The control of iron oxidation state during FeO and olivine crystal growth
Crystal growth experiments (micro-pulling down or Czochralski methods, respectively) and DTA/TG measurements with Fe2+ containing olivines (fayalite--forsterite solid solution) and with FeO (wustite) are performed. For both substances the oxygen partial pressure p_O2 of the growth atmosphere had to be adjusted within the stability region of Fe2+ for all temperatures ranging from room temperature to the melting point. The formation of Fe3+ (Fe3O4, Fe2O3) had to be avoided. The adjustment of p_O2 could be obtained by a mixture of argon, carbon dioxide and carbon monoxide. Thermodynamic equilibrium calculations show, that mixtures of an inert gas (e.g. argon) with another gas or gas mixture that supplies oxygen at elevated temperature (e.g. CO2/CO) are superior to the use of inert gases with constant oxygen admixture. The reason is that the Ar/CO2/CO mixture adjusts its oxygen concentration with temperature in a way similar to that needed for the stabilization of Fe2+.