FE-MG CARBONATE SOLID SOLUTION EXPERIMENTS LEAD TO FE-MG OXIDE SOLID SOLUTION EXPERIMENTS

The mixing properties of siderite (sid;FeCO₃) - magnesite (magn; MgCO₃) solid solutions were to be measured via displaced equilibrium experiments performed in a piston-cylinder apparatus. The univariant reaction sid + hematite (hem; Fe₂O₃)=magnetite (mt; Fe₃O₄) + CO₂ is displaced to higher temperatures (at constant pressure) by the increasing mole % of magn in carbonate. Crystalline materials with AgC₂O₄ as CO₂ source, buffered at hem/mt in a double capsule, were held at 640°C - 660°C and 11 kbar for 3-5 days. All hem disappeared and mt apparently grew as shown by XRD analysis. However, I suggest magnesioferrite (mfer; MgFe₂O₄) formed from the hem and carbonate. Further research shows that mfer or mfer-mt solid solutions are stable at an oxygen fugacity (f_O2) of hem/mt or higher (at 1000°C). At lower f_O2 (or more Mg-rich compositions) a mfer-mt solid solution coexists with magnesiowustite (mwu; (Fe,Mg)O) (Zalazinskii et al. 1975, Russ. Jour. Phys. Chem. 49, 914-915). These phase relationships are under investigation now in this lab. Fe-Mg carbonate experiments as designed are not possible.

Applications: Controversy surrounds Martian meteorite ALH84001, especially the interpretation of carbonate globules and associated mt grains. The nanometer-sized, chemically pure mt grains have been cited as evidence for former life on Mars (McKay et al. 1996, Science 273, 924-930). However, they may have formed from Fe,Mg carbonate by increase in temperature or increase in environmental f_O2 (Koziol 2003, XXXIV LPSC, Abst. #1128). If mt formed from sid-magn, this suggests a low f_O2 (near mt/wustite) at formation. At higher f_O2 mfer or mfer-mt solid solutions are stable. In a more general application, more study of Fe,Mg oxide stability (i.e. mwu) at high pressure and various temperatures and f_O2 could shed light on phase relations in the mantles of Earth and Mars.

This research supported by NSF grant # EAR-9805873 and NASA grants # NAG5-8199 & NAG5-9808.