ALBERT

Description: Stable paleomagnetic information in meteoritic metal is carried by the “cloudy zone”: ~1–10 μm‐wide regions containing islands of ferromagnetic tetrataenite embedded in a paramagnetic antitaenite matrix. Due to their small size and high coercivity (theoretically up to ~2.2 T), the tetrataenite islands carry very stable magnetic remanence. However, these characteristics also make it difficult to image their magnetic state with the necessary spatial resolution and applied magnetic field. Here, we describe the first application of X‐ray holography to image the magnetic structure of the cloudy zone of the Tazewell IIICD meteorite with spatial resolution down to ~40 nm and in applied magnetic fields up to ±1.1 T, sufficient to extract high‐field hysteresis data from individual islands. Images were acquired as a function of magnetic field applied both parallel and perpendicular to the surface of a ~100 nm‐thick slice of the cloudy zone. Broad distributions of coercivity are observed, including values that likely exceed the maximum applied field. Horizontal offsets in the hysteresis loops indicate an interaction field distribution with half width of ~100 mT between the islands in their room temperature single‐domain state, providing a good match to first‐order reversal curve diagrams. The results suggest that future models of remanence acquisition in the cloudy zone should take account of strong interactions in order to extract quantitative estimates of the paleofield.

Description: Plain Language Summary: Magnetic fields played a significant role in the formation of the solar system and the evolution of the early planetary bodies in the first few million years after solar system formation. Knowledge about magnetic fields in the early solar system can be obtained from meteorites. Some meteorite types contain abundant iron‐nickel alloy that contains nanoscale “cloudy zone” regions (named after their appearance in an optical microscope) that can preserve magnetic information over 4.5 billion years. The cloudy zone is a complex material consisting of magnetically stable nanoscale particles embedded in a nonmagnetic matrix in very close proximity to one another. The fine scale and extreme magnetic stability of the cloudy zone make it challenging to study using conventional magnetic microscopy techniques. Here, we apply X‐ray holography for the first time to image the magnetization of individual magnetic particles and how they respond to magnetic fields. This new approach enables us to measure the magnetic properties of individual nanoscale particles, providing the first direct measurement of their magnetic stability and the strength of particle interactions. These measurements will improve our understanding of the magnetic information carried by the cloudy zone, and of how to extract information about solar system magnetic fields.

Description: Key Points: X‐ray holography enables magnetization of natural samples to be imaged with ~40 nm resolution and in applied magnetic fields up to ±1.1 T. Meteoritic cloudy zone consists of strongly interacting single‐domain particles with single‐particle coercivities up to 1 T. Average interaction fields between particles in the cloudy zone are of the order 100–200 mT.

Description: European Commission (EC) http://dx.doi.org/10.13039/501100000780

Description: Europen Commission

Description: European Research Council (ERC) http://dx.doi.org/10.13039/501100000781

Description: European Research Council under the European Union's Seventh Framework Programme

Description: Glacial meltwaters export substantial quantities of dissolved and dissolvable amorphous silicon (DSi and ASi), providing an essential nutrient for downstream diatoms. Evidence suggests that glacially exported DSi is isotopically light compared to DSi in non-glaciated rivers. However, the isotopic fractionation mechanisms are not well constrained, indicating an important gap in our understanding of processes in the global Si cycle. We use rock crushing experiments to mimic subglacial physical erosion, to provide insight into subglacial isotope fractionation. Isotopically light DSi (δ30SiDSi) released following initial dissolution of freshly ground mineral surfaces (down to −2.12 ± 0.02 ‰) suggests mechanochemical reactions induce isotopic fractionation, explaining the low δ30SiDSi composition of subglacial runoff. ASi with a consistent isotopic composition is present in all mechanically weathered samples, but concentrations are elevated in samples that have undergone more intense physical grinding. These experiments illustrate the critical role of physical processes in driving isotopic fractionation and biogeochemical weathering in subglacial environments. Understanding perturbations in high latitude Si cycling under climatic change will likely depend on the response of mechanochemical weathering to increased glacial melt.

