Some Unresolved Problems in the Structural Chemistry of the "FeS" System

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EAR-9909145PariseThe structural chemistry and phase transformation behavior of compounds in the iron-sulfur system are important in settings from the Earth's core to crust where, for example, pyrite (FeS2) formation from mackinawite (FeS) is a key process in several geochemical cycles. While the structures of many of the phases of importance to planetary surfaces and interiors are known, significant questions remain. These include resolution of conflicting proposals for phase diagrams, crystal structures (nuclear and electronic) and the mechanisms of phase transformations between the compounds with stoichiometry close to FeS. The research funded by this grant will concentrate on the following key questions: 1) what are the structures adopted by FeS at high temperature and pressure? Reports of structural changes at intermediate temperatures and pressures will be studied using diamond anvil high pressure cells and a newly installed detector system on the Stony Brook large volume high pressure device. 2) What is the nature of the intermediate magnetic structure at high pressure? 3) When FeS is precipitated in low temperature aerobic or anaerobic environments, what are the determining factors promoting stability, the transformation to pyrite, or to other competing phases? These questions are addressed using in situ techniques, which replicate conditions of relevance to earth environments. Parts of this project rely on neutron and synchrotron X-ray sources in the US and in the UK, where these in situ experiments will be carried out.