Identification Of Magnetite Research Articles

Discovering the colours of industrial heritage characterisation of paint coatings from the powerplant at the Levada de Tomar

AbstractThe paint coatings of three energy generators from the 20th‐century powerplant at Levada de Tomar, Portugal, were investigated using micro‐Raman and micro‐X‐ray fluorescence spectroscopies and scanning electron microscopy with energy dispersive spectroscopy. This multi‐analytical approach was used to identify the colouring agents, thus providing a chronological chromatic pallet and allowing to infer on the use of the three energy generators. Together with traditional pigments like Prussian blue, red iron oxide, and carbon black, pigments used in industrial areas like copper phthalocyanine and toluidine red were identified as colouring agents. Complex paint systems of the oldest equipment (1924) were revealed as well as maintenance procedures of the equipment that worked during a longer time (1944–1990). Powdery carbon black layers, resulting from incomplete hydrocarbon combustion and present between metallic substrates and coating layers, suggested the inexistence of paint coatings replacement after the powerplant shutdown. The identification of magnetite as a corrosion product of iron alloy substrate revealed that corrosion developed after the engine shutdown and not during the operation period. The results obtained highlight the potentialities of scientific‐based approach and Raman spectroscopy to the industrial heritage study, an emergent cultural area.

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Scanning electron microscope backscatter electron image of a small magnetite‐bearing brecciated rock fragment located in Apollo 16 regolith breccia sample 60016, as discussed in Joy et al. (pp. 1157–1172). The identifi cation of magnetite in a lunar sample provides direct evidence for oxidized conditions on the Moon. The lower color image shows the location of different mineral phases in the sample noted in the color scale at left (black areas represent pores, fractures, or phases that cannot be identifi ed). Image courtesy of Katherine Joy.

Identification of magnetite in lunar regolith breccia 60016: Evidence for oxidized conditions at the lunar surface

AbstractLunar regolith breccias are temporal archives of magmatic and impact bombardment processes on the Moon. Apollo 16 sample 60016 is an “ancient” feldspathic regolith breccia that was converted from a soil to a rock at ~3.8 Ga. The breccia contains a small (70 × 50 μm) rock fragment composed dominantly of an Fe‐oxide phase with disseminated domains of troilite. Fragments of plagioclase (An95‐97), pyroxene (En74‐75, Fs21‐22,Wo3‐4), and olivine (Fo66‐67) are distributed in and adjacent to the Fe‐oxide. The silicate minerals have lunar compositions that are similar to anorthosites. Mineral chemistry, synchrotron X‐ray absorption near edge spectroscopy (XANES) and X‐ray diffraction (XRD) studies demonstrate that the oxide phase is magnetite with an estimated Fe3+/ΣFe ratio of ~0.45. The presence of magnetite in 60016 indicates that oxygen fugacity during formation was equilibrated at, or above, the Fe‐magnetite or wüstite–magnetite oxygen buffer. This discovery provides direct evidence for oxidized conditions on the Moon. Thermodynamic modeling shows that magnetite could have been formed from oxidization‐driven mineral replacement of Fe‐metal or desulphurisation from Fe‐sulfides (troilite) at low temperatures (<570 °C) in equilibrium with H2O steam/liquid or CO2 gas. Oxidizing conditions may have arisen from vapor transport during degassing of a magmatic source region, or from a hybrid endogenic–exogenic process when gases were released during an impacting asteroid or comet impact.

The palaeomagnetism of glauconitic sediments

The palaeoenvironmental significance of glaucony has long been appreciated, but accurate palaeomagnetic dating of events recorded by glauconitic horizons requires an understanding of how glauconitic sediments acquire a remanent magnetization. Pure glauconitic minerals are paramagnetic, but glauconite grains are large and slow-forming (over periods that can exceed 100kyr), with complex and variable morphologies. It is, thus, possible that small magnetic grains within glaucony particles may carry a significant fraction of the remanence in weakly magnetized sediments. Any remanence carried by glauconitic grains may therefore represent the geomagnetic field at a time significantly later than the time of deposition, or a time-averaged signal over some or all of the formation period. We investigated this problem using weakly magnetic Palaeocene glauconitic siltstones from southern New Zealand. We disaggregated the rock and separated it magnetically into glauconitic and non-glauconitic fractions. Results from stepwise isothermal remanent magnetization (IRM) acquisition, alternating-field demagnetization, temperature dependence of magnetic susceptibility, and stepwise thermal demagnetization of a triaxial IRM were used to demonstrate that the remanent magnetization is carried by single-domain or pseudo-single-domain magnetite in the non-glauconitic sediment fraction, and that the glauconite grains themselves make no contribution to the remanent magnetization. However, accurate measurement of the primary remanence is complicated by a strong viscous overprint and mineral alteration during thermal demagnetization studies. Identification of magnetite as the remanence carrier in sediments within a reducing diagenetic environment gives confidence that the remanence has a depositional origin. Glauconite does not carry a remanence; therefore, its effect is to dilute and weaken the overall magnetization. Furthermore, the use of rock magnetic parameters may be problematic when glauconite concentrations are (as in the studied sediments) orders of magnitude greater than remanence carrier concentrations, because in such cases the glauconite susceptibility can dominate that of the remanence carriers.

Identification of magnetite and hematite in rocks by magnetic observation at low temperature

A method of identification of coarse-grained hematite and magnetite based on the recognition of their distinctive low-temperature transitions is presented. Specimens are magnetized to saturation, first at room temperature and then at −196°C. The transitions are observed as discontinuities in the curve of the change of isothermal remanent magnetization (IRM) with increasing temperature, from −196°C to 20°C. At −140°C the low-temperature IRM of magnetite is almost completely lost. At −20°C about one-half of the room-temperature IRM of hematite is recovered owing to the memory effect of hematite. The basic characteristics of the memory effects, upon which the method of identification is based, have been investigated with a vibration magnetometer. The memory of room-temperature magnetization has been described previously for hematite and magnetite. It has also been found that under the conditions of the present experiments there is a reciprocal relationship across the transition temperature between the low- and ordinary-temperature IRM of magnetite. On passing through the transition from high to low temperature, the remanence from the preceding cycle of high-temperature events is alone memorized; all earlier magnetizations are lost. Similarly, the last low-temperature IRM is alone memorized when magnetite is heated through the transition temperature. The method of identification has revealed the presence of hematite in a lightly metamorphosed red sediment and of magnetite in a red sediment baked by an igneous intrusion and in a red arkose, but neither coarse hematite nor magnetite has yet been found in unmetamorphosed red sandstones. The detection of both hematite and magnetite by this method is limited by the dependence of the magnetic transitions upon grain size and the presence of impurities.