Description: Stable paleomagnetic information in meteoritic metal is carried by the ‘cloudy zone’: ~1‐10 μm wide regions containing islands of ferromagnetic tetrataenite embedded in a paramagnetic antitaenite matrix. Due to their small size and high coercivity (theoretically up to ~2.2 T), the tetrataenite islands carry very stable magnetic remanence. However, these characteristics also make it difficult to image their magnetic state with the necessary spatial resolution and applied magnetic field. Here, we describe the first application of X‐ray holography to image the magnetic structure of the cloudy zone of the Tazewell IIICD meteorite with spatial resolution down to ~40 nm and in applied magnetic fields up to ± 1.1 T, sufficient to extract high‐field hysteresis data from individual islands. Images were acquired as a function of magnetic field applied both parallel and perpendicular to the surface of a ~100 nm thick slice of the cloudy zone. Broad distributions of coercivity are observed, including values that likely exceed the maximum applied field. Horizontal offsets in the hysteresis loops indicate an interaction field distribution with half width of ~100 mT between the islands in their room‐temperature single‐domain state, providing a good match to first‐order reversal curve diagrams. The results suggest that future models of remanence acquisition in the cloudy zone should take account of strong interactions in order to extract quantitative estimates of the paleofield.

Description: A nanoscale intergrowth of fine tetrataenite particles in an iron rich matrix, known as the `cloudy zone' has recently been recognised as a stable paleomagnetic recorder. It is found in meteorites containing iron-nickel alloy that have developed the characteristic Widmanst\"atten pattern. However, the close particle proximity and high magnetocrystalline anisotropy of the cloudy zone make it a highly unconventional material to use in paleomagnetic studies. Open questions about the formation of the cloudy zone, its exact composition and structure, and how it acquires a remanence remain unanswered. To use the cloudy zone as a reliable and accurate paleomagnetic recorder, its properties have to be well understood. The particle size in the cloudy zone has been measured to be between $\sim$500 nm and $\sim$10 nm, however, it is rarely higher than $\sim$150 nm. This fine lengthscale makes it difficult to study the cloudy zone with conventional methods. No known single method can provided the solution to all current problems concerning the use of the cloudy zone as a paleomagnetic recorder. Therefore, to explore the properties of the cloudy zone a multi-method approach using advanced nanoscale investigation techniques and theoretical calculations was adopted. The formation of the cloudy zone within the context of the Fe-Ni phase diagram was studied using Monte Carlo simulations. These simulations were supplemented by Density Functional Theory (DFT) calculations. DFT was also used to explore the preferred chemical ordering schemes of the matrix as well as its ground magnetic state. The 3D structure of the cloudy zone was imaged with sub-nanometer resolution using Atom Probe Tomography (APT). This was one of the first applications of APT to image meteoritic metal. Accurate composition measurements of the matrix and tetrataenite as well as kamacite were made. Synchrotron M\"ossbauer spectroscopy was used to provide high spatial resolution information of the magnetic state of the matrix and tetrataenite in the cloudy zone as well as the surrounding metal. The matrix was conclusively demonstrated to be paramagnetic at room temperature as a bulk material. X-Ray holography was used for the first time to directly image magnetisation of individual paleomagnetic remanence carriers under high applied fields. In-field hysteresis behaviour of individual particles in the cloudy zone was measured by directly imaging the sample magnetisation with a resolution of $\sim$25 nm. The cloudy zone was found to consist of strongly interacting single domain particles. The experimental observations were supported by modelling. The combined approach of multiple methods was capable of providing answers to some of the important questions about the cloudy zone. The cloudy zone was found to be highly stable against remagnetisation by applied external fields. If the particle size is below $\sim$80-50 nm the cloudy zone was found to consist of isolated single-domain tetrataenite particles sitting in a paramagnetic matrix. The matrix might become ferrimagnetic at very low temperatures. Depending on how the magnetisation is measured, the measurement might be affected by the matrix changing its magnetic state to ferromagnetic at surfaces. This phenomena should not affect the overall stability of the cloudy zone as a paleomagnetic recorder. Due to the close proximity of the tetrataenite particles, there are strong magnetostatic interactions between them. This finding demands that new methods be developed for correct interpretation of the remanence recorded in the cloudy zone.

