P.F. Schofield 1, A.D. Smith 2
1 Department of Mineralogy, Natural History Museum, London SW7 5BD, UK
2 CCLRC Daresbury Laboratory, Daresbury, Warrington WA4 4AD, UK
Valence state ratios of the first row transition metals can be used as a measure of the redox conditions of geological, planetary and environmental processes.
Additionally, the textural relationships between minerals provides a record of the formation history, stability and alteration events of natural materials. By measuring and imaging the crystal chemistry of multivalent elements in minerals, whilst maintaining the spatial integrity of the mineral microstructure, a detailed understanding of the complex processes involved with the formation of natural materials can be developed. Moreover, the properties, reactivity and stability of these materials in their current environments can be accurately modelled.
X-ray Photo-Emission Electron Microscopy (XPEEM) has the capability to provide precisely this sort of information. With an area selectivity ranging from the nanometer scale up to several hundreds of microns, quantitative valence state images can be obtained, non-destructively, from the same material prepared for electron and optical microscopy. Valence state imaging of oxide and sulphide mineral intergrowths,including weathered magnetite, Mn-hollandite hydrothermal veins, oxidised magnesiowustite and polygenetic Co-Cu-Fe sulphide microtextures demonstrate the efficacy of the technique and its application.
More detailed XPEEM studies have been performed on oceanic materials such as metal sulphide precipitates exhumed from hydrothermal vents present at mid-ocean ridges and Mn nodules from the Madeira Abyssal Plain in the Atlantic Ocean. The speciation of oxygen has been investigated in the Santa Catharine meteorite and studies of chromite grains in Martian basalts may provide information relating to the evolution of magma chambers on Mars.
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