Electron Microscopy and Diffraction Analyses of Nano-Crystalline Phases in Planetary Materials

This work focuses on the mineralogical and petrological investigation of two important and common high-pressure phases in impact rocks: coesite, a high-pressure polymorph of SiO2, which can be used as diagnostic indicator of impact cratering in quartz-bearing target rocks, and TiO2 II, a high-pressure polymorph of TiO2, a common accessory phase in many rocks of the crust. The ejecta samples studied here are from two different impact events: Kamil crater (Egypt), a small-scale simple crater of 45 m in diameter for the study of coesite, and the large-scale Australasian tektite/microtektite strewn field for the study of both coesite and TiO2 II. I show evidence for coesite formation through direct quartz-to-coesite transformation in subsolidus conditions. In Kamil crater, coesite occurs as individual and spheroidal grains representing the first crystallization seeds of coesite. In the Australasian samples, coesite occurs as individual grains or aggregate, either with subrounded or subhedral shape. In both impact sites, quartz and coesite, when in direct contact, show a recurrent reciprocal orientation, with the {10-11} or {-1011} plane families of quartz almost parallel with the plane (010) of coesite. A possible way to transform quartz into coesite is through a martensitic-like transformation, with a relative structural shift of quartz plains along {10-11} that turn into coesite (010) plains.The 3D ED study of TiO2 II reveals that this phase has a primitive orthorhombic lattice with the cell parameters a = 4.7(1) Å, b = 5.5(1) Å, c =5.0(1) Å, and space group Pbcn. This is the same structure as the srilankite (Zr,Ti)O2. So, the name TiO2 II should be used referring to the Ti endmember of srilankite.