EDS mapping of major elements Si, O, Mg and Fe. a) Dark field image of the ROI 1 in Fig. 10a with the orange box indicating the EDS mapping region. The area surrounded by the yellow box is used for drift corrections; b) EDS mapping results of Si, O, Mg and Fe. The brightness of the pixels is correlated with the relative abundance of the corresponding element. The brighter spots in the Fe distribution map are consistent with the white spots in the enclosed area of the orange box, showing taht the nanoparticles are indeed nanophase irons.