Bulk modulus of Fe-rich olivines corrected for non-hydrostaticity

In situ X-ray diffraction was used to measure the isothermal bulk modulus at room conditions (K_T0) of synthetic olivines with different iron contents. The chemical formulae of the olivine samples were ${({\mathrm{Fe}}_x,{\mathrm{Mg}}_{1?x})}_2\mathrm{Si}{\mathrm{O}}_4$ with x = 0.45; 0.64; 0.82; 1, with 1% standard deviation (referenced as Fa45, Fa64, Fa82 and Fa100, respectively). All experiments were performed in the multi-anvil apparatus installed at NSLS beamline X17B2, to pressures up to about 7 GPa. Unit-cell volumes under hydrostatic conditions and differential stresses present in the samples were calculated using the method developed by Singh et al. (1998), and pressures measured using NaCl as a standard were then corrected for these stresses. Using a second-order Birch–Murnaghan equation of state, we obtained the isothermal bulk modulus of each composition: $K_{T0}^{\mathrm{Fa45}}=131.4\pm 2.6$ GPa, $K_{T0}^{\mathrm{Fa64}}=132.1\pm 3.1$ GPa, $K_{T0}^{\mathrm{Fa82}}=136.3\pm 1.7$ GPa and $K_{T0}^{\mathrm{Fa100}}=134.8\pm 1.4$ GPa. These values combined with data available in the literature show that the K_T0 of Fe-rich olivines increases very slowly with the Fe content, but possibly not in a simple linear trend.     

Surface chemistry and reactivity of plant phytoliths in aqueous solutions

In order to better understand the reactivity of plant phytoliths in soil solutions, we determined the solubility, surface properties (electrophoretic mobilities and surface charge) and dissolution kinetics of phytoliths extracted from fresh biomass of representative plant species (larch tree and elm, horsetail, fern, and four grasses) containing significant amount of biogenic silica. The solubility product of larch, horsetail, elm and fern phytoliths is close to that of amorphous silica and soil bamboo phytoliths. Electrophoretic measurements yield isoelectric point pH_IEP = 0.9, 1.1, 2.0 and 2.2 for four grasses, elm, larch and horsetail phytoliths respectively, which is very close to that of quartz or amorphous silica. Surface acid–base titrations allowed generation of a 2-pK surface complexation model (SCM) for larch, elm and horsetail phytoliths. Phytoliths dissolution rates, measured in mixed-flow reactors at far from equilibrium conditions at 1 ≤ pH ≤ 8, were found to be very similar among the species, and close to those of soil bamboo phytoliths. Mechanistic treatment of all plant phytoliths dissolution rates provided three-parameters equation sufficient to describe phytoliths reactivity in aqueous solutions:$R(\text{mol}/{\text{cm}}^2/\text{s})=6?{10}^{?16}?a_{\text{H+}}+5.0?{10}^{?18}+3.5?{10}^{?13}?a_{\text{OH}?}^{0.33}$Alternatively, the dissolution rate dependence on pH can be modeled within the concept of surface coordination theory assuming the rate proportional to concentration of > SiOH₂⁺, > SiOH⁰ and > SiO^? species. In the range of Al concentration from 20 to 5000 ppm in the phytoliths, we have not observed any correlation between their Al content and solubility, surface acid–base properties and dissolution kinetics.It follows from the results of this study that phytoliths dissolution rates exhibit a minimum at pH ～ 3. Mass-normalized dissolution rates are similar among all four types of plant species studied and these rates are an order of magnitude higher than those of typical soil clay minerals. The minimal half life time of larch and horsetail phytoliths in the interstitial soil solution ranges from 10–12 years at pH = 2–3 to < 1 year at pH above 6, comparable with mean residence time of phytoliths in soil from natural observations.     

Temporal dynamics of AVS and SEM in sediment of shallow freshwater floodplain lakes

Acid volatile sulfide (AVS) is an operationally defined sulfide fraction, which is considered important for trace metal fate in reduced sediments. Understanding AVS formation rates is important for the management of metal polluted sediment. However, little is known about the fate and dynamics of AVS in spatially and seasonally variable freshwater environments. The authors monitored in situ AVS formation and degradation and simultaneously extracted metals (SEM) in two floodplain lakes and compared this to AVS formation rates in laboratory experiments with the same sediment. In the laboratory experiments, the formation rates of AVS were studied at 20 °C for initially oxidized sediments that were: (a) untreated; (b) enriched with extra ${\mathrm{SO}}_4^{2-}$; and (c) treated with sodium-azide (biocide). In the field, AVS concentrations were highly variable and were significantly correlated to surface water temperature and O₂ concentrations as well as to sediment composition. Between February and August, AVS formation was approximately linear at a rate of 0.07 μmol g⁻¹ d⁻¹. Degradation rates differed drastically between the lakes due to different degradation mechanisms. In one lake AVS removal was caused by reworking and oxygenation of the sediments by bream (Abrami brama), at a rate of 0.25 μmol g⁻¹ d⁻¹. In the other lake AVS removal was caused by desiccation, at a rate of ±2.6 μmol g⁻¹ d⁻¹. This illustrates the large differences that can be found between similar lakes, and the importance of biological processes. In the laboratory, concentrations of AVS with and without ${\mathrm{SO}}_4^{2-}$ addition were similar during the first weeks, and increased at a rate of 0.15 μmol g⁻¹ d⁻¹. However, ${\mathrm{SO}}_4^{2-}$ addition increased the AVS concentration at the end of the experiment, whereas sodium-azide eliminated AVS formation, as expected. This suggests that AVS formation was ${\mathrm{SO}}_4^{2-}$-limited in the laboratory as well as in these shallow freshwater lakes.