Reassessing the role of magnetite during natural hydrogen generation

Abstract

Interactions between water and ferrous rocks are known to generate natural H2 in oceanic and continental domains via the oxidation of iron. Such generation has been mainly investigated through the alteration of Fe2+-silicate and some Fe2+-carbonates. So far, magnetite (α-Fe3O4) has never been considered as a potential source mineral for natural H2 since it is considered as a by-product of every known chemical reaction leading to the formation of H2, despite it bears 1/3 of Fe2+ in its mineral lattice. This iron oxide is rather seen as a good catalyst for the formation of H2. Recently, hydrogen emissions were observed in the surroundings of banded iron formations (BIF) that are constituted of, among other minerals, magnetite. Thus, this work is an attempt to constrain the true potential of magnetite by means of batch reactor experiments and additional thermodynamic calculations. It explores theoretical and experimental reaction pathways of magnetite during water-rock interactions, focusing on low temperatures (T < 200°C). For the purpose of the experiments, gold capsules filled with magnetite powders were run at 80°C and 200°C. Gas products were analyzed using gas chromatography (GC) while solid products were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, and scanning electron microscopy (SEM). After experimental alteration, high amounts of H2 were quantified while mineralogical transitions were observed by SEM. It showed self-reorganization of the primary iron oxide resulting in sharp-edge and better crystalized secondary minerals. In parallel, XRD analyses showed tiny changes between the patterns of the initial powder and the solid products of reaction. Finally, Mössbauer spectroscopy revealed that the starting magnetite was partly converted to maghemite (γ-Fe2O3), a metastable Fe-oxide only containing Fe3+. Major implications arise from these results. Concerning H2 exploration, this work provides evidence that natural hydrogen can be generated at near-ambient temperature. It also infers that magnetite-rich lithologies such as BIF should be targeted while looking for H2 source rocks. In addition, these outcomes could be of major interest for mining companies as they provide key elements to understand the formation of BIF-hosted iron ores.

A new continental hydrogen play in Damara Belt (Namibia)

“…They usually consist in Fe 2+ -bearing minerals that are accepted to source H 2 during water–rock interactions such as magnetite (α − Fe 3 O 4 ) or siderite (FeCO 3 ), according to the equations Eqs. ( 1 ), ( 2 ) 10 , 32 : …”

“…In detail, we propose that magnetite gets partially converted at depth into maghemite (γ-Fe 2 O 3 ), a metastable Fe-oxide only containing Fe 3+ , before reaching its stable state corresponding to hematite. Importantly, such a reaction has only been tested successfully at T ≤ 200 °C up to now 10 . In addition, the destabilization of other Fe 2+ -rich minerals (e.g.…”

“…They therefore concluded that H 2 may be generated during banded iron formation weathering. In addition, a recent experimental alteration study demonstrated that a synthetic powder of magnetite generates H 2 at 80 °C and 200 °C during its alteration 10 . Paradoxically, Neoproterozoic banded iron formations are assumed to contain very low amounts of Fe 2+ in the literature (e.g.…”

“…Paradoxically, Neoproterozoic banded iron formations are assumed to contain very low amounts of Fe 2+ in the literature (e.g. 11 , 12 ), which sparks an ongoing debate regarding the connection between H 2 generation and iron oxidation 10 . In that case, other mechanisms could be considered such as radiolysis of H 2 O and/or deep-seated H 2 generation.…”

Characterizing Natural Hydrogen Occurrences in the Paris Basin From Historical Drilling Records

“…However, a significant aspect of the Lizard complex composition is that the dunite is largely serpentinized and contains magnetite (Leake & Styles, 1984). Recent findings by Geymond et al (2023) have revealed the potential of magnetite to facilitate H 2 production at relatively lower temperatures through hydrothermal alteration processes. Therefore, in this geological context, serpentinized dunite, particularly rich in magnetite, may represent a viable source rock for H₂ generation.…”