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. The spatial distribution oftransition metal valence states is of broad interest in the microanalysis ofgeological and environmental samples. An example is rock varnish, a naturalmanganese (Mn)-rich rock coating, whose genesis mechanism remains a subjectof scientific debate. We conducted scanning transmission X-ray microscopywith near-edge X-ray absorption fine-structure spectroscopy (STXM-NEXAFS)measurements of the abundance and spatial distribution of different Mnoxidation states within the nano- to micrometer thick varnish crusts. Suchmicroanalytical measurements of thin and hard rock crusts require samplepreparation with minimal contamination risk. Focused ion beam (FIB) slicingwas used to obtain ∼100 –1000 nm thin wedge-shaped slices of thesamples for STXM, using standard parameters. However, while this preparationis suitable for investigating element distributions and structures in rocksamples, we observed artifactual modifications of the Mn oxidation states atthe surfaces of the FIB slices. Our results suggest that the preparationcauses a reduction of Mn4+ to Mn2+ . We draw attention tothis issue, since FIB slicing, scanning electron microscopy (SEM) imaging,and other preparation and visualization techniques operating in thekilo-electron-volt range are well-established in geosciences, but researchersare often unaware of the potential for the reduction of Mn and possibly otherelements in the samples.

Artifacts from manganese reduction in rock samples prepared by focused ion beam (FIB) slicing for X-ray microspectroscopy