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Structure and Reactivity of Terrestrial and Extraterrestrial Pyrrhotite
(2012)
- Pyrrhotite (Fe1-xS) is a non-stoichiometric iron monosulfide common in terrestrial rocks, ore deposits, and many extraterrestrial materials. The non-stoichiometry due to metal vacancies relates to a variety of composition-dependent crystallographic superstructures, but little of the existing structural and microstructural complexity has been explored yet. This thesis investigates the occurrences and nature of pyrrhotite superstructures, examines the related nano- and microstructural phenomena, and explores their effects on chemical reactivity. The goal is to comprehend the relations of the nanoscale real structure of pyrrhotite to its physicochemical properties. A central tool in these studies is analytical transmission electron microscopy (TEM), which has been extensively used to study terrestrial and extraterrestrial samples. In three studies, published or submitted as scientific research articles, it is shown that structural complexity of pyrrhotites is a widespread feature in terrestrial and extraterrestrial materials and is strongly interrelated with its physicochemical properties and environments of formation and alteration. A new model based on translation interface modulation is being introduced to provide a realistic description of the structural state of natural NC-pyrrhotites. Novel insights into the thermodynamically stable phase assemblages in the Fe-S system at ambient temperatures are presented and the crystallography and connected thermochemistry of pyrrhotites are deployed to reach new conclusions about the petrogenetic history of chondritic meteorites and the alteration processes they were involved in. Finally, an experimental alteration study reveals for the fist time quantitatively that the vacancy superstructures and anisotropy of pyrrhotites have tremendous effects on their kinetic behaviors during dissolution under acidic and oxidizing conditions. Intrinsic reactivity differences between 4C- and NC-pyrrhotite are clearly resolved and discusses in the framework of the newly established structure model.
