Secondary Fe(III) oxyhydroxides play a key role in controlling the mobility and bioavailability of trace metals in acidic, sulfate-rich soils, such as mining and smelter sites. Schwertmannite, jarosite, goethite, and ferrihydrite are the most common mineral phases identified in such soils. A good understanding of the precipitation and transformation of these minerals in soils is very important for predicting the mobility and long-term stability of trace metals associated with them. In the present study, bulk powder X-ray diffraction (XRD), scanning electron microscopy (SEM), synchrotron based micro- X-ray diffraction (µ-XRD), and micro X-ray fluorescence (µ-SXRF) spectroscopy were used to investigate precipitates fromthe surface horizon of an organic soil (Histosol) at a site that once contained a lead smelter. Soil samples were collected from 0 to ≈10 cm depth during both wet and dry seasons. Goethite and akaganeite were identified as the major mineral phases in this soil. Schwertmannite and jarosite were also occasionally identified, particularly in the soil samples from dry periods. The peaks in the akaganeite XRD pattern were significantly broadened and the relative intensities of some major peaks were distinctly different compared with the diffraction pattern of synthetic akaganeite, possibly due to the effects of pH and the incorporation of sulfate. The SEM and µ-XRD data support the hypothesis that the goethite in the precipitates is not the product of direct precipitation froms olution but the transformation of previously precipitated schwertmannite or akaganeite.

The Mineo pallasite is a relatively poorly known meteorite, which shows interesting features that are not fully understood, such as the occurrence of iron oxide regions bordering both the olivine grain boundaries and the (Fe,Ni) metal. In this study, the Fe oxides have been characterised by Raman spectroscopy, electron microprobe analysis, field emission scanning electron microscopy, transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and 3D electron diffraction (3D ED). The combination of TEM–EDS and 3D ED yields a reliable identification of the chemical and crystallographic features of the cryptocrystalline portion of the sample investigated, enabling the Fe-oxide regions to be positively identified as goethite, FeO(OH).

The occurrence of goethite was unambiguously associated with terrestrial alteration, also confirmed by the presence of calcite, detected by TEM-EDS and 3D ED. Goethite contains minor elements such as Na, Si and Ca, probably coming from alumino-silicates in the terrestrial environment, and Ni associated with the (Fe,Ni) metal. The observation of goethite along olivine grain boundaries, as an alteration product of the (Fe,Ni) metal diagenesis, is also very intriguing as it might be related to the (Fe,Ni) metal intruded into the sub-micrometric olivine fragments during pallasite formation. Further work is needed to extensively analyse the texture and composition of olivine/metal boundaries.