INITIAL CHARACTERIZATION OF SYNTHETIC GOETHITE POST-SHOCK COMPRESSION USING SCANNING ELECTRON MICROSCOPY INDICATES SHOCK RESISTANCE

The effects of shock compression on goethite have not been well studied. Goethite (FeO(OH)) is one of the most common iron oxyhydroxide minerals and is abundant in soils from the weathering of other iron-rich minerals. Goethite forms acicular crystals and are elongated along the {001} plane. Within the structure there are channels along {001} because of the vacant rows of octahedral sites. When subjected to shock these channels may potentially collapse and rebound and combined with acicular habit may allow the goethite structure to compact offering some shock resistance.

Shock compression of the goethite sample was achieved by using an inverted shock microscope and laser driven flyer plates. A high-energy laser to shoots small aluminum discs at high velocity towards the sample causing compression upon impact. A Nd:YAG laser of wavelength 1064 nm, with a maximum pulse energy of 2.5 J, pulse width of 20 ns, and a high value of M²=40 is used as the launch laser. In this experiment, 25 µm Al flyer plates with 3.5 km/s impact velocities to produce planar shock waves of 5 ns duration in the sample were used. Initial results of the shock compression experiment were then examined using a scanning electron microscope (SEM) and an energy-dispersive X-ray spectrometer (EDS).

Post-shock goethite shows the same “bird’s nest” texture observed prior to shock and thus it is maintained after shock. The acicular shaped grains vary in size but are on average 0.5-1-micron long. Sparse equant iron-rich areas show higher contrast than goethite indicating a possible phase transformation has occurred locally. Other unique features were observed such as aluminum and iron spherules and pure elemental iron in a few instances. Based off the initial characterization, goethite shows it is resistant to shock compression. Further TEM and X-ray diffraction analysis is planned. This study opens comparative investigations of several oxide/oxyhydroxide minerals.