THE EFFECT OF SHOCK COMPRESSION ON THE CRYSTAL STRUCTURE OF CRYPTOMELANE

Shock compression effects in minerals can be studied by utilizing the laser-driven flyer plate experiment. This is a dynamic compression technique where a millimeter-scale metal disc is launched at a sample, maintaining speeds of 1-4 km/s, and impacts it to subject it to high pressure. Mechanical shock effects in geological samples are studied with this experimental method, along with powder X-ray diffraction (PXRD), spectroscopy, and microscopy techniques to probe potential structural and chemical changes. There are many applications for mechanical shock studies in geology, including investigating catalysis properties, studying soil development on planetary bodies, and researching events where a hypervelocity impact is associated, such as meteorite impacts.

Cryptomelane, a manganese oxide and member of the hollandite supergroup, has double chains of edge-sharing Mn-octahedra and is commonly found in a fibrous habit. The structure is a tunnel topology and crystals are often tens of nanometers wide and several micrometers long. Thus extreme pressure has the potential to induce effects such as crystallographic disorder, complete phase transformation, or melt or glass formation. Owing to the ubiquity of manganese oxides in both ocean and terrestrial settings, as well as the numerous potential applications of the cryptomelane tunnel structure, the mineral is an excellent candidate for novel shock experiment studies.

PXRD, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and laser fluorescence spectroscopy were used to characterize the structure, morphology, and fluorescence of both pre and post-shock crystals of cryptomelane. After experiencing mechanical shock, PXRD analysis of the recovered sample fragments showed narrowing of Bragg peaks, while TEM analysis and electron diffraction of post-shock samples also showed that there was little destruction to the original structure. In fact, the original fibrous morphology remained intact, with the crystals forming small bundles. The lack of abundant structural reorganization could have further implications for the use of cryptomelane in various interdisciplinary areas of science and engineering. These findings have additional implications for the stability of cryptomelane in soils and surfaces of planetary bodies.