Search

Intracellular magnetite crystal formation by magnetotactic bacteria has emerged as a powerful model for investigating the cellular and molecular mechanisms of biomineralization, a process common to all branches of life. Although magnetotactic bacteria are phylogenetically diverse and their crystals morphologically diverse, studies to date have focused on a few, closely related species with similar crystal habits. Here, we investigate the process of magnetite biomineralization in Desulfovibrio magneticus sp. RS1, the only reported species of cultured magnetotactic bacteria that is outside of the [alpha]-Proteobacteria and that forms bullet-shaped crystals. Using a variety of high-resolution imaging and analytical tools, we show that RS-1 cells form amorphous, non-crystalline granules containing iron and phosphorus before forming magnetite crystals. Using NanoSIMS (dynamic secondary ion mass spectroscopy), we show that the iron-phosphorus granules and the magnetite crystals are likely formed through separate cellular processes. Analysis of the cellular ultrastructure of RS-1 using cryo-ultramicrotomy, cryo-electron tomography, and tomography of ultrathin sections reveals that the magnetite crystals are not surrounded by membranes but that the iron-phosphorus granules are surrounded by membranous compartments. The varied cellular paths for the formation of these two minerals lead us to suggest that the iron-phosphorus granules constitute a distinct bacterial organelle. bacterial organelle | biomineralization | magnetotactic bacteria | dynamic secondary ion mass spectroscopy | magnetite doi/10.1073/pnas.1001290107

Permian plutonism is widespread in the U.S. Appalachians, but not in the Canadian sector of the Appalachians. The German Bank pluton, sampled in two boreholes offshore southwestern Nova Scotia, gave a U-Pb age of 300 [+ or -] 1 Ma, determined by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) on thermally annealed and chemically etched zircon grains. Electron-microprobe dating of monazite yielded an age of 308 [+ or -] 13 Ma. Late Paleozoic plutonism in the Canadian Appalachians was dominated by northwestward subduction of the Rheic Ocean, whereas the Rheic Ocean in the U.S. Appalachians closed by southeasterly subduction. A slab tear therefore developed at the southwestern margin of the Rheic Ocean slab beneath the Canadian Appalachians. The high Sr and Ba metaluminous magnetite granite of the German Bank pluton resulted from melting of underplated lower crust and/or lithospheric mantle by a rising asthenospheric diapir related to this slab tear. Geochemically, the German Bank pluton is similar to most 300-265 Ma plutons of the southern Appalachians, except in its strongly negative Nd isotopic composition that results from the underthrusting of a ca. 2 Ga lower crustal block of Saharan craton. doi: 10.1130/B30031.1

Ordering of [Fe.sup.3+] and [Fe.sup.2+] cations between octahedral and tetrahedral sites in synthetic members of the magnetite-ulvospinel ([Fe.sub.3][O.sub.4]-[Fe.sub.2]Ti[O.sub.4]) solid-solution series was determined using Fe [L.sub.2,3]-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti [L.sub.2,3]-edge and Fe K-edge X-ray absorption spectroscopy (XAS), and unit-cell parameters. Microprobe analyses, cell edges, and chemical FeO determinations showed that bulk compositions were stoichiometric magnetite-ulvospinel solid solutions. XMCD showed that the surface was sensitive to redox conditions, and samples required re-equilibration with solid-solid buffers. Detailed site-occupancy analysis gave [Fe.sup.2+]/[Fe.sup.3+] XMCD-intensity ratios close to stoichiometric values. [L.sub.2,3]-edge XAS confirmed that [Ti.sup.4+] was restricted to octahedral sites. XMCD showed that significant [Fe.sup.2+] only entered the tetrahedral sites when Ti content was >0.40 atoms per formula unit (apfu), whereas [Fe.sup.2+] in octahedral sites increased from 1 apfu in magnetite to a maximum of ~1.4 apfu when Ti content was 0.45 apfu. As Ti content increased, a steady increase in [Fe.sup.2+] in tetrahedral sites was observable in the XMCD spectra, concurrent with a slow decrease in [Fe.sup.2+] in octahedral sites. Calculated magnetic moments decreased rapidly from magnetite (4.06 [[micro].sub.B]) to USP45 (1.5 [[micro].sub.B]), then more slowly toward ulvospinel (0 [[micro].sub.B]). Two synthesized samples were maghemitized by re-equilibrating with an oxidizing buffer. XMCD showed that [Fe.sup.2+] oxidation, with concomitant vacancy formation, was restricted to octahedral sites. Through the direct measurement of Fe oxidation states, XMCD results can be used to rationalize the magnetic properties of titanomagnetites, along with oxidized titanomaghemitized analogs, in Earth's crustal rocks. Keywords: Titanomagnetite, titanomaghemite, cation site ordering, vacancy ordering, X-ray absorption spectroscopy, X-ray magnetic circular dichroism, magnetic moment DOI: 10.2138/am.2010.3343

