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The characterization of magnetic minerals in rocks often uses methods that measure induced magnetization. When rocks, sediments or soils contain two magnetic phases, in which one has a high saturation magnetization (M S), for example magnetite, and the other a low M S, for example hematite, the induced magnetization will be dominated by the stronger phase. An earlier study by Frank and Nowaczyk has shown that even when magnetite makes up <10 wt per cent of the ferromagnetic content, it will mask hematite. This makes identification of phases with low M S difficult to identify. We conduct a systematic study of synthetic mixtures of single domain magnetite and hematite with a broad spectrum of particle size, using hysteresis properties, acquisition of isothermal remanent magnetization (IRM) and first-order reversal curve distributions (FORC). Hysteresis parameters and FORC distributions do not vary significantly from the pure magnetite sample for hematite concentrations ≤90 wt per cent. IRM is not saturated for hematite concentration of 30 wt per cent or higher. Principle component analysis (PCA) of the processed FORCs, detects the presence of hematite for concentrations 70 wt per cent at the very least. Our results illustrate the difficulty in identifying hematite when it is found together with magnetite. IRM acquisition is the most sensitive method for identifying hematite when it occurs together with magnetite. [ABSTRACT FROM AUTHOR]

Magnetite is an iron oxide found in rocks. Its magnetic properties are used for paleoclimatic reconstructions. It can also be synthesized in the laboratory to exploit its magnetic properties for bio- and nanotechnological applications. However, although the magnetic properties depend on particle size in a well-understood manner, they also depend on the structure of the oxide, because magnetite oxidizes to maghemite under environmental conditions. The dynamics of this process have not been well described. Here, a study of the alteration of magnetite particles of different sizes as a function of their storage conditions is presented. Smaller nanoparticles are shown to oxidize more rapidly than larger ones, and that the lower the storage temperature, the lower the measured oxidation. In addition, the magnetic properties of the altered particles are not decreased dramatically, thus suggesting that this alteration will not impact the use of such nanoparticles as medical carriers. [ABSTRACT FROM AUTHOR]

In this study, the contribution of the paramagnetic, ferrimagnetic and haematite components to the magnetic anisotropy is separated by means of high-field torque magnetometry. Torque measurements at different fields, which are high enough to saturate the ferrimagnetic minerals, however, still low enough that the torque resulting from the haematite is linear with field, allow for the separation of the three magnetic anisotropy components. The method has been applied to haematite single crystals in which no paramagnetic or ferrimagnetic components have been found contribute to the torque signal. The mean direction of the poles to the crystallographic basal plane in the haematite single crystals is subparallel to the minimum-susceptibility direction measured in low-field. The separation analysis has also been applied to highly deformed red beds from the Lower Glarus nappe complex (Switzerland). No ferrimagnetic phases are present in the rocks and, therefore, they cannot contribute to the anisotropy of magnetic susceptibility. The magnetic fabric arises from a paramagnetic subfabric carried by the phyllosilicate minerals and haematite, in which the basal planes of both phases are in the cleavage plane. The measured magnetic lineation seen in low-field anisotropy of magnetic susceptibility appears to be an apparent lineation that arises from a weak girdling of haematite and the paramagnetic minerals conforming the rock. [ABSTRACT FROM AUTHOR]

Abstract: This study focuses on archeomagnetic directional data that are obtained from combustion levels in the Riparo Gaban rock shelter (northern Italy) in order to assess the degree of disturbance in the sediments and to provide new information to evaluate evolution of the geomagnetic field in the mid-Holocene. Combustion levels consist of ash, rubefied levels, or partially burned material and mixtures of these, or polygenic material. Rock magnetic investigations show that superparamagnetic to single-domain magnetite is the main carrier of the remanent magnetization, but small amounts of hematite and maghemite may also be present in the different materials. Rock magnetic parameters, e.g., the Koenigsberger ratio Q or the reversibility of thermomagnetic can be related to the degree of burning, and the dispersion parameter k to the degree of preservation. The grouping of archeomagnetic directions of a specific level is strongly dependent on the degree of physical disturbances. Directional means from thirteen levels can be compared with reference curves for paleosecular variation based on geomagnetic field models and other archeomagnetic data. The data from this study are in agreement with other archeomagnetic data from the Balkans in the time period 4900 BC to 4600 BC. [Copyright &y& Elsevier]