PRELIMINARY ELECTRON MICROSCOPY INVESTIGATION COMPARING SYNTHETIC AND NATURALLY OCCURING CRYPTOMELANE

Cryptomelane is a natural mineral as well as a synthetic applied material that is receiving increasing attention for environmental applications. There are comparatively few electron microscopy studies of natural cryptomelanes and thus there is a poor understanding of the variability of cryptomelane textures. Most electron microscopy studies of synthetic cryptomelane are limited to basic textural SEM and TEM investigations. Here are presented comparative analyses of an exemplary natural sample and synthetic samples.

The Xiangtan manganese deposit in China has large amounts of cryptomelane and is a source of Mn, but may also be an industrial mineral source of cryptomelane. Field emission scanning electron microscopy (FESEM) investigation under variable pressure conditions using nitrogen as the compensating gas indicates a botryoidal texture of round spheres that range 1-5 micrometers in diameter, as well as a dendritic “tree” textures. The acicular fibers of the dendritic texture are approximately 1-3 μm in length and tens of nanometers wide. A dendritic texture is suggests that Mn-rich waters precipitated in cavities and pores. Observed textures illustrate complexities of the phases present.

Bright field TEM imaging of synthetic cryptomelane indicates crystals are blade or tabular fibers that are several hundreds of nanometers in length and tens of nanometers in width. Morphologies are consistent with monoclinic symmetry as are electron diffraction patterns. From grain mount and ion-milled material structural imaging is possible. Some defects have been observed with some sample material showing apparent omission of octahedral rows and other examples showing possible offset and twinning. Less commonly regions of sample consist of polycrystalline aggregates with crystals approximately 10 to 20 nanometers in diameter.

TEM investigation indicates that the diversity of textures and defects of cryptomelane may be more extensive than previously documented. TEM investigation indicates a range of textures and microtextures may impact cation exchange reactions and may be relevant to explaining variation in environmental properties and technology development.