We document an example of serpentinization of olivine and orthopyroxene that produced virtually no magnetite, but instead relatively Fe-rich yellow-colored lizardite ([X.sub.Fe] = 0.08 to 0.17), and the native Fe-Ni-Co metals, awaruite and wairauite. Lizardite's identity was confirmed by micro-Raman spectroscopy, although peaks are broad. Electron microprobe analyses of the lizardite yield a continuous compositional trend of formula contents suggestive of the progressive uptake of [Fe.sup.3+] exclusively on M sites, where it is charge balanced by vacancies. Although these observations are unusual, this secondary mineral assemblage can be explained in terms of the likely intensive variables [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.], and [MATHEMATICAL EXPRESION NOT REPRODUCIBLE IN ASCII.] attending the alteration. The absence of magnetite in serpentinization does not signify a lack of oxidation. By forming the hydrated phase-component ferri-lizardite instead of magnetite from the fayalite and ferrosilite components, the yield of hydrogen is reduced by two-thirds. The usual inverse correlation of rock density with magnetic susceptibility is unlikely to be the case in this kind of serpentinization. Keywords: Serpentinite, ferrian lizardite, olivine-websterite, micro-Raman, hydrogen, magnetic susceptibility DOI: 10.2138/am.2009.3301

The iron oxides hematite, magnetite, and goethite were studied with density functional theory to establish a consistent set of structures for both the bulk mineral and key surfaces, characterize surface relaxation, and predict and test calculated scanning tunneling microscopy (STM) images. Spinpolarized, plane-wave pseudopotential calculations were carried out on recognized terminations of the hematite (0001) and goethite (010) surfaces and on two terminations of magnetite (111), derived from bulk structures optimized with the same simulation parameters. In the bulk, geometry optimizations having different spin configurations were compared, to find that even without an on-site Coulomb correction, the expected spin states were found to have lowest energy: antiferromagnetic in hematite and goethite and ferrimagnetic in magnetite. However, magnetite shows a conducting minority spin. All four surfaces showed structural relaxation consistent with previous work. The 1/2-monolayer termination (octahedral and tetrahedral Fe) of magnetite (111) underwent slightly more relaxation than the 1/4-monolayer termination, with consequently lower surface energy. A calculated STM image for 1/4-monolayer magnetite is compared to an observed image at positive bias and suggests that the tetrahedral Fe dominates the image. STM images are predicted for hematite and goethite to aid interpretation of future experimental work. Keywords: Magnetite, hematite, goethite, surface, STM, electronic structure

This study demonstrates physical adsorption of polymer coated nano-sized magnetite ([Fe.sub.3][O.sub.4]) onto multiwalled carbon nanotube (MWNT) surface. A new hybrid material was made following four consecutive procedures of co-precipitation, polymer coating, functionalized MWNT, and ultrasonication method. Polymer coated magnetite particle (MaPAm) composites with MWNT nanohybrid (MaPAmNT) were acquired. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) have been used to investigate their nanostructure and morphology, thus verifying that polyacrylamide coated magnetite particles were well adsorbed onto surface of the MWNT. From thermogravimetric analysis (TGA) data, we also confirmed compositions of the MaPAmNT. Magnetorheological (MR) properties of the MaPAmNT under an external magnetic field were investigated, exhibiting typical MR behavior of yield stress and shear stress. Index Terms--Magnetite, magnetorheological fluid, multiwalled carbon nanotube (MWNT), nanohybrid, polymer.

