Fig. 2: Identification of magnetite (Mag) in the iron-sulfide grains.

a Transmission electron microscopy (TEM) bright-field image of a region of a spherical iron-sulfide grain focused-ion-beam section in which magnetite and pure metallic iron particles have co-precipitated within the troilite–pyrrhotite (Tro + Po) matrix with some scattered hexagonal vesicles. b Quantitative transmission electron microscopy–energy-dispersive line-profile results obtained from the position indicated by the arrow in (a). c, d O, K, and Fe L_2,3 electron energy-loss spectra of a magnetite particle (Fe₃O₄) embedded in the studied iron-sulfide grain. The spectra of the prepared standards (Fe₂O₃ and Fe₂SiO₄) are marked by asterisks. e Results of the best fit (coefficient of determination of 0.99) for the Fe-oxidation-state ratio of the magnetite particle using the Fe L₃ edge, showing an approximate Fe³⁺ to Fe²⁺ ratio of 2:1 in the magnetite particle. f High-magnification high-angle annular dark-field image of the magnetite particle, showing a close spatial relationship with metallic iron. g High-resolution TEM image of a magnetite particle. The fast Fourier transform pattern of this grain is shown in the insert at the bottom left of the figure. h Atomic-resolution annular dark-field scanning TEM image of a magnetite particle. A magnetite structure model along the [\(\bar{{{{{{\bf{1}}}}}}}\)11] zone axis is superimposed on the image.