The Effect of Shock Compression on the Crystal Structure of Cryptomelane (K-OMS-2)

Cryptomelane K-OMS-2 is a manganese oxide that occurs in low temperature geologic environments and is readily synthesized in the lab. A reflux method was used to prepare cryptomelane K-OMS-2 for shock compression experiments using laser driven flyer plates. Powder X-ray diffraction data indicates that cryptomelane K-OMS-2 is shock resistant and largely retains crystallinity, with some major reflections increasing in peak sharpness with others broadening slightly. Transmission electron microscopy data indicates particle size and morphology of shocked material is also largely retained but crystals have amorphous regions near the surfaces, have deformed lattice fringes, and have streaked electron diffraction characteristics. Raman spectroscopy indicates that shock has altered the structure of cryptomelane, with the 570 cm⁻¹ band-shift probably corresponding to a slight decrease in bond length and a dramatic change in band intensity for the 641 cm⁻¹ may be evidence of increasing disorder, and the shift towards lower frequency could be resulted from an increased bond length. The mechanism for shock resistance for these materials is proposed to be a thixotropic rebound where nanoporosity allows for crystals to migrate and the texture then returns to the approximate original position after the shock wave has passed. This investigation opens new areas of research in shock research for mineral stability.