Nuria Sánchez-Pastor

Raman Study of Synthetic Witherite–Strontianite Solid Solutions

ABSTRACT Characterization of zoned crystals of a synthetic witherite–strontianite solid solution (BaxSr1-xCO3) was carried out using electron microprobe analysis and Raman spectroscopy. The sample was obtained by coprecipitation using the silica gel method. As each carbonate crystal from this preparation showed the whole range of intermediate compositions BaxSr1-xCO3, 0.1 ≤ x ≤ 0.9, the solid solution could be studied for single crystals. Peak-shape analysis of the Raman bands showed that the peak shifts depend on the replacement of the Ba and Sr cations introducing different radii and masses. We observed a shift to higher wave numbers for an increase of the SrCO3 content.

Raman spectroscopic characterization of a synthetic, non-stoichiometric Cu–Ba uranyl phosphate

Crystals of phases belonging to the autunite group (general formula X(2+)(UO2)2(X(5+)O4)2·nH2O), specifically the uranyl phosphates (X(5+)=P) metauranocircite (X(2+)=Ba(2+)), metatorbernite (X(2+)=Cu(2+)) and a barian metatorbenite phase (X(2+)=Cu(2+)/Ba(2+)), have been synthesized in a silica gel medium and characterized by Raman spectroscopy. The Raman spectra showed bands in the range 750-1100 cm(-1), which were attributed to the ν1 and ν3 (PO(4))(3-) and (UO(2))(2+) stretching vibrations. By using the wavenumbers of the most intense and well defined ν1 (UO(2))(2+) vibration, the U-O bonds lengths were calculated for the three uranyl phosphate minerals. The results are in good agreement with previous single crystal structure analysis data. Bands in the spectra from 350 to 700 cm(-1) were attributed to the (PO(4))(3-) bending modes. Moreover, in the range 70-350 cm(-1), two groups of bands could be defined. The first group, with vibrations at lower wavenumbers, was attributed to the lattice modes and the second group, from 150 to 350 cm(-1), was assigned to the ν2 (UO(2))(2+) bending mode. Finally, in the case of the barian metatorbernite, bands in the range 1500-3800 cm(-1) were assigned to the OH stretching and the ν2 bending vibrations of water molecules. In this phase, all the vibrations show bandshifts when compared to the vibrations in metatorbernite. These bandshifts can be related to transitional Cu-O and Ba-O bond lengths.

In Situ Nanoscale Observations of Metatorbernite Surfaces Interacted with Aqueous Solutions

Metatorbernite (Cu(UO(2))(2)(PO(4))(2)·8H(2)O) has been identified in contaminated sediments as a phase controlling the fate of U. Here, we applied atomic force microscopy (AFM) to observe in situ the interaction between metatorbernite cleavage surfaces and flowing aqueous solutions (residence time = 1 min) with different pHs. In contact with deionized water the features of (001) surfaces barely modify. However, changes are remarkable both under acidic and basic conditions. In acidic solutions (pH = 2.5) metatorbernite surface develops a rough altered layer and large pits nucleate on it. The altered layer shows a low adhesion and is removed by the AFM tip during the scanning. The large pits spread rapidly, at few tens of nm/s, indicating a collapse of the structure. The combination of dissolution and the presence of defects in the metatorbernite structure can explain both the collapse process and the alteration of the surfaces under acidic conditions. Other mechanisms such as ion exchange reactions remain speculative. In NaOH solutions (pH = 11.5) metatorbernite dissolves by formation of etch pits bounded by steps parallel to [100], the direction of the most straight periodic bond chains (PBCs) in metatorbernite structure. These steps retreat at ∼0.15 nm/s. Under these conditions dissolution is promoted by the formation of stable uranyl carbonate complexes in solution.

On the effect of carbonate on barite growth at elevated temperatures

Abstract The effect of carbonate on the growth of barite {001} surfaces from aqueous solutions supersaturated with respect to barite (Ωbarite ~ 12) was studied by hydrothermal atomic force microscopy (HAFM) and Raman spectroscopy at temperatures ranging from 25 to 70 °C. The experiments showed that the effects of carbonate depend on the specific location of growth. For mono-layers growing on pristine barite, the carbonate-additive promotes growth and the spreading rate of two-dimensional islands increases with temperature. However, growth is inhibited in layers growing on surfaces, which grew in carbonate-containing solution. The threshold carbonate concentration necessary to completely inhibit growth is inversely correlated with temperature. Raman spectroscopy revealed the presence of carbonate within crystals, which grew in carbonate-containing solution. Judging by these findings, incorporation of carbonate into the structure of growing barite as a thermally activated process likely is a controlling factor, which inhibits barite growth. Thus the study shows that additives can exert opposing effects on growth not only depending on additive concentration but also depending on the specific growth location. The implication of this work, therefore, is that bimodal effects of additives on crystal growth occur more frequently than generally recognized. The insights into the mechanisms of such bimodal effects of additives can significantly contribute to the understanding and predictability of the kinetics of macro-scale processes such as barite scale formation or the behavior of barium sulfate in CO2-sequestration fluids

Vaterite Stability in the Presence of Chromate

ABSTRACT Vaterite aggregates grown in Cr(VI)-bearing silica gel were studied by combining scanning electron microscopy and Raman spectroscopy. All the vaterite samples were spherical aggregates consisting of lens-shaped individuals, and their morphology was distinctive for vaterite. Raman spectra corresponding to the aggregates grown in gels containing high Cr(VI) concentrations showed the typical bands of vaterite. However, in contrast to their morphology, the Raman spectra of the aggregates grown in the presence of lower Cr(VI) concentrations exclusively showed bands characteristic of calcite. These aggregates are interpreted as calcite pseudomorphs formed after vaterite through a replacement process. The replacement involves the interface coupling of the dissolution of vaterite and the precipitation of the stable polymorph calcite. The fact that only those vaterite aggregates formed in the presence of low Cr(VI) concentrations transformed into calcite indicates the presence of high Cr(VI) concentrations in the growth medium contributes to stabilize vaterite and prevents its transformation into calcite in the short term.