Cronstedtite: H2 generation and new constraints on its formation conditions

This work investigated the formation conditions of cronstedtite, the most Fe-rich serpentine, using a steel autoclave lined with Teflon at 90 °C, over 167 days, allowing for in situ monitoring of pH, T and P, as well as for gas sampling. From a starting mineralogical mixture composed of quartz and metal iron, cronstedtite crystallized in association with, magnetite, akaganeite, ferrihydrite and amorphous silica. This mineralogical association was not observed in previous syntheses and it is likely related to the experimental conditions of this study. These conditions also affected the composition and structure (polytypic sequence and crystallinity degree) of cronstedtite, which are accurately characterized for the first time. Experimental data and thermodynamic modelling indicate that this Fe-serpentine formed under neutral conditions, -62 < logfO2 < −59, and redox potential ranging from −0.46 to −0.41 Volts. During the experiment, H2 was generated by iron oxidation in presence of water, explaining the increase of pressure in the autoclave until 6.8 bar at 90 °C. The total amount of H2 generated was of 0.23 mol. The results of this study are useful to improve the understanding of cronstedtite formation in natural and anthropic environments. They also showed that formation conditions of cronstedtite seem to be compatible with those needed for hydrogenotrophic life, as previously supposed by other works.