Early stage amorphous precursors provide a low energy pathway for carbonate mineralization. Many natural deposits of carbonate minerals and biogenic calcium carbonate (both amorphous and crystalline) include significant amounts of Mg. To understand the role of magnesium-containing amorphous precursors in carbonate mineralization, we investigated the energetics and structure of synthetic amorphous Ca–Mg carbonates with composition Ca1−xMgxCO3·nH2O (0⩽x⩽1) using isothermal acid solution calorimetry and synchrotron X-ray scattering experiments with pair distribution function (PDF) analysis. Amorphous magnesium carbonate (AMC with x=1) is energetically more metastable than amorphous calcium carbonate (ACC with x=0), but it is more persistent (crystallizing in months rather than days under ambient conditions), probably due to the slow kinetics of Mg2+ dehydration. The Ca1−xMgxCO3·nH2O (0⩽x⩽1) system forms a continuous X-ray amorphous series upon precipitation and all intermediate compositions are energetically more stable than a mixture of ACC and AMC, but metastable with respect to crystalline carbonates. The amorphous system can be divided into two distinct regions. For x=0.00–0.47, thermal analysis is consistent with a homogeneous amorphous phase. The less metastable compositions of this series, with x=0.0–0.2, are frequently found in biogenic carbonates. If not coincidental, this may suggest that organisms take advantage of this single phase low energy amorphous precursor pathway to crystalline biogenic carbonates. For x⩾0.47, energetic metastability increases and thermal analysis hints at nanoscale heterogeneity, perhaps of a material near x=0.5 coexisting with another phase near pure AMC (x=1). The most hydrated amorphous phases, which occur near x=0.5, are the least metastable, and may be precursors for dolomite formation.
Energy-dependent phenomenological optical model potentials based on a single-folding model approach for the α- 12 C and deuteron- 12 C systems were obtained for incident energies between 10 and 100 MeV/nucleon. The Lane-consistent Bruyères Jeukenne–Lejeune–Mahaux model nucleon–nucleus potentials were used in these calculations. With only three free parameters, these potentials account satisfactorily for both angular distributions of elastic scattering cross sections and total reaction cross sections of both projectile–target systems within the energy range studied. Comparisons were made between the present potentials and other systematic phenomenological potentials.
The simultaneous homogeneous and heterogeneous precipitation of hydrous Fe(III) oxides was investigated in the presence of environmentally ubiquitous anions (nitrate, chloride, and sulfate). Experiments were conducted with 10–4 M Fe(III) at acidic pH (pH = 3.7 ± 0.2), which often occurs at acid mine drainage sites or geologic CO2 storage aquifers near injection wells. Quartz was used as a model substrate for heterogeneous precipitation. Small angle X-ray scattering (SAXS) and grazing incidence SAXS (GISAXS), atomic force microscopy (AFM), and dynamic light scattering (DLS) measurements were conducted. In situ SAXS/GISAXS quantified the size, total particle volume, number, and surface area evolutions of the primary nanoparticles formed in the nitrate and chloride systems. In both systems, the heterogeneously precipitated particles were smaller than the homogeneously precipitated particles. Compared with chloride, the volume of heterogeneously precipitated hydrous Fe(III) oxides on the quartz surface was 10 times more in the nitrate system. After initial fast heterogeneous nucleation in both nitrate and chloride systems, nucleation, growth, and aggregation occurred in the nitrate system, whereas Ostwald ripening was the dominant heterogeneous precipitation process in the chloride system. In the sulfate system, fast growth of the heterogeneously precipitated particles and fast aggregation of the homogeneously precipitated particles led to the formation of particles larger than the detection limit of GISAXS/SAXS. Thus, the sizes of the particles precipitated on quartz surface and in solution were analyzed with AFM and DLS, respectively. This study provides unique qualitative and quantitative information about the location (on quartz surfaces vs in solutions), size, volume, and number evolutions of the newly formed hydrous iron oxide particles in the presence of quartz substrate and ubiquitous anions, which can help in understanding the fate and transport of pollutants in the environment.