Entropy Drives Calcium Carbonate Ion Association

The understanding of the molecular mechanisms underlying the early stages of crystallisation is still incomplete. In the case of calcium carbonate, experimental and computational evidence suggests that phase separation relies on so-called pre-nucleation clusters (PNCs). A thorough thermodynamic analysis of the enthalpic and entropic contributions to the overall free energy of PNC formation derived from three independent methods demonstrates that solute clustering is driven by entropy. This can be quantitatively rationalised by the release of water molecules from ion hydration layers, explaining why ion association is not limited to simple ion pairing. The key role of water release in this process suggests that PNC formation should be a common phenomenon in aqueous solutions

Neutron imaging with fission and thermal neutrons at NECTAR at MLZ

The instrument NECTAR is located at beam port SR10 of the neutron source FRM II at the Heinz Maier-Leibnitz Zentrum (MLZ). With a pair of moveable uranium plates placed in front of the entrance window of the beam tube, a fission neutron spectrum with a mean energy of 1.9 MeV can be used for neutron imaging applications. Via remote control these plates can be removed and a thermal neutron spectrum (mean energy at 28 meV) gets available for experiments. While the fission neutron spectrum is regularly used, some upgrades of the instrument are necessary to make the thermal neutron spectrum routinely available for user experiments. This includes additional equipment like a new sample stage and a second detector system foreseen to extend the capabilities of NECTAR. The current state of the instrumentation and necessary changes for the future thermal beam option and its usage for standard user experiments will be presented. First measurements were carried out with a temporary flight tube installed and a compact detector (510 mm × 180 mm x 180 mm) for thermal neutrons with a spatial resolution in the range of 100 μm. The feasibility of the thermal beam option could already be verified at an L/D ratio of 240 and a thermal neutron flux of 7.92·106 cm−2 s−1. The thermal neutron beam option adds a pure thermal neutron spectrum – Maxwell spectrum originating from the moderator without alteration by a secondary source or converter – to the energy ranges available for neutron imaging at MLZ instruments. It also offers a unique possibility to combine two quite different neutron energy ranges at a single instrument including their respective advantages. The thermal neutron beam option is funded by BMBF in the frame of research project 05K16VK3

Nanoscale Anatomy of Iron-Silica Self-Organized Membranes: Implications for Prebiotic Chemistry

Iron-silica self-organized membranes, so-called chemical gardens, behave as fuel cells and catalyze the formation of amino/carboxylic acids and RNA nucleobases from organics that were available on early Earth. Despite their relevance for prebiotic chemistry, little is known about their structure and mineralogy at the nanoscale. Studied here are focused ion beam milled sections of iron-silica membranes, grown from synthetic and natural, alkaline, serpentinization-derived fluids thought to be widespread on early Earth. Electron microscopy shows they comprise amorphous silica and iron nanoparticles of large surface areas and inter/intraparticle porosities. Their construction resembles that of a heterogeneous catalyst, but they can also exhibit a bilayer structure. Surface-area measurements suggest that membranes grown from natural waters have even higher catalytic potential. Considering their geochemically plausible precipitation in the early hydrothermal systems where abiotic organics were produced, iron-silica membranes might have assisted the generation and organization of the first biologically relevant organics

Characterization of relativistic electron bunch duration and travelling wave structure phase velocity based on momentum spectra measurements on the ARES linac at DESY

The ARES linac at DESY aims to generate and characterize ultrashort electron bunches (fs to sub-fs duration) with high momentum and arrival time stability for the purpose of applications related to accelerator R&D, e.g. development of advanced and compact diagnostics and accelerating structures, test of new accelerator components, medical applications studies, machine learning, etc. During its commissioning phase, the bunch duration characterization of the electron bunches generated at ARES has been performed with an RF-phasing technique relying on momentum spectra measurements, using only common accelerator elements (RF accelerating structures and magnetic spectrometers). The sensitivity of the method allowed highlighting different response times for Mo and Cs2Te cathodes. The measured electron bunch duration in a wide range of machine parameters shows excellent agreement overall with the simulation predictions, thus demonstrating a very good understanding of the ARES operation on the bunch duration aspect. The importance of a precise in-situ experimental determination of the phase velocity of the first travelling wave accelerating structure after the electron source, for which we propose a simple new beam-based method precise down to sub-permille variation respective to the speed of light in vacuum, is emphasized for this purpose. A minimum bunch duration of 20 fs rms, resolution-limited by the space charge forces, is reported. This is, to the best of our knowledge, around 4 times shorter than what has been previously experimentally demonstrated based on RF-phasing techniques with a single RF structure. The present study constitutes a strong basis for future time characterization down to the sub-fs level at ARES, using dedicated X-band transverse deflecting structures.Comment: 17 pages, 11 figures. To be submitted to Physical Review Accelerators and Beam

