The control of iron oxidation state during FeO and olivine crystal growth

TL;DR

This study investigates controlling the iron oxidation state during FeO and olivine crystal growth by adjusting oxygen partial pressure with specific gas mixtures, ensuring Fe2+ stability and avoiding Fe3+ formation.

Contribution

It demonstrates the effectiveness of using temperature-dependent gas mixtures to maintain Fe2+ stability during crystal growth, improving control over oxidation states.

Findings

Gas mixtures like Ar/CO2/CO effectively stabilize Fe2+.

Thermodynamic calculations support temperature-dependent oxygen regulation.

Avoidance of Fe3+ formation is achieved through controlled atmospheres.

Abstract

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+.