H2 Detector Responses and Mineralogical Changes During Hydrothermal Treatment of Magnetite-Bearing Banded Iron Formations

Geological hydrogen is increasingly investigated as a potential subsurface energy resource, yet the reactivity of Fe-rich geological materials during H2-generating water–rock interactions remains poorly constrained. Magnetite-bearing banded iron formations (BIFs) represent large Fe-rich geological reservoirs with potential relevance to geological H2 generation. Hydrothermal experiments were conducted on five natural BIF samples and a magnetite-rich lodestone reference at 150, 200, and 230 °C for 30 days. All reacted samples showed H2 detector responses above procedural blanks, with responses generally increasing with temperature. The magnetite-rich lodestone exhibited the highest response (379 ppm at 230 °C). Quantitative X-ray diffraction revealed decreases in magnetite accompanied by hematite formation, while scanning electron microscopy showed progressive surface alteration. These results suggest that temperature, mineralogy, and mineral accessibility influence the hydrothermal reactivity of Fe-rich materials. Because gas measurements were conducted under open-reactor conditions and no replicate experiments were performed, the measured values are treated as individual H2 detector responses rather than quantitative H2 yields or validated comparative measurements. This preliminary screening study documents H2 detector responses and Fe-mineral transformations during hydrothermal treatment of natural BIFs, providing a basis for further controlled investigation of their relationship.