Distribution of Uranium Isotopes in Sandy Deposits by Sequential Extraction

The sequential extraction procedure is used to reconstruct the processes of redistribution of uranium isotopes in the mineral phases of the aquifer of the Vendian sandy-argillaceous deposits developed in the coastal territory of the European North of Russia. This aquifer has large resources of drinking and mineral groundwater which, however, are used in extremely limited quantities. This is due to the very complex nature of the hydrochemical conditions, and uranium-isotopic methods are used to clarify these conditions. The following mineral phases of core samples were characterized: adsorbed trace materials and carbonate minerals; ferrihydrate, amorphous minerals of Fe, Al and Si, and secondary U minerals; crystalline iron minerals; clay and some refractory minerals; and all remaining resistant minerals. The most resistant minerals, whose fraction ranges from 70.58 to 96.4%, have a minimum average uranium concentration of 0.47 ppm. This uranium is practically conserved in the rock, as a result of which the average ratio of its 234U:238U isotopes is close to equilibrium. In the remaining fractions, uranium is redistributed by groundwater along their flow lines from recharge areas in watersheds to areas of discharge into river valleys. Its maximum concentration (12.89 ppm) measurement is carried out by coprecipitation with iron hydroxides, then the uranium is adsorbed and precipitated with carbonates (9.14 ppm). The average 234U:238U is maximum in adsorbed trace materials and carbonate minerals (2.39 ± 0.36) and is close to that in fresh groundwater (2.8 ± 0.42). It is also increased in the amorphous minerals of Fe (1.53 ± 0.23). In general, there is a dependence of the 234U:238U activity ratio in rock on the degree of participation of groundwater in the deposition of hydrogenic uranium isotopes into the cracks and pores of these rocks. The results obtained contribute to the refinement of such a parameter as the retardation factor, which is necessary for understanding the processes of migration and concentration of uranium in sedimentary basins.

On mechanisms of mesocrystal formation: magnesium ions and water environments regulate the crystallization of amorphous minerals

Elucidating the emergence of crystalline superstructures from amorphous precursors, hydration environments and ionic constituents can guide transformation and structuration reactions towards distinct micro- and nano-structures.

On the paucity of amorphous minerals in the sensitive postglacial marine clays

The abundance and role of amorphous minerals in the sensitive marine clays of eastern Canada has been a subject of investigation since 1974. Unfortunately, the Segalen "selective" extraction procedure (alternating HCl and NaOH), which was used in most of the experiments, is very aggressive. By attacking both oxide minerals (which were presumed to be amorphous) and crystalline phyllosilicates, its use led to major overestimates of the possible amorphous mineral content. Less aggressive "selective" extraction procedures indicate that less than 1% of the sensitive Champlain Sea sediments is oxide minerals, and Mössbauer spectroscopy has detected only crystalline iron oxides, which are probably of detrital origin. The amount of amorphous oxide minerals is at most a small fraction of 1%. Crystalline iron oxides, in the concentrations naturally present, have been demonstrated to influence the rheology of the sensitive marine clays, but it seems improbable that the very small amounts of amorphous minerals that may be present are geotechnically important. Key words : sensitive marine clays, iron oxides, amorphous minerals, selective extraction.