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Notes: Grain-size separation of the 〈5 μm fraction of a natural haematite into six fractions has been accomplished by means of centrifuging. Separation was repeated up to 10 times under the same centrifuging conditions to obtain grain-size fractions with a minimal quantity of undesired grains smaller than the lower limit for each fraction (the so-called tailing). to calculate the grain-size distribution of each fraction the total grain-size range is divided into 0.1 μm increments; the travelling distance for each increment is calculated using Stokes' Law starting with a homogeneous suspension for each separation run. the travelling distance is recalculated to a virtual suspension volume, which in turn is related to the actually extracted suspension volume. In this way a normalized distribution is obtained for 0.1 μm increments for each separation run. to arrive at the grain-size distribution for each fraction the calculation is repeated for the appropriate number of runs under the same centrifuging conditions and the normalized distribution is fitted with an actually determined grain-size distribution for the total grain-size range. Electron microscope observation of the separated fractions yielded good agreement between calculated and observed grain-size ranges. With these well-determined grain-size fractions rockmagnetic parameters can be determined for the geologically important 〈5 μm grain-size range for weakly magnetic material, like haematite or goethite.

Notes: The magnetic behaviour during stepwise demagnetization of a set of artificial samples, containing well-defined natural goethite from five localities (chemical composition, crystallite size and grain size are known) is reported. Differential Scanning Calorimetry measurements indicated that the goethites converted to haematite between 260° and 360°C. In this temperature range a small decrease in initial susceptibility and an additional remanence decay leading toward an extra bending point in the thermal decay curve are observed. Because titanomagnetite or pyrrhotite are absent in the samples, this extra bending point is tentatively interpreted as being due to recrystallization of the minor haematite already present in the original goethite concentrates, facilitated by water made available through the goethite/haematite conversion. At much higher temperatures, at some 600°C, a self-reversal of the remanence is observed in most specimens.The haematite derived from the goethites between 260° and 360°C appeared to be prone to further alteration at successive higher demagnetization temperatures. Chemical alterations in the magnetic mineral suite, starting at temperatures from 380°C upward–depending on the grain size of the original goethite–were inferred from an increase in initial susceptibility. The susceptibility behaviour showed an appreciable variation, due to the creation of varying trace amounts of magnetite, which appeared to be already present after the 400°C step. After the final 685°C demagnetization step the magnetic mineralogy was usually dominated by magnetite. The observed variation is thought to be the result of local within-specimen differences in availability of the amount and possibly also the composition of the vapour phase. The vapour phase is due to chemical reactions between the matrix constituents: waterglass and calcite, leading to a CO2/H2O vapour phase. Reducing capacity, necessary for the magnetite formation, is envisaged to be created by decomposition of trace amounts of organic matter at relatively low temperatures, up to some 400°C. At higher temperatures (over 550°C) decomposition of traces of ferrous iron bearing clayminerals presumably donates the ferrous iron.Differences in the magnetite/haematite ratio as well as in the properties of both minerals were assessed in some detail with more or less routine rockmagnetic methods: acquisition of the isothermal remanent magnetization at room temperature (in fields up to 11.2MA m-1 or 14 T), AF demagnetization and low-temperature cycling of the saturation remanence. This was done after the 400°C step to study the magnetic properties of the newly formed haematite and after the 685°C step to evaluate the properties of the magnetite and more evolved haematite.After the 400°C step the haematite appeared to be very fine-crystalline and magnetically extremely hard. Its saturation remanence is considerably lower than that of well-crystalline haematite. After the 685°C step its hardness has decreased and its saturation remanence slightly increased. Chemical differences between the goethites do not show up as distinct haematite properties, only Mn-bearing goethite yields a softer haematite type.The magnetite formed in all goethite samples seems to have more or less identical properties depending on its thermal growth history. It has a maximum blocking temperature of some 560°C, which does not point to a high vacancy content. High values for the magnetite/haematite saturation remanence ratios indicate magnetite grain sizes from very close to the superparamagnetic threshold size to approximately 1μm, i.e. the goethite grains have been converted to intimately intergrown haematite/magnetite aggregates.

Notes: Grain-size dependence of some remanent magnetic properties of a natural low-temperature haematite is reported. The haematite was obtained from a gossan in Montarnu (France) and upgraded by magnetic separation. Seven grain-size fractions were prepared ranging from 10 down to 〈0.25 μm by means of microprecision sieving and centrifuging. The haematite has an average crystallite size of 60 nm and contains minor contents of Si, Al and H2O. Its unit cell dimensions are very close to those of ideal haematite.The magnetic properties of the haematite are strongly dependent on its thermal history. For the original unheated haematite fractions, the isothermal saturation remanence (Jrs) decreases with grain size. The remanent coercive force (Hcr), remanent acquisition coercive force (Hcr,) and median destructive field of the saturation remanence (H1/2I) are maximal in the 1.0-0.5 μm fraction. This grain-size range can be understood as the SD threshold size in analogy with ferrimagnetic minerals. Jrs/(Hcr x Xin) is a useful grain-size indicator. Lowtemperature cycling of Jrs shows identical behaviour for all fractions; the remanence decreases slightly