The surface modified magnetite superparamagnetic nanoparticles were synthesized by using a co-precipitation method. The surfaces of magnetite nanoparticles were encapsulated by the poly-ethyleneglycole (PEG). The crystalline structure and the composition of core magnetite nanoparticles were partially changed from [Fe.sub.3][O.sub.4] to [alpha]-[Fe.sub.2][O.sub.3] after surface coating with a very thin outer shell of PEG. The magnetite and the surface-coated magnetite particles were well monodispersed. The average diameters of the nanoparticles were about 10.4 nm and 11 nm, respectively, determined by using transmission electron microscopy and dynamic light scattering method. The superparamagnetic behaviors were examined by measuring magnetization hysteresis loops and zero-field cooling and field cooling magnetizations as a function of temperature from 5 K to 300 K. The blocking temperatures of the magnetite and the surface coated magnetite were around 245 K and 130 K, respectively. The superparamagnetic behaviors of the magnetite and the surface coated magnetite nanoparticles were discussed in the context of the change of magnetizations with temperature. Index Terms--Magnetic particle, magnetite, magnetization, polymer, superparamagnetic.

X-ray magnetic circular dichroism (XMCD) experiments were carried out to compare the [Fe.sup.2+] [Fe.sup.3+] ratio in nanomagnetite chemically produced from lepidocrocite and nanomagnetite biogenically produced by the Fe-reducing bacterium Shewanella putrefaciens. Together with TEM imaging, these measurements showed that the biotic magnetite nanoparticles were of excellent quality, with small size dispersion and high crystallinity. From the XMCD measurements, it could be shown that the biogenic nanomagnetite contained a higher amount of [Fe.sup.2+] than the abiogenic nanomagnetite. Keywords: Nanomagnetite, XMCD, Shewanella putrefaciens, biomineralization

Variations in rock magnetic properties are responsible for the many linear, short-wavelength, low-amplitude magnetic anomalies that are spatially associated with faults that cut Neogene basin sediments in the Rio Grande rift, including the San Ysidro normal fault, which is well exposed in the northern part of the Albuquerque Basin. Magnetic-susceptibility measurements from 310 sites distributed through a 1200-m-thick composite section of rift-filling sediments of the Santa Fe Group and prerift Eocene and Cretaceous sedimentary rocks document large variations of magnetic properties juxtaposed by the San Ysidro fault. Mean volume magnetic susceptibilities generally increase upsection through eight map units: from 1.7 to 2.2E-4 in the prerift Eocene and Cretaceous rocks to 9.9E-4-1.2E-3 in three members of the Miocene Zia Formation of the Santa Fe Group to 1.5E-3-3.5E-3 in three members of the Miocene-Pleistocene Arroyo Ojito Formation of the Santa Fe Group. Rock magnetic measurements and petrography indicate that the amount of detrital magnetite and its variable oxidation to maghemite and hematite within the Santa Fe Group sediments are the predominant controls of their magnetic property variations. Magnetic susceptibility increases progressively with sediment grain size within the members of the Arroyo Ojito Formation (deposited in fluvial environments) but within members of the Zia Formation (deposited in mostly eolian environments) reaches highest values in fine to medium sands. Partial oxidation of detrital magnetite is spatially associated with calcite cementation in the Santa Fe Group. Both oxidation and cementation probably reflect past flow of groundwater through permeable zones. Magnetic models for geologic cross sections that incorporate mean magnetic susceptibilities for the different stratigraphic units mimic the aeromagnetic profiles across the San Ysidro fault and demonstrate that the stratigraphic level of dominant magnetic contrast changes with different exposure levels into the fault. These data indicate that tectonic juxtaposition of primary variations of magnetic properties of strata across the fault is the source of the associated magnetic anomaly. This study indicates that magnetic anomalies over faults and folds can be generated by sediments (1) deposited within tectonic basins having volcanic or basement source areas rich in magnetite, (2) having depositional environments with sufficient but varying energy to transport dense magnetic minerals and cause stratigraphic changes of magnetic properties, and (3) having magnetic minerals preserved owing to their youth or nonreactive geochemical environments. Keywords: rock magnetism, magnetic anomaly, Rio Grande rift, faulting, New Mexico.

