Search

Recent paleomagnetic studies have constrained the strength and longevity of the magnetic field generated by the solar nebula, which has broad implications for the early evolution of the solar system. Paleomagnetic evidence was recorded by nanoscale iron inclusions in olivine crystals in the Semarkona LL 3.0 chondrite. These dusty olivines have been shown to be credible carriers of ancient magnetic remanence. The small scale of the iron inclusions presents several challenges for defining their fundamental magnetic properties. Here we present the first correlative study of the response of these magnetic structures under applied laboratory fields. Results show that the majority of particles are in a single-vortex state and exhibit stable magnetic behavior in applied fields up to 200 mT. Experimental observations using Lorentz microscopy and magnetic transmission X-ray microscopy are shown to compare well with the results of finite-element micromagnetic simulations derived from 3-D models of the particles obtained using electron tomography. This correlative approach may be used to characterize the fundamental magnetic behavior of many terrestrial and extraterrestrial paleomagnetic carriers in the single-vortex to multivortex size range, which represent the vast majority of stable magnetic carriers in rocks and meteorites.

Palaeomagnetic measurements of meteorites (1-5) suggest that, shortly after the birth of the Solar System, the molten metallic cores of many small planetary bodies converted vigorously and were capable of generating magnetic fields (6). Convection on these bodies is currently thought to have been thermally driven (7, 8), implying that magnetic activity would have been short-lived (9). Here we report a time-series palaeomagnetic record derived from nano magnetic imaging (10) of the Imilac and Esquel pallasite meteorites, a group of meteorites consisting of centimetre-sized metallic and silicate phases. We find a history of long-lived magnetic activity on the pallasite parent body, capturing the decay and eventual shutdown of the magnetic field as core solidification completed. We demonstrate that magnetic activity driven by progressive solidification of an inner core (11-13) is consistent with our measured magnetic field characteristics and cooling rates (14). Solidification-driven convection was probably common among small body cores (15), and, in contrast to thermally driven convection, will have led to a relatively late (hundreds of millions of years after accretion), long-lasting, intense and widespread epoch of magnetic activity among these bodies in the early Solar System., The pallasites are slowly cooled (2-9 K per million years, Myr; ref. 14) meteorites consisting of olivine crystals embedded in an Fe-Ni matrix (16), which have been suggested to originate [...]

In this study, we examine the effects of reheating on finely exsolved hematite-i|menite intergrowths from the ~91 Ma Ecstall pluton using reflected light microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). As a result of the emplacement of the younger adjacent ~52 Ma Quottoon pluton, samples closer to a thermal boundary have experienced greater degrees of thermal alteration. Five main microstructural features characterize hematite-ilmenite intergrowths from the Ecstall: (I) exsolution lamellae of hematite and ilmenite; (II) oxidation of ilmenite to form hematite, rutile, and other Ti-rich phases; (III) 20-50 nm magnetite precipitates in hematite; (IV) futile blitz texture; and (V) exsolution of hematite in rutile. Based on spatial relationships, textures II through V appear to be related to reheating of the Ecstall by the Quottoon, and samples up to ~14 km from the thermal boundary intrusive contact have been affected. We propose a mechanism, similar to that of Kontny and Dietl (2001), in which reheating has driven [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] conditions across the hematite-magnetite buffer to lower [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] resulting in the reduction of hematite. Higher temperatures also enhanced oxidation in ilmenite. The formation of magnetite altered the bulk magnetic properties of these samples, increasing NRM intensity. This study underscores the need to consider a pluton's post-emplacement thermal history before making tectonic interpretations based on paleomagnetic data. Keywords: Electron microscopy, hematite-ilmenite, magnetite, mineral intergrowths, Ecstall pluton DOI: 10.2138/am.2010.3191

When suspended in solution, clay platelets coated with nanometer-scale magnetite particles behave as magnetorheologic fluids that are important to a variety of industrial applications. Such dual-phase assemblages are also similar to natural aggregates that record the direction and intensity of the Earth's magnetic field in lake and marine depositional environments. This study characterizes the mineralogical structure and magnetic behavior of montmorillonite platelets coated with aggregates of nanometer-scale magnetite crystals. The distribution of magnetite crystal sizes in three different clay-magnetite assemblages was directly measured using conventional transmission electron microscopy and agrees within error with estimates derived from magnetic hysteresis measurements. Magnetic hysteresis and low field susceptibility measurements combined with electron holography experiments indicate that all three samples behave superparamagnetically at room temperature, and show increasing levels of single domain behavior as the samples are cooled to liquid nitrogen temperatures. At such low temperatures, magnetostatic interactions are observed to partially stabilize otherwise superparamagnetic grains in flux closure structures. Keywords: Magnetite, clay, smectite, TEM, electron holography, electron tomography, granulometry, magnetic properties

