Hydrogen Production By Anaerobic Oxidation of Fayalite Ferrous Ions
In the absence of atmospheric oxygen (O2), in deep geological conditions prevailing far away from Earth atmosphere, hydrogen (H2) is produced by the anaerobic oxidation of ferrous ions (Fe2+) present in the crystal lattice of the iron-endmember fayalite by the protons (H+) of water.
Considering three formula units of fayalite (Fe2(SiO4)) for the purpose of stoechiometry and reaction mass balance, four ferrous ions will undergo oxidation by water protons while the two remaining will stay unoxidised. Neglecting the orthosilicate anions not involved in the redox process, it is then possible to schematically write the two half-redox reactions as follows:
- 4 (Fe2+ → Fe3+ + e–) (oxidation of ferrous ions)
- 2 (H2O + 2 e– → O2– + H2) (reduction of protons into hydrogen)
This leads to the global redox reaction involving ferrous ions oxidation by water:
- 4 Fe2+ + 2 H2O → 4 Fe3+ + 2 O2– + 2 H2
The two unoxidised ferrous (Fe2+) ions still available in the three formula units of fayalite finally combine with the four ferric (Fe3+) cations and oxide anions (O2–) to form two formula units of magnetite (Fe3O4).
Finally, considering the required rearrangement of the orthosilicate anions into free silica (SiO2) and free oxide anions (O2–), it is possible to write the complete reaction of anaerobic oxidation and hydrolysis of fayalite according to the following mass balance:
- 3 Fe2SiO4 + 2 H2O → 2 Fe3O4 + 3 SiO2 + 3 H2
- fayalite + water → magnetite + quartz + hydrogen
This reaction closely resembles the Schikorr reaction observed in the anaerobic oxidation of the ferrous hydroxide in contact with water:
- 3 Fe(OH)2 → Fe3O4 + 2 H2O + H2
- ferrous hydroxide → magnetite + water + hydrogen
Read more about this topic: Serpentinite, Formation and Petrology
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