Volcanic activity at Mt. Vulture lasted about 750 ka and produced Si[O.sub.2]-undersaturated volcanic rocks that can be classified as old (~700 ka), intermediate (~600-550 ka), and young (~130 ka). The intermediate deposits consist of pyroclastic falls and flows and lavas with compositions ranging from phonolite to foidite. A recent revision of the stratigraphic setting allowed these deposits to be classified into one synthem (the Barile Synthem) and further subdivided into four subsynthems (Toppo S. Paolo, Rionero, Vulture-S. Michele, and Ventaruolo). In the present investigation, trioctahedral micas from sample VUT 191 in the Vulture-S. Michele Subsynthem are considered. The host rock has modal diopside (20.2%), analcime (22.8%), plagioclase (27.8%), hauyne (5%), phlogopite (8.9%), and magnetite (6.3%). The micas were studied using chemical (EPMA, C-H-N, SIMS), structural (SCXRD), and spectroscopic (Mossbauer) methods. EPMA of 36 crystals from thin sections and 6 discrete crystals selected for the structural analysis showed remarkable compositional variability, as follows (in wt%): Si[O.sub.2] = 33.14-38.01, [Al.sub.2][O.sub.3] = 15.56-20.45, MgO = 13.[O.sup.2-]20.81, Fe[O.sub.tot] = 6.34-14.08, Ti[O.sub.2] = 2.34-6.02, [K.sub.2]O = 6.03-9.48, [Na.sub.2]O = 0.50-0.78, and BaO = 0.89-4.06; all crystals proved to be phlogopite. Elemental C-H-N analyses yielded [H.sub.2]O = 2.86 [+ or -] 0.36 wt%. The water content was also determined by SIMS on two single crystals, labeled VUT 191_2 and VUT 191_19, which yielded values of 3.81 [+ or -] 0.12 and 1.72 [+ or -] 0.08 wt% [H.sub.2]O, respectively. Mossbauer investigation showed that all the iron in VUT191 mica is octahedral with [Fe.sub.2+] 25.5% and [Fe.sup.3+] = 74.5%, confirming that Vulture micas are particularly [Fe.sup.3+]-rich, as also found from previous investigations. Structure refinements using anisotropic displacement parameters were performed in space group C2/m and converged at 1.89 < R [less than or equal to] 3.17, 2.09 [less than or equal to] [R.sub.w] [less than or equal to] 3.43%. All of the analyzed micas belong to the 1M polytype but exhibit remarkable variations in the c parameter from 10.1569(4) to 10.2458(4) [Angstrom]. The chemical and structural parameters indicate that the studied micas can be divided into two groups: the first encompassing strongly dehydrogenated micas affected mainly by Ti-oxy [[sup.IV][M.sup.2+] + 2(OH) [left and right arrow] WTi4+ + 2[0.sup.2-] + [H.sub.2]] and [M.sup.3+]-oxy [[sup.IV] [M.sup.2+] + (OH)-[left and right arrow] W[M.sup.3+] + [O.sup.2-] + 1/2 [H.sub.2], with [M.sup.3+] = [Fe.sup.3+], [Al.sup.3+] substitutions. The second group consist of samples in which vacancy-bearing mechanisms, 2 [sup.VI][M.sup.2+] [left and right arrow] [sup.VI][Ti.sup.4+]+ [sup.VI] [cube root]and 3[sup.IV][M.sup.2+] [left and right arrow] 2'a[M.sup.3+] +[sup.VI][cube root] occur. Keywords: Volcanic phlogopite, Mossbauer spectroscopy, structure refinement, CHN, SIMS, crystal chemistry, substitution mechanisms

