3D electron diffraction for the charaterization of cryptocrystalline micro-samples: Applications to planetary sciences

Cutting-edge research topics in Earth and planetary sciences often require the study of small cryptocrystalline polyphasic samples, typically characterized by nano-scale intergrowths, like the products of high-pressure/ high-temperature experiments, fast nucleating minerals at non-equilibrium conditions or micro-sized extraterrestrial materials. Conventional optical imaging and X-ray crystallographic tools may be not sufficient for the proper characterization of such samples. The development of efficient probes able to investigate the nanoworld is therefore crucial to further our understanding of the mineralogical and geochemical processes that regulate Earth and extraterrestrial environments. Over the last ten years, electron diffraction (ED) evolved from a qualitative method restricted to few TEM users, to a robust protocol for phase identification and ab-initio structure determination. Such changes have been possible due to the development of automatic and semi-automatic routines for 3D data collection (Mugnaioli & Gemmi, 2018). This methodology is in principle equivalent to single-crystal X-ray diffraction, but can be performed on crystals 10 to 1000 times smaller. In this contribution, we show recent applications of ED in planetary sciences. In particular, how ED allowed the mineralogical screening of the carbonaceous chondrite CM Paris (Pignatelli et al., 2018) and of a hydrated chondritic micrometeorite (CP94-050-052) through the polytypic description of submicrometric phyllosilicate grains. Moreover, we will present an extensive petrographic and crystallographic study of quartz-coesite mineralogical association in impact ejecta from Kamil Crater, Egypt (Folco et al., 2018), and from the Australasian tektite strewn field. We believe that the extensive application of modern ED techniques on micro-to-nanometer extraterrestrial samples has the potential for significant breakthroughs in our understanding of the Solar System’s formation and evolution. Also, it will allow the thorough exploitation of the evidence already enclosed in the micrometeorite collection recovered within the Progetto Nazionale Ricerche in Antartide (PNRA) and in the forthcoming European space missions. ED will also significantly support other sources of information based on remote sensing and spectroscopy and will therefore ensure better constraints in numerical modeling studies.