Gucsik, and 6 co-authors, reported in 2015 that quartz grains from the Ries impact structure containing shock-induced microstructures were investigated using Scanning Electron Microscopy in cathodoluminescence (SEM-CL), secondary electron (SEM-SE) and back-scattered electron (SEM-BSE) modes as well as Mott–Seitz analysis. The purpose of this study is to evaluate the mechanism by which CL detects Planar Deformation Features (PDFs) in quartz, which is one of the most important indicators of shock metamorphism in rock-forming minerals.
PDFs are micron-scale features not easily identified using optical microscopy or scanning electron microscopy. The CL spectrum of PDFs in quartz that has suffered relatively high shock pressure shows no or a relatively weak emission band at around 385 nm, whereas an emission band with a maximum near 650 nm is observed independent of shock pressure. Thus, the ~385 nm intensity in shocked quartz demonstrates a tendency to decrease with increasing shock metamorphic stage, whereas the 650 nm band remains fairly constant. The result indicates that the emission band at 385 nm is related to the deformed structure of quartz as PDFs.
The authors concluded that cathodoluminescence (CL) microscopy on PDFs in shocked quartz grains from the Ries impact crater, Germany, shows these features as dark lines (no visible light emitted). The CL spectra of these samples exhibit broad bands at ~385 nm in the near-ultraviolet to blue range, and at ~650 nm in the red range, which might be associated with Al centers and a non-bridging oxygen hole center (NBOHC), respectively. This indicates that recombination centers or traps for photon emission are more closely-spaced within the BG, which is probably caused by the partial amorphization of the quartz and/or an instantaneous increase in temperature up to 500 °C. Thus, the intensity of the emission band at 385 nm has potential as a shock indicator of shock in quartz.
Concerning the CL spectral features as well as Mott–Seitz analysis, we conclude that the phase transformation of quartz leads to small symmetry and electronic structure breaking displacements of atoms resulting in non-radiative processes in the luminescence emission. The authors noted that the CL technique employed here is a useful and powerful tool for the identification of shock-induced micro-deformation in rock-forming minerals. The authors have interpreted the lack of emission in PDFs in shocked quartz as a lack of recombination of monovalent ions and/or Al centers within the SiO4 lattice structure. This is in agreement with the high degree of disorder in the amorphous materials as detected by micro-Raman spectroscopy. SEM-BSE imaging shows no compositional zoning at the boundaries between PDFs and the quartz host, nor evidence of any nucleation or growth of new product phase indicating the glassy infilling of the PDFs is truly amorphous. However, quartz high-pressure polymorphs, stishovite and coesite, also appear dark under SEM-CL, but these minerals have distinct Raman spectra, ruling out the likelihood of a polymorphic phase transformation.
Future detailed analysis using SEM-CL on PDFs, stishovite and coesite are planned. Structurally, the PDFs are brittle fractures along crystallographic planes, containing an infilling of amorphous material involving a syndeformational shear component. These features combined with the lack of Al and/or monovalent recombination indicate the dominance of combined or competing pressure or strain rate control on PDF formation via disequilibrium martensitic transformation of the shocked quartz, rather than displacive or reconstructive mineral transformations (Sharp and de Carli 2006). The observations and inferences from this study show that SEM-CL, combined with other compositional and spectroscopic data, may prove a fundamental tool in analyzing and mapping physico-chemical microscale defect distributions and deformation processes of interest to geoscientists.
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