Magnetotactic bacteria, which most commonly live within the oxic-anoxic transition zone (OATZ) of aquatic environments, produce intracellular crystals of magnetic minerals, specifically magnetite or greigite. The crystals cause the bacteria to orient themselves passively with respect to the geomagnetic field and thereby facilitate the bacteria's search for optimal conditions within the sharp chemical gradients of the OATZ. The bacteria may also gain energy from the redox cycling of their crystals. Because magnetotactic bacteria benefit from their magnetic moments, natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells. These traits also allow crystals produced by magnetotactic bacteria (called magnetofossils when preserved in sediments) to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism. Magnetofossils are recognizable based on their narrow size and shape distributions, distinctive morphologies with blunt crystal edges, chain arrangement, chemical purity, and crystallographic perfection. This article presents a scheme for rating magnetofossil robustness based on these traits. The magnetofossil record extends robustly to the Cretaceous and with lesser certainty to the late Archean. Because magnetotactic bacteria predominantly live in the OATZ, the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column. The magnetofossil record therefore provides an underutilized archive of paleoenvironmental information. Several studies have demonstrated a relationship between magnetofossil abundance and glacial/interglacial cycles, likely mediated by changes in pore water oxygen levels. More speculatively, a better-developed magnetofossil record might provide constraints on the long-term evolution of marine redox stratification. More work in modern and ancient settings is necessary to explicate the mechanisms linking the abundance and character of magnetofossils to ancient biogeochemistry. Keywords: bacteria; magnetite; greigite; biomineralization; biogeochemistry

Mineral assemblages in volcanic rocks record both pre-eruptive conditions and changes experienced by magma as it rises. Titanomagnetite in andesitic magmas is especially sensitive to changes in temperature and oxygen fugacity immediately prior to and during eruptions. Two end-member eruption states can be distinguished by examining titanomagnetite textures in erupted rocks. Slow-ascent eruptions--characterized by near-stagnant magma bodies and slow effusion of lava domes--show solid-state exsolution of titano-hematite/ilmenite lamellae within titanomagnetite hosts. By contrast, fast-ascent eruptions--characterized by rapid chilling of magma in sub-Plinian eruptions--contain titanomagnetites without such exsolution features. This mineralogical distinction is particularly useful in examining very fine-grained distal tephra layers where other characteristic properties of the two eruptions types are not present. Such tephra records in lake deposits typically provide the most precise long-term eruption records from andesitic volcanoes. Using an example from Mount Taranaki, New Zealand, we show that by classifying eruption styles within such sequences, the underlying magmatic system processes at a volcano can be elucidated and separated from other environmental factors such as vent/crater configuration. Keywords: titanomagnetite, exsolution, andesite volcanism, Mount Taranaki

Highly oriented magnetite ([Fe.sub.3][O.sub.4]) thin films are produced by reactive sputtering in a mixture of hydrogen and argon. The magnetoresistance (MR) results have shown strong coupling among [Fe.sub.3][O.sub.4] nanoparticles originated from Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction and dipolar interaction.

Understanding demagnetization by hypervelocity impacts is crucial for the interpretation of planetary magnetic anomalies and remanent magnetization in meteorites. We describe an innovative approach for investigating the effects of impacts on the remanent magnetization of geologic materials. It consists of the combination of pulsed laser impacts and Superconducting Quantum Interference Device (SQUID) microscopy. Laser impacts are nondestructive, create shocks with peak pressures as high as several hundred GPa, and allow well-calibrated modeling of shock wave propagation within the impacted samples. High-resolution SQUID microscopy quantitatively maps the magnetic field of room-temperature samples with an unprecedented spatial resolution of ~100 [micro]m. We present shock modeling and magnetic field data obtained for two laser impacts on a magnetite-bearing basalt sample. Magnetic measurements show a demagnetized area at the impact locations. We also show that high-resolution magnetic measurements combined with impact modeling provide a continuous relation between the demagnetization intensity and the peak pressure undergone by the sample. This promising technique will allow for the investigation of the demagnetization behavior of a variety of geological materials upon impacts, with implications for our understanding of the magnetization of extraterrestrial materials and of terrestrial impact structures. Keywords: impact demagnetization, SQUID microscopy, laser impact, magnetite.

In this paper, variation in magnetoresistance (MR) by transition-metal (TM) doping in magnetite ([Fe.sub.3][O.sub.4]) has been investigated. The samples ([T.sub.x][Fe.sub.3-x][O.sub.4], T = V, and Cr) were polycrystalline and prepared as thin films by a sol-gel method. As the TM composition (x) increases, the MR strength is reduced but the reduction rate with x differs significantly for the two TM-doping cases. For the V-substituted samples, the MR is reduced rapidly with x and no significant MR is detected above x = 0.11. On the other hand, the Cr-substituted samples exhibit the MR effect up to x = 0.49. Such difference in MR strength between the two TM-doping cases is attributable to the difference in the intrinsic properties of the ternary ferrites such as electronic structure and carrier spin polarization. Index Terms--Electronic structure, magnetite, magnetoresistance (MR), spin polarization.

Nano-crystalline magnetite powder has been prepared successfully by self- propagating high-temperature synthesis (SHS) reaction using Fe, [Fe.sub.2][O.sub.3] and sodium perchlorate as starting materials. The synthesized powder has been subjected to intense milling using a planetary ball mill up to 10 h. The effects of Fe/[Fe.sub.2][O.sub.3] molar ratio as well as the milling time on the powder particle characteristics were investigated using XRD, SEM, TEM and VSM techniques. XRD results showed that the non-stoichiometric molar ratio (2.5 Fe+0.5 [Fe.sub.2][O.sub.3]) is more favorable to synthesize magnetite phase. The phase composition was not affected significantly by milling process while both mean particle and crystallite size were decreased. Mean crystallite size of magnetite reduced from 90 to 40 nm after 10 h milling. Saturation magnetization ([M.sub.s]) increased from 47.6 in non-milled sample to 69.6 emu/g in milled sample for 10 h, while the coercivity ([H.sub.c]) reduced from 365 to 120 Oe after milling. Index Terms--Ferrimagnetic materials, materials processing, nanotechnology, soft magnetic materials.

Byline: O.E. Ayala-Valenzuela, J.A. Matutes-Aquino, R. Betancourt-Galindo, O. Rodriguez-Fernandez In this work, we present the synthesis and measurement of complex magnetic susceptibility I(I) = I'(I) - I'(I) of a magnetite magnetic fluid, oleic acid-kerosene. Measurements were developed applying different magnetic polarising fields by means of an electromagnet. Short-circuited transmission line technique was used. Complex magnetic susceptibility measurements of magnetic fluids are presented as a function of both frequency and external polarising magnetic field in ranges from 60 MHz to 6 GHz and from 0 Oe to 1212 Oe, respectively. Resonance frequencies in the range from 2.09 GHz to 5.06 GHz and maximum frequencies in the range from 1.19 GHz to 4.68 GHz as a function of the bias polarising field were measured for the magnetic fluid. From resonance measurements for different polarising fields, a mean anisotropy field value of A= 767Oe was obtained. Also a mean value of anisotropy constant = 12.2x104erg/cm3 and gyromagnetic ratio I[sup.3] = 16.07 x 106 1/sOe were determined.

Four kinds of magnetic particles having average sizes of 7, 18, 40, and 80 nm are synthesized and their heating ability when they are dispersed in an agar gel and exposed to an ac magnetic field at 120 kHz, a noninvasive frequency for anticancer hyperthermia is investigated. It is revealed that the particles 18 nm in average diameter gives the highest heating ability, though they exhibit narrow hysteresis loops as compared to the particles having average diameters of 40 and 80 nm.

A theory based on the discrete-dipole approximation, which accounts for optical coupling between magnetite is presented. Qualitative correlations between theoretical calculations and experimental data suggest that the shifts in spectral peak position depend on both interparticle distance and geometrical configuration.