Ferromanganese formations are widespread in the Earth’s aquatic environment. Of all
the mechanisms of their formation, the biogenic one is the most debatable. Here, we studied the
Fe-Mn crusts of hydrothermal fields near the underwater volcano Puy de Folles (rift valley of the
Mid-Atlantic Ridge). The chemical and mineralogical composition (optical and electron microscopy
with EDX, X-ray powder diffraction, X-ray fluorescence analysis, Raman and FTIR spectroscopy, gas
chromatography—mass spectrometry (GC-MS)) and the magnetic properties (static and resonance
methods, including at cryogenic temperatures) of the samples of Fe-Mn crusts were investigated. In
the IR absorption spectra, based on hydrogen bond stretching vibrations, it was concluded that there
were compounds with aliphatic (alkane) groups as well as compounds with double bonds (possibly
with a benzene ring). The GC-MS analysis showed the presence of alkanes, alkenes, hopanes, and
steranes. Magnetically, the material is highly coercive; the blocking temperatures are 3 and 13 K.
The main carriers of magnetism are ultrafine particles and X-ray amorphous matter. The analysis
of experimental data allows us to conclude that the studied ferromanganese crusts, namely in their
ferruginous phase, were formed as a result of induced biomineralization with the participation of
iron-oxidizing and iron-reducing bacteria.