The suppression of fluorescence peaks in energy-dispersive X-ray diffraction

A novel method to separate diffraction and fluorescence peaks in energy- dispersive X-ray diffraction (EDXRD) is described. By tuning the excitation energy of an X-ray tube source to just below an elemental absorption edge, the corresponding fluorescence peaks of that element are completely suppressed in the resulting spectrum. Since Bremsstrahlung photons are present in the source spectrum up to the excitation energy, any diffraction peaks that lie at similar energies to the suppressed fluorescence peaks are uncovered. This technique is an alternative to the more usual method in EDXRD of altering the scattering angle in order to shift the energies of the diffraction peaks. However, in the back-reflection EDXRD technique [Hansford (2011). J. Appl. Cryst. 44, 514–525] changing the scattering angle would lose the unique property of insensitivity to sample morphology and is therefore an unattractive option. The use of fluorescence suppression to reveal diffraction peaks is demonstrated experimentally by suppressing the Ca K fluorescence peaks in the back-reflection EDXRD spectra of several limestones and dolomites. Three substantial benefits are derived: uncovering of diffraction peak(s) that are otherwise obscured by fluorescence; suppression of the Ca K escape peaks; and an increase in the signal-to-background ratio. The improvement in the quality of the EDXRD spectrum allows the identification of a secondary mineral in the samples, where present. The results for a pressed-powder pellet of the geological standard JDo-1 (dolomite) show the presence of crystallite preferred orientation in this prepared sample. Preferred orientation is absent in several unprepared limestone and dolomite rock specimens, illustrating an advantage of the observation of rocks in their natural state enabled by back-reflection EDXRD

A role for diatom-like silicon transporters in calcifying coccolithophores

Biomineralization by marine phytoplankton, such as the silicifying diatoms and calcifying coccolithophores, plays an important role in carbon and nutrient cycling in the oceans. Silicification and calcification are distinct cellular processes with no known common mechanisms. It is thought that coccolithophores are able to outcompete diatoms in Si-depleted waters, which can contribute to the formation of coccolithophore blooms. Here we show that an expanded family of diatom-like silicon transporters (SITs) are present in both silicifying and calcifying haptophyte phytoplankton, including some globally important coccolithophores. Si is required for calcification in these coccolithophores, indicating that Si uptake contributes to the very different forms of biomineralization in diatoms and coccolithophores. Significantly, SITs and the requirement for Si are absent from highly abundant bloom-forming coccolithophores, such as Emiliania huxleyi. These very different requirements for Si in coccolithophores are likely to have major influence on their competitive interactions with diatoms and other siliceous phytoplankton

Calcium sulfate precipitation pathways in natural and engineering environments

International audienceThe solution-mediated formation of calcium sulfate minerals, i.e. gypsum, anhydrite and bassanite, is a common process in both natural and engineered settings. It plays a key role in the global sulfur cycle and serves as an indicator of past environmental conditions on Earth and Mars. Products relying on the crystallization of these minerals have been employed since antiquity, and today they are an essential part of a wide array of industrial applications. Accordingly, the fundamental aspects of calcium sulfate mineralization have been the focus of intensive research during the past century. However, a recent flurry of studies addressing alternative, i.e. non-classical, nucleation and growth mechanisms has spurred a revisit of the precipitation pathway of the most common phase, gypsum. The newly obtained data sketch a far more complex picture of the mineralization process than previously assumed. This has important consequences for the interpretation of calcium sulfate deposits, both from a geochemical and industrial point of view. In order to shed light on this issue, we discuss in this review both recent and long-standing observations of abiotic formation routes of calcium sulfate minerals as a function of the physicochemical solution properties. By integrating both the classical and non-classical perspectives on crystallization we put forward a unified model for calcium sulfate crystallization. Using this model, we (re)-evaluate the phase stability and transformations taking place in the CaSO 4-H 2 O system. Next, we look into the formation of calcium sulfate minerals occurring in close association with the biosphere. Employing the abiotic case scenario as a benchmarking tool, the possible influence and/or control exerted by biological activity (and its byproducts) on the precipitation pathway is critically reviewed. Finally, we point out the central issues that need to be resolved if we wish to fully understand and control the formation of calcium sulfate solids in natural and engineered environments