Understanding the structure of minerals at the atomic scale: a new perspective enabled by advanced microscopy

Abstract

From oxides to ores and rocks, minerals are the most prevalent materials on Earth. The majority of their properties are the direct result of their microstructure. The investigation of their structure at the nano and micron scale is routinely carried out using techniques such as optical and electron microscopy, X-ray diffraction or secondary ion mass spectrometry. However, these techniques are usually limited in resolution, either spatially or chemically. More recently, atom probe tomography (APT) has emerged as a powerful microscopy technique that can provide 3D maps showing the position and atomic mass of individual atoms with sub-nanometre resolution. The non-conductive character of most minerals, both thermally and electrically, makes their investigation by APT challenging, from sample preparation to data interpretation. However, with the relatively recent development of focused ion beam sample preparation techniques and ultra-violet laser-assisted local electrode atom probe, the APT study of large band gap materials such as oxides has become more successful in the last decade. Advanced microscopy techniques such as transmission Kikuchi diffraction (TKD) or electron backscattered diffraction (EBSD) can also be used in combination with APT, and bring a new perspective to the investigation of the atomic scale structure of minerals, leading to a better understanding of their structure – properties relationships. The overall purpose of this thesis is to develop and apply new methods and techniques for the characterization of the structure of minerals at the atomic scale. This is achieved by means of various advanced microscopy techniques, which are applied to a selection of important scientific questions. By using a combination of APT, TKD, EBSD and transmission electron microscopy we investigate intergranular corrosion in stainless steels, the atomic structure of dental enamel and the robustness of zircon as a geological dating accessory. In this work, intergranular corrosion mechanisms in a commercial austenitic stainless steel (ASS) were revealed using EBSD and correlative TKD/TEM. Characterization by APT of the intergranular iron-chromium spinel formed during corrosion of the ASS revealed new insights at the atomic scale on its role towards the fast corrosion rate of the ASS. With the combined use of EBSD, TKD and APT, the atomic scale distribution of trace elements within dislocations in deformed mineral zircons was investigated for the first time to review the robustness of zircon for radiogenic dating. By using APT and TEM, new structural and elemental analysis of human dental enamel at the atomic scale provided unprecedented information for our understanding of human tooth decay.