Lorentz transmission electron microscopy has been used to study fine-scale exsolution microstructures in ilmenite-hematite, as part of a wider investigation of the lamellar magnetism hypothesis. Pronounced asymmetric contrast is visible in out-of-focus Lorentz images of ilmenite lamellae in hematite. The likelihood that lamellar magnetism may be responsible for this contrast is assessed using simulations that incorporate interfacial magnetic moments on the (001) basal planes of hematite and ilmenite. The simulations suggest qualitatively that the asymmetric contrast is magnetic in origin. However, the magnitude of the experimental contrast is higher than that in the simulations, suggesting that an alternative origin for the observed asymmetry cannot be ruled out. Electron tomography was used to show that the lamellae have lens-like shapes and that (001) planes make up a significant proportion of the interfacial surface that they share with their host. Keywords: Hematite, ilmenite, lamellar magnetism, Lorentz electron microscopy, electron tomography, transmission electron microscopy

Magnetotactic bacteria comprise several aquatic species that orient and migrate along geomagnetic field lines. This behavior is based on the presence of intracellular ferrimagnetic grains of the minerals magnetite ([Fe.sub.3][O.sub.4]) or greigite ([Fe.sub.3][S.sub.4]). Whereas the structural and magnetic properties of magnetite magnetosomes have been studied extensively, the properties of greigite magnetosomes are less well known. Here we present a study of the magnetic microstructures, chemical compositions, and three-dimensional morphologies and positions of Fe-sulfide crystals in air-dried cells of magnetotactic bacteria. Data were obtained using several transmission electron microscopy techniques that include electron holography, energy-filtered imaging, electron tomography, selected-area electron diffraction, and high-resolution imaging. The studied rod-shaped cells typically contain multiple chains of greigite magnetosomes that have random shapes and orientations. Many of the greigite crystals appear to be only weakly magnetic, because the direction of their magnetic induction is almost parallel to the electron beam. Nevertheless, the magnetosomes collectively comprise a permanent magnetic dipole moment that is sufficient for magnetotaxis. One of the cells, which is imaged at the point of dividing, contains multiple chains of both equidimensional Fe-sulfide and elongated Fe-oxide crystals. The equidimensional and elongated crystals have magnetic properties that are consistent with those of greigite and magnetite, respectively. These results can be useful for obtaining a better understanding of the function of magnetotaxis in sulfide-producing cells, and they have implications for the interpretation of the paleomagnetic signals of greigite-bearing sedimentary rocks. Keywords: Greigite, magnetotactic bacteria, transmission electron microscopy, electron holography, electron tomography, magnetic properties, biomineralization

The use of sequential catalytic growth (SCG) to encapsulate iron, nickel-iron and iron-cobalt phosphide catalyst nanoparticles periodically along the entire lengths of carbon nanotubes is demonstrated. The results show that the SCG mechanism can be used to grow carbon nanotubes that contain regularly spaced room-temperature-ferromagnetic nanoparticles.

Ferrihydrite and a vernadite-like mineral, in samples collected from the riverbeds and floodplains of the river draining the largest mining-contaminated site in the United States (the Clark Fork River Superfund Complex), have been studied with transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. These poorly crystalline minerals are environmentally important in this system because contaminant heavy metals (As, Cu, Pb, and/or Zn) are always associated with them. Both two- and six-line ferrihydrite have been identified with selected area electron diffraction. For the vernadite-like mineral, the two d values observed are approximately between 0.1 and 0.2 A larger than those reported tier vernadite, the Mn hydrous oxide that is thought to have a birnessite-like structure, but which is disordered in the layer stacking direction. In several field specimens, the ferrihydrite and vernadite-like minerals are intimately mixed on the nanoscale, but they also occur separately. It is suggested that the vernadite-like mineral, found separately, is produced biogenically by Mn-oxidizing bacteria, whereas the same mineral associated with ferrihydrite is produced abiotitally via the heterogeneous oxidation of [Mn.sup.2+.sub.aq] initially on ferrihydrite surfaces. Evidence from this study demonstrates that the vernadite like mineral sorbs considerably more toxic metals than does ferrihydrite, demonstrating that it may be a good candidate for application to heavy-metal sorption in permeable reactive barriers.

