Geometry-driven controls on hydrothermal natural hydrogen generation from magnetite mineral

Despite recent efforts to investigate hydrothermal natural hydrogen production from magnetite, the effects of sample geometry and surface area on hydrogen yields remain inadequately characterized. This study compares hydrogen production from natural magnetite prepared as an intact slab versus fine powder under identical experimental conditions (pH 9, 200 °C, 103.4 bar, 60 days). The results show that magnetite powder yielded 0.052 mmol g−1 rock hydrogen, approximately fivefold higher than the slab. Comprehensive characterization of the magnetite slab revealed increased surface roughness after reaction. Quantitative XRD analysis showed substantial phase conversion from magnetite to hematite, consistent with oxidation of Fe 2+ to Fe 3+ . SEM imaging indicated pronounced textural restructuring, including the development of euhedral crystal faces and enhanced microporosity, reflecting dissolution and reprecipitation processes. These results demonstrate that powders maximize hydrogen output through abundant reactive sites, whereas slabs, which are more representative of intact geological formations, exhibit lower specific yields governed by diffusion limitations and passivating hematite layers. These findings are particularly significant for both natural hydrogen production and geological hydrogen storage in magnetite bearing banded iron formations (BIFs) and other magnetite rich lithologies. • Magnetite powder yields 5 × more H 2 than slabs under hydrothermal conditions. • Surface passivation by hematite coating limits hydrogen generation in slabs. • Geometry and surface area critically control natural H 2 production rates.