Magnetite

About: Magnetite is a research topic. Over the lifetime, 10277 publications have been published within this topic receiving 278071 citations.

Cyclicity in the Main and Upper Zones of the Bushveld Complex, South Africa: Crystallization from a Zoned Magma Sheet

Abstract: The major element composition of plagioclase, pyroxene, olivine, and magnetite, and whole-rock Sr/Sr data are presented for the uppermost 2.1 km of the layered mafic rocks (upper Main Zone and Upper Zone) at Bierkraal in the western Bushveld Complex. Initial Sr/Sr ratios are near-constant (0.7073 ± 0.0001) for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation. The 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite (magnetite–ilmenite–apatite) layers and changes up-section from gabbronorite (An72 plagioclase; Mg# 74 clinopyroxene) to magnetite–ilmenite–apatite–fayalite ferrodiorite (An43; Mg# 5 clinopyroxene; Fo1 olivine). The overall fractionation trend is, however, interrupted by reversals characterized by higher An% of plagioclase, higher Mg# of pyroxene and olivine, and higher V2O5 of magnetite. In the upper half of the succession there is also the intermittent presence of cumulus olivine and apatite. These reversals in normal fractionation trends define the bases of at least nine major cycles. We have calculated a plausible composition for the magma from which this entire succession formed. Forward fractional crystallization modeling of this composition predicts an initial increase in total iron, near-constant SiO2 and an increasing density of the residual magma before magnetite crystallizes. After magnetite begins to crystallize the residual magma shows a nearconstant total iron, an increase in SiO2 and decrease in density. We explain the observed cyclicity by bottom crystallization. Initially magma stratification developed during crystallization of the basal gabbronorites. Once magnetite began to crystallize, periodic density inversion led to mixing with the overlying magma layer, producing mineralogical breaks between fractionation cycles. The magnetitite and nelsonite layers mainly occur within fractionation cycles, not at their bases. In at least two cases, crystallization of thick magnetitite layers may have lowered the density of the basal layer of melt dramatically, and triggered the proposed density inversion, resulting in close, but not perfect, coincidence of mineralogical breaks and packages of magnetitite layers.

Properties of magnetic fluid particles

Abstract: Very fine particles of magnetite, nickel ferrite, and cobalt ferrite were produced by grinding coarser powders in a ball mill with a carrier fluid and a surfactant. The particles were examined by means of chemical analysis, electron microscopy, x-ray diffraction, magnetic measurements, and Mossbauer spectroscopy. Properties were determined before and after removing the surfactant coating. The most significant observation was that in some systems a large fraction of the spins was pinned in extremely high anisotropy fields as a result of bonding to the surfactant molecules. Anomalous magnetic hysteresis behavior was also noted when the surfactant coating was present.

A simple inorganic process for formation of carbonates, magnetite, and sulfides in Martian meteorite ALH84001

Abstract: We show experimental evidence that the zoned Mg-Fe-Ca carbonates, magnetite, and Fe sulfides in Martian meteorite ALH84001 may have formed by simple, inorganic processes. Chemically zoned carbonate globules and Fe sulfides were rapidly precipitated under low-temperature (150 °C), hydrothermal, and non-equilibrium conditions from multiple fluxes of Ca-Mg-Fe-CO2-S-H2O solutions that have different compositions. Chemically pure, single-domain, defect-free magnetite crystals were formed by subsequent decomposition of previously precipitated Fe-rich carbonates by brief heating to 470 °C. The sequence of hydrothermal precipitation of carbonates from flowing CO2-rich waters followed by a transient thermal event provides an inorganic explanation for the formation of the carbonate globules, magnetite, and Fe sulfides in ALH84001. In separate experiments, kinetically controlled 13C enrichment was observed in synthetic carbonates that is similar in magnitude to the 13C enrichment in ALH84001 carbonates.

Size-controlled preparation of magnetite nanoparticles in the presence of graft copolymers

Abstract: Stable aqueous iron oxide nanoparticle dispersions were prepared by coprecipitation of ferrous (Fe2+) and ferric (Fe3+) aqueous solution by a base in the presence of graft copolymers, poly(glycerol monoacrylate)-g-poly(PEG methyl ether acrylate) (PGA-g-PEG). PGA-g-PEG was prepared by acidic hydrolysis of poly(solktal acrylate)-g-poly(PEG methyl ether acrylate), which was synthesized by copolymerization of solktal acrylate and PEG methyl ether acrylate by atom transfer radical polymerization (ATRP). The size of the magnetite nanoparticles can be controlled from 4 nm to 18 nm by varying the graft density of the graft copolymers. Structural characterization using X-ray diffraction showed the presence of only the magnetite phase in the nanoparticles. Thermogravimetric analysis confirmed the presence of the graft copolymers on the magnetite surface. The magnetic characterization of the nanoparticles showed that they were superparamagnetic at room temperature.