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Journal of Petrology Advance Access published online on June 13, 2005
Journal of Petrology, doi:10.1093/petrology/egi038
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received August 9, 2004
Accepted March 16, 2005

Article

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

A. FEENSTRA 1*, S. SÄMANN 2, and B. WUNDER 1

1 GEOFORSCHUNGSZENTRUM POTSDAM, DEPARTMENT 4, TELEGRAFENBERG, D-14473 POTSDAM, GERMANY
2 FREIE UNIVERSITÄT BERLIN, DEPARTMENT OF EARTH SCIENCES, MALTESERSTRASSE 74-100, D-12249 BERLIN, GERMANY

* To whom correspondence should be addressed.
A. FEENSTRA, E-mail: feenstra{at}gfz-potsdam.de


  Abstract

The mutual solubility in the system corundum-hematite [{alpha}-(Al, Fe3+)2O3] was investigated experimentally using both synthetic and natural materials. Mixtures of {gamma}-Al2O3 and {alpha}-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 {alpha}-(Al0·88Fe0·123+O3) coexists with the orthorhombic phase Fe0·533+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 (2{sigma} uncertainties): WHCor = 89 ± 10 kJ/mol; WSCor = 23 ± 8 J/(Kmol); WHHem = 91 ± 14 kJ/mol; WSHem = 14 ± 10 J/(Kmol). Application of the corundum-hematite solution as a solvus geothermometer is limited because of the scarcity of suitable rock compositions.

Keywords: corundum; hematite; corundum-hematite miscibility gap; experimental study; Margules model; metabauxite.
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