SHOCK COMPRESSION EFFECTS ON THE CRYSTAL STRUCTURE OF BIRNESSITE
The basic structural unit of birnessite is sheets of MnO6 octahedra, with the interlayers occupied by cations K, Na, Mg, and water depending on the phase. Natural birnessite is typically poorly crystalline and occurs in a wide variety of geologic settings; synthetic samples, however, can have a platy morphology. The extreme pressures induced by dynamic compression have the potential to cause structural disorder, phase transformations, collapse of the interlayer, or melt or glass formation. These possibilities, as well as the relative abundance of birnessite-group members on the surface of the earth and potentially other planetary bodies, make the mineral a natural choice for shock experiment studies.
Scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), and laser fluorescence spectroscopy have been used to characterize the morphology, structure, and chemical composition of the birnessite both before and after mechanical shock. SEM and TEM analysis of recovered post-shock sample fragments shows that the birnessite and associated materials show little to no crystallinity and appear to not have new crystalline phases. This is in contrast to cryptomelane which has been shocked under similar conditions and largely retains its crystal structure, however with demonstrable disorder. These findings have a variety of implications on the behavior of birnessite-group minerals in soil development and natural hypervelocity impact events, as well as their usage in interdisciplinary areas of materials science.