PRELIMINARY ELECTRON MICROSCOPY INVESTIGATION OF SYNTHETIC CRYPTOMELANE

Cryptomelane is a Mn-oxide belonging to the hollandite group that occurs in low temperature geologic environments. Cryptomelane is of great interested because of its cation exchange capacity and catalytic properties in synthetic form. The idealized empirical chemical formula is K(Mn⁴⁺, Mn²⁺)₈O₁₆. However, Mn³⁺ cations are known or are likely to occur in some samples. The mineral is reported to have monoclinic I2/m and tetragonal I4/m polymorphs. In summary the crystal structure consists of four rows of double manganese octahedra (2 x 2) that create a tunnel, which has sites for K⁺ cations, which may be more generally referred to, as A sites. These K⁺ cations can exchange for molecular water and other cations.

Transmission electron microscopy (TEM) investigations of cryptomelane in the literature are of variable quality. Here are presented results of a TEM study on lacey carbon grain mounts and ion-milled sample material generated from a fumaric acid synthesis method. Bright field TEM imaging of grain mounts indicate crystals are blade or tabular fibers that are several hundreds of nm in length and tens of nm 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 will consist of polycrystalline aggregates with crystals approximately 10 to 20 nm in diameter. Observed microtextures may impact cation exchange reactions and may be relevant to explaining variation in environmental properties and technology development. These preliminary results indicate further TEM study involving scanning transmission electron microscopy – energy dispersive spectroscopy and electron energy loss spectroscopy are warranted.