Chemical and oxidation-state imaging of mineralogical intergrowths: The application of X-ray photo-emission electron microscopy (XPEEM)

Abstract

We describe the application of X-ray photo-emission electron microscopy (XPEEM) to studies of minerals and mineral intergrowths in which the spatial integrity of the sample must be maintained in order to retain the chronological context of the study. Chemical imaging, oxidation state imaging and area selective spectroscopy of minerals are described for a range of samples that includes oxides, sulphides and silicates. Oxidation state images with spatial resolutions between ∼200nm and ∼500nm are presented for Fe3+ in igneous pyroxenes, for Mn2+/Mn4+ in deep sea Mn nodules, and for Fe2+/Fe3+ in a skeletal intergrowth of magnetite in magnesiowüstite.Spatially resolved, quantitative L-edge XANES analysis from XPEEM image stacks is reported for a chromite grain from a Martian basalt and for a chemically-zoned metamorphic garnet crystal. In the Martian chromite grain the Fe3+/ΣFe and Cr3+/ΣCr ratios were found to be constant at 0.18±0.02 and 1.0 respectively. The Fe3+/ΣFe ratio across a metamorphic garnet was found to increase from 0.07±0.02 at the centre of the crystal to 0.13±0.02 at the rim while the Mn2+/ΣMn ratio remained at 1.0 throughout. It is shown that for complex mineral intergrowths XPEEM can be used for mineral identification at a length-scale of a few 10s of nm.While the spatial resolution from which high quality L-edge XANES spectra have been extracted from mineralogical samples to date is about 100nm, XPEEM offers an achievable resolution approaching 30nm in the soft X-ray region (2000eV and below). The non-destructive nature of XPEEM is of particular importance for natural and synthetic samples of high scientific value that may be required for further analysis by other microscopy, chemical analysis or isotope techniques. XPEEM can be used as a stand-alone spectromicroscopy method for the study of mineralogical samples or can be combined with other well established synchrotron methods such as hard X-ray, microfocus XANES spectroscopy and soft X-ray, scanning transmission X-ray microscopy.