DIFFRACTION CHARACTERISTICS OF RARE EARTH ELEMENT (REE) - REACTED SYNTHETIC CRYPTOMELANE: COMPLEXITIES IN A NEW GEOMATERIALS GROUP

Cryptomelane is a manganese oxide common to rock varnish, soils and other Mn-rich environments near Earth’s surface. It is an octahedral molecular sieve (OMS-2) material. Although relatively little is known regarding the interaction of rare earth elements (REE) with cryptomelane, these processes may be important for understanding the controls of distribution of REE in these near earth-surface environments. We investigated the interaction of selected REE with a synthetic cryptomelane using nitrate solutions at approximate standard-state conditions.

Transmission electron microscopy (TEM) investigation indicates that reactions of the selected REE nitrate solutions with cryptomelane behave differently. There is a clear catalytic precipitation among the Ce-reacted and Eu-reacted materials. Electron diffraction data is consistent with cerianite (CeO₂) and Eu precipitates are amorphous. La-reacted products seem to be simple cation exchange reactions.

The un-reacted synthetic cryptomelane and REE-reacted products were investigated with powder X-ray diffraction (XRD) using long scan times (~12 hours) owing to the poor crystallinity of the samples. XRD data indicate that there are differences between the starting material and the Ce-reacted products, but they are functionally indistinguishable due to the overlapping of the major peaks of cryptomelane and cerianite. Although limited in number of samples, La-reacted and Ce-reacted samples showed clear peak broadening or splitting of peaks for the first reflection (110) of cryptomelane indicating possible effects of cation exchange, particle size reduction, or overlap or near overlap with phases that cannot be clearly identified. These results may produce limits for comparison with TEM data, but they indicate that, because the systems are so complex, TEM investigation is necessary to identify which phases are present in the reactions. Scanning transmission electron microscopy – energy dispersive spectroscopy analysis combined with additional XRD investigation, including synchrotron powder diffraction analysis, is required to better understand this emerging geomaterials group.