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)₂^brucite = 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.