COMPARING THE EFFECTS OF DYNAMIC SHOCK COMPRESSION OF TWO MAJOR OXIDE MINERALS: CRYPTOMELANE AND GOETHITE

Cryptomelane (K(Mn4+, Mn2+)8O16) and goethite (α-FeOOH) are common minerals that appear in many geologic environments such as soils and desert rock varnish. Two recent publications in our lab group explored the effects of dynamic shock compression (DSC) of each mineral in detail using multiple analytical tools such as confocal raman spectroscopy (RS), powder X-ray diffraction (pXRD), and transmission electron microscopy (TEM). Both minerals showed promise for being shock resistant, however, cryptomelane underwent less chemical and structural changes compared to goethite. Laser-driven DSC was performed for each experiment using Al flyer plates to contact the minerals at 3.5 km/s. Based on Hugoniot calculation and impedance matching, cryptomelane was found to be shocked at ~36.8 GPa, and goethite was shocked at ~ 41.6 GPa. Confocal RS indicates that cryptomelane underwent possible structural changes due to 2 peaks at 570 cm-1 and 641 cm-1 inverting in addition to peak shifts attributed to changes in Mn-O bond lengths. Changes of intensity also occurred on the post shocked samples indicating phase transitions. Confocal RS showed significant changes in peaks indicating structural changes from the mineral transforming to hematite (Fe2O3). Interestingly, neither material became amorphous after the shock compression experiments verified by pXRD. No major changes were observed in pXRD patterns for cryptomelane. However, goethite underwent some phase transformations to hematite in addition to iron aluminum phases being detected due to the Al flyer plate. TEM found both materials mostly unchanged after shock compression. Cryptomelane and goethite showed a “birds nest” aggregate of crystals before and after DSC leading to a hypothesis of a thixotropic-like model being proposed as the dominant mechanism occurring. Amorphous regions near the edges and surface of the cryptomelane and goethite were detected after examining the lattice fringes of each mineral. Electron diffraction patterns also identified a change in crystallinity in both minerals and can be observed by streaking in the patterns. These studies have led to important conclusions and future research directions regarding structural integrity and potential mechanochemical applications of the minerals. Future research will investigate metal-doped versions of cryptomelane and goethite systematically.