Richard C. Hugo

Structural characterization of the clay mineral illite-1M

This work reports the structural characterization of illite-1M from northern Hungary, with the first attempt to refine the structure model and locate the interlayer water molecule. Structural characterization was accomplished using state-of-the-art analytical methods available for clays. The results illustrate the status of techniques for clay structure determination, as well as providing a structural model for illite. The chemical formula for the illite-1M under investigation can be written as K0.78Ca0.02Na0.02(Mg0.34Al1.69FeIII0.02)[Si3.35Al0.65]O10(OH)2·nH2O. Structure simulations with WILDFIRE yielded a model with 30% of cis-vacant layers and an expandability percentage of 10%. The value of the percentage of expandability was confirmed with NEWMOD, with which the best simulation was obtained with 90% of di-octahedral mica with K (80% site population) in the interlayer region and 10% of expandable layers. The best structure simulation obtained with DIFFaX was also obtained with a population of K atoms of 80%, six cells along c (in agreement with the results of a transmission electron microscopy study) and an average dimension of the particles in the ab plane of 300 nm. Besides the determination of the basic structure unit (the results are consistent with those obtained with the local information provided by a fit of the pair distribution function data) and the model of disorder, refinement with DIFFaX+ allowed the calculation of a possible position for the interlayer water molecule. Although physically sound, both the observed tetrahedral layer corrugation and the location of the water molecule need further experimental evidence, because the final fit of the observed pattern is still imperfect. The reasons for this misfit are discussed.

The Role of Extracellular Polymeric Substances in the Silicification of Calothrix: Evidence from Microbial Mat Communities in Hot Springs at Yellowstone National Park, USA

We provide nanoscale evidence of the role of sheath exopolymers in the silicification of the sheathed cyanobacteria Calothrix. Electron microscope observations of silicified Calothrix cells revealed that silica accretes directly onto EPS sheath fibrils to produce an open web of silica particles that could remain permeable to nutrients and waste products. We also found that silicified Calothrix cells from different microhabitats contained morphologically distinct silica particles. Differences in silicification texture suggest that environmental variables may influence silicification at the nanoscale. We develop a framework based on aggregation kinetics to address silicification processes in Calothrix and other sheathed cyanobacteria.

Mixed-Valent Fe Films ('Schwimmeisen') on the Surface of Reduced Ephemeral Pools

Floating, mixed-valent Fe films have been observed worldwide in wetlands, ferrous iron-rich seeps, and in seasonally reduced soils, but are usually misidentified as oil or biofilms. There has been little characterization or explanation of their formation. Along the Oregon coast such films were found on ephemeral pools where Fe(II)-rich groundwater (∼100 µM Fe) has been discharged at the base of Pleistocene sand dunes. Fe(II) oxidized to Fe(III) at the air-water interface to form ∼100–300 nm thick films. Analyses indicated that the films contained both Fe(III) and Fe(II) in a ratio of 3:1; Si was the other main cation; OH was the main anion and some C was also identified. The film morphology was flat under optical and electron microscopy with some attached floccules having a string-like morphology. Energy-filtered electron diffraction patterns showed three diffraction rings at 4.5, 2.6 and 1.4 Å in some places and two rings (2.6 and 1.4 Å) in others. Upon further oxidation the films became 2-line ferrihydrite. We are proposing the name ‘schwimmeisen’ for the floating, mixed-valent Fe film.

Shock-induced mobilization of metal and sulfide in planetesimals: Evidence from the Buck Mountains 005 (L6 S4) dike-bearing chondrite

Abstract The conditions under which metal cores formed in silicate-metal planetary bodies in the early Solar System are poorly known. We studied the Buck Mountains 005 (L6) chondrite with serial sectioning, X‑ray computed microtomography, and optical and electron microscopy to better understand how metal and troilite were redistributed as a result of a moderately strong (shock stage S4) shock event, as an example of how collisional processes could have contributed to differentiation. The chondrite was recovered on Earth in multiple small pieces, some of which have a prominent, 1.5-3 mm wide holocrystalline shock melt dike that forms a jointed, sheet-like structure, as well as an associated shock vein network. The data suggest that metal and troilite within the dike were melted, sheared, and transported as small parcels of melt, with metal moving out of the dike and along branching veins to become deposited as coarser nodules and veins within largely unmelted host. Troilite also mobilized but partly separated from metal to become embedded as finer-grained particles, vein networks, and emulsions intimately intergrown with silicates. Rock textures and metal compositions imply that shock melts cooled rapidly against relatively cool parent body materials, but that low-temperature annealing occurred by deep burial within the parent body. Our results demonstrate the ability of shock processes to create larger metal accumulations in substantially unmelted meteorite parent bodies, and they have implications for the formation of iron meteorites and for core formation within colliding planetesimals.

Phase Analysis of Large EDS Datasets with Matlab

Today’s SEM experimentalist can acquire prodigious amounts of data in short amounts of time. High-stability FEG-SEMs equipped with high-throughput SDD detectors are widely available. Commercial microanalysis systems control motorized specimen stages, acquiring hundreds of spatially aligned fields of view (FOV) in an overnight or weekend SEM session [1]. The resulting mosaic of EDS datacubes can easily comprise hundreds of gigabytes.