The interfaces between fine-scale exsolution lamellae of hematite and ilmenite from an igneous rock in Rogaland, Norway have been studied by conventional transmission electron microscopy (TEM), high-resolution TEM, and energy-filtered TEM (EFTEM), to investigate the lamellar magnetism hypothesis for the origin of the unusual magnetic properties of this rock. Very fine hematite and ilmenite lamellae, less than 50 nm in length and parallel to (001) of their host, were abundant throughout the hemo-ilmenite sample. Dark-field and EFTEM observations indicated the hematite and ilmenite have very sharp structural and compositional interfaces with their hosts. The interfaces between the coarse hematite and ilmenite lamellae (length > 1 [micro]m) and their hosts have some interface dislocations to relieve elastic coherency strain. On the other hand, very fine lamellae (length < 50 nm) have no interface dislocations and are perfectly coherent. The interface dislocations for lamellae on the order of 100 nm in length are distributed heterogeneously, and more than 80% of the length of the interfaces seen by TEM is dislocation free. Thus, most of the interfaces in the sample are coherent. These results are in accord with the predictions of Monte Carlo simulations of the exsolution process (Harrison and Becker 2001) and with the hypothesis that coherent and sharp structural and compositional interfaces are the origin of lamellar magnetism (Robinson et al. 2002).

Biotite dissolution in the laboratory and in nature was examined and compared to elucidate certain aspects of the weathering processes. Batch dissolution experiments of fresh biotite [([K.sub.091] [Na.sub.0.01])([Mg.sub.0.40] [Fe.sub.2.07] [Mn.sub.0.05] [Al.sub.0.14] [Ti.sub.0.19])([Si.sub.2.82] [Al.sub.1.18])(O.sub.10[(OH).sub.2] in granite were carried out at 150[degrees]C for 1 to 56 days. Examination by scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) revealed that, in the early stage, dissolution proceeds from the edges of crystals inward and secondary minerals such as Fe oxide are precipitated mostly at the edges, with only a few secondary minerals found on the basal surfaces. A dissolution experiment using a mixture of biotite and muscovite, done at 150[degrees]C for 7 days, indicated that hematite crystals formed mostly at the edges of biotite but not on muscovite. This observation suggests that released Fe is precipitated before it diffuses into the bulk solution. Because a dissolution rate at the edge is larger by two orders of magnitude than at the basal surface (Turpault and Trotignon 1994), precipitation before diffusion of dissolved elements to bulk solution well explains the preferential secondary mineralization at the edges in the laboratory experiments. SEM-EDS of fresh to slightly weathered biotites revealed that early stage weathering proceeds in the same way as in the laboratory with the edges being preferentially weathered and secondary minerals being precipitated mostly at the edges. Because of the similarity between the occurrence of secondary minerals in the laboratory experiments and in nature, the laboratory results elucidate the early stage weathering conditions, namely that (1) supersaturation with respect to secondary minerals in a solution occurs around biotite, i.e., the solution is poorly connected to a main flow pathway of water, and (2) once supersaturation is achieved, secondary minerals are precipitated mainly at the edge before some released elements diffuse into the solution. The immobility of water immediately adjacent to primary minerals partly explains the large difference in dissolution rate between the laboratory and natural samples. A phlogopite dissolution experiment was done at 150[degrees]C for 5.5 days to examine the effect of biotite composition on vermiculitization. High-resolution transmission electron microscopy revealed that vermiculite layers are formed by layer-by-layer transformation within phlogopite after 5.5 day dissolution but not within biotite after 56 day dissolution or within slightly weathered biotite. A comparison of our data with results of other studies indicates that the formation of vermiculite layers between biotite layers occurs even in the early stage when biotite is not Fe-rich [more than 0.8 Mg per [O.sub.10][(OH).sub.2]]. When biotite is Fe-rich [less than 0.4 Mg per [O.sub.10][(OH).sub.2]]--like the present one-released Fe is precipitated as, for example, Fe hydroxide, and vermiculite is rarely formed because of limited availability of Mg. Thus, a higher Mg content in biotite facilitates the formation of vermiculite, at least in the early stage. Because vermiculite dissolves at a much slower rate than biotite, Mg-rich biotite dissolves at a slower rate than Fe-rich biotite. In the late stage, biotite dissolution still continues from the edge and within biotite, which results in a fine comb-like texture for weathered biotite; vermiculite occurs as a domain within chemically and structurally altered biotite as well, as at the edge even during weathering of Mg-poor biotite.