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Shock deformation of calcite, a possible tool for reconstruction of the size of deeply eroded impact structure. Agoudal impact structure (Morocco) case study

Study from Ries crater in Germany showed that peaks in X-ray diffraction patterns of shocked carbonates are broadened due to increased domain size and microstrain and the deformation is correlated with peak pressure. It is expected that shock-induced deformations of calcite unit-cell decreases with increasing distance from the impact structure centre, which would allow for estimation of the structure size by analysis of diffractograms of a number of samples from a deeply eroded structure. We have tested this hypothesis by analyzing target limestone samples from various locations within the Agoudal impact structure in Morocco. The XRD powder analysis revealed peak broadening in X-ray diffraction pattern, as well as peak shift that indicates shock induced effects in calcite. The observed peak broadening was determined by calculation of Full Width at Half Maximum (FWHM). The domain size and strain are determined after Williamson-Hall method by calculations in XPowderX software. Also, unit-cell parameters were refined from peak positions. Comparison of X-ray diffraction patterns from different locations in the Agoudal impact structure revealed that shock-induced effects decrease with distance from the presumed impact centre.

X-ray diffraction, calcite, peak broadening, microstrain

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