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Experimental alteration of ferroan brucite at temperature below 150°C: new thermodynamic and kinetic constaints on H2 production during ultramafic rock alteration at low temperature

The alteration of ferroan brucite, a common by-product of serpentinization, has been proposed as a H2 source at low temperature. Here, synthetic ferroan brucite with Fe/(Fe+Mg) = 0.2 was reacted with pure water at temperatures ranging from 348 to 573 K in 29 experiments either conducted in gold capsules or in Ti-based reactors. H2 production monitoring with time and characterization of the reaction products revealed the occurrence of the following reaction: 3 Fe(OH)2brucite = Fe3O4 + H2 + 2 H2O. This reaction proceeded completely in ~ 2 months at 378 K and was thermally activated. The small grain size of the synthetic brucite (40-100 nm) was similar to observations in natural samples, and was probably responsible for the high reaction rate measured. H2 production reached a plateau and Fe-bearing brucite also precipitated as a reaction product, suggesting the achievement of equilibrium. The thermodynamic properties of Fe(OH)2 were refined based on the experimental dataset and differ by less than 5 % from previous estimates. However, ferroan brucite is predicted to be stable at an hydrogen activity one order of magnitude lower than previously calculated. As a result, significant H2 production during ferroan brucite alteration at low temperature requires efficient fluid renewal. Such a mechanism strongly differs from olivine serpentinization which can occur even at high activity in H2 and thus with limited water renewal.


William Carlin1, Benjamin Malvoisin2, Fabrice Brunet2, Bruno Lanson2, Nathaniel Findling2, Martine Lanson2, Tiphaine Fargetton3, Laurent Jeannin3, Olivier Lhote4
1Univ. Grenoble Alpes, USMB, CNRS, IRD, UGE, ISTerre, France;Storengy (ENGIE), France; 2Univ. Grenoble Alpes, USMB, CNRS, IRD, UGE, ISTerre, France; 3Storengy (ENGIE), France; 4Engie Research, ENGIE, France
GeoBerlin 2023