AN ANALYSIS OF THE EFFECTS OF SAMPLE PROCESSING ON X-RAY POWDER DIFFRACTION PEAKS AND THE IMPLICATIONS FOR STUDIES OF SHOCK METAMORPHOSED CARBONATES

Impact craters are formed when shock waves propagate through target rock during an impact event and commonly affect the crystal lattices of existing minerals. Silicates have been the subject of more studies of shock metamorphosed materials because they are more abundant in the Earth’s crust and are deformed in unique ways unlike carbonates. Shock metamorphism can produce disorder within carbonate crystalline lattices that can be measured by X-ray diffraction (XRD). Diffraction peaks of shocked materials have been shown to display reduced peak intensity and broadened peak widths. It has been suggested that the amount of peak broadening may be a good indicator of the shock pressures experienced in the target rock. When rock samples are processed for XRD they are ground into fine powders to minimize crystallographic orientation effects in the diffraction data.

This study assessed the effects of grinding method and grind times on diffraction peaks of unshocked dolostone in order to observe potential changes in the diffraction peaks caused by grinding. A sample of Neoproterozoic Beck Springs Dolomite, Inyo County, CA was cut into 6 aliquots. Each aliquot was ground by 3 methods: using a mechanical pulverizer, by hand using a mortar and pestle, and by hand in solution with alcohol in a mortar and pestle. Using these 3 methods of grinding, the aliquots were ground for increasing amounts of time (ranging from 3 to 18 min). Results indicate that using the mechanical pulverizer produces the most predictable and consistent changes in peak intensity and broadening with increased grind time and is therefore the recommended method to use.

To further test this newly proposed method of sample processing, the mechanical pulverizer was then used on samples of naturally shocked dolostone from the Serpent Mount Impact Structure located in southern, Ohio. Results indicate that when ground for 3 min in the mechanical pulverizer samples from the central uplift have lower intensities and greater peak broadening than samples from the crater rim. This is consistent with what is expected and indicates that grinding in the mechanical pulverizer produces the most predictable amount of deformation to the lattice and should therefore be the method used in future studies of shock metamorphosed carbonates.