Description: Iron and stony‐iron meteorites form the Widmanstätten pattern during slow cooling. This pattern is composed of several microstructures whose length‐scale, composition and magnetic properties are dependent upon cooling rate. Here we focus on the cloudy zone: a region containing nanoscale tetrataenite islands with exceptional paleomagnetic recording properties. We present a systematic review of how cloudy zone properties vary with cooling rate and proximity to the adjacent tetrataenite rim. X‐ray photoemission electron microscopy is used to compare compositional and magnetization maps of the cloudy zone in the mesosiderites (slow cooling rates), the IAB iron meteorites and the pallasites (intermediate cooling rates), and the IVA iron meteorites (fast cooling rates). The proportions of magnetic phases within the cloudy zone are also characterized using Mössbauer spectroscopy. We present the first observations of the magnetic state of the cloudy zone in the mesosiderites, showing that, for such slow cooling rates, tetrataenite islands grow larger than the multidomain threshold, creating large‐scale regions of uniform magnetization across the cloudy zone that render it unsuitable for paleomagnetic analysis. For the most rapidly cooled IVA meteorites, the time available for Fe‐Ni ordering is insufficient to allow tetrataenite formation, again leading to behavior that is unsuitable for paleomagnetic analysis. The most reliable paleomagnetic remanence is recorded by meteorites with intermediate cooling rates ( urn:x-wiley:ggge:media:ggge22125:ggge22125-math-0001 2–500 °C Myr urn:x-wiley:ggge:media:ggge22125:ggge22125-math-0002) which produces islands that are “just right” in both size and degree of Fe‐Ni order.

Description: Saponite is a clay mineral of the smectite group that finds applications in the chemical industry as a catalyst or catalyst precursor as well as in nanocomposites used for structural or catalytic applications. Saponite of controlled composition, crystallinity, particle size, and morphology would be highly beneficial to industry; however, such materials are not found in a sufficiently pure form in nature. Synthetic methods to produce saponite with specific properties are currently lacking as the understanding of the mechanisms controlling its formation, crystalline properties and particle morphology, is limited. Understanding the saponite formation mechanism is crucial for the development of a highly tuned and controlled synthesis leading to materials with specific properties. Here, we report a new chemical reaction mechanism explaining the nucleation and kinetics of saponite growth at different pHs, at 95–100 °C, and under the influence of pH-modifying additives explored via a combination of X-ray scattering methods and infrared spectroscopy. Our results show that the main factor affecting the nucleation and growth kinetics of saponite is the pH, which has a particularly significant impact on the rate of initial nucleation. Non-uniform reactivity of the aluminosilicate gel also significantly affects saponite growth kinetics and causes a change in the rate-determining step as seen in graphical abstract. The most crystalline saponite is obtained when the nucleation is suppressed by a low initial pH (〈7), but the reaction is performed at a higher pH of about 9. The stacking of the saponite sheets can be further improved by a separate postsynthesis treatment with an alkali (NaOH) solution. A simple, ambient pressure method for synthesizing a highly crystalline saponite is proposed that could be easily upscaled for industrial purposes.

Description: Advanced in situ techniques based on electrons and X-rays are increasingly used to gain insights into fundamental processes in liquids. However, probing liquid samples with ionizing radiation changes the solution chemistry under observation. In this work, we show that a radiation-induced decrease in pH does not necessarily correlate to an increase in acidity of aqueous solutions. Thus, pH does not capture the acidity under irradiation. Using kinetic modeling of radiation chemistry, we introduce alternative measures of acidity (radiolytic acidity π* and radiolytic ion product KW*), that account for radiation-induced alterations of both H+ and OH– concentration. Moreover, we demonstrate that adding pH-neutral solutes such as LiCl, LiBr, or LiNO3 can trigger a significant change in π*. This provides a huge parameter space to tailor the acidity for in situ experiments involving ionizing radiation, as present in synchrotron facilities or during liquid-phase electron microscopy.