The upper 15−20 m of a 200 m thick lateritic weathering profile on Precambrian itabirites of Capanema, Brazil, reveals a genetic pathway for the formation of hematitic and goethitic nodules in the ferruginous crust through a very fine grain Al-hematite and Al-goethite mixture, called here the brick-red-material (brm). This evolution develops between the soft saprolite and a 10 m thick indurated ferruginous crust. The soft saprolite retains the original structures of the itabirite and is characterized by almost complete dissolution of quartz, the development of goethite septa, and the partial dissolution of primary hematite. Near the contact with the overlying ferruginous crust, the brm is gradually filling voids as well as replacing primary hematite and goethite in the saprolite. In the upper indurated crust, the brm transforms into coarse structureless ferruginous nodules (aluminous hematites and goethites) and is the precursor of the hematito-goethitic nodules of the crusts. Crystallization of newly-formed Al-goethite and Al-hematite within the brm occurs without detectable amounts of amorphous iron oxides of ferrihydrite precursors.

Room temperature Mössbauer and electron-probe measurements of Fe3+/ΣFe in chromite from the mantle section of the Oman ophiolite define two groups of samples: a low Fe3+/ΣFe group (with Fe3+/ΣFe = 0.21–0.36) have cr# = Cr/(Cr + Al) in the range 0.49–0.75, whereas a smaller more geographically localized high Fe3+/ΣFe group (with Fe3+/ΣFe = 0.71–0.78) have a more restricted range of cr# ratios of 0.72–0.75. The low Fe3+/ΣFe chromitites have very variable Fe3+/ΣFe ratios. They are thought to have crystallized from melts that have interacted with depleted mantle and thereby acquired their variable Fe3+/ΣFe ratio. The high Fe3+/ΣFe chromitites are restricted to one small area of the mantle and their high oxidation state is thought to be post magmatic. They are either the product of later heating, related to melt flux or interaction with a later oxidising melt. A difference in oxygen fugacity between the MORB-depleted harzburgite host, which is at the quartz–fayalite–magnetite (QFM) buffer and the later chromite-bearing melts (QFM + 2) implies that there is a real difference in the oxidation state of the MORB and arc-magma sources.