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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

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

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

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

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.