89 10 kJ/mol; WCor 1⁄4 23 8 J/(Kmol); WHem 1⁄4 91 14 kJ/mol; WHem 1⁄4 14 10 J/(Kmol). Application of the corundum– hematite solution as a solvus geothermometer is limited because of the scarcity of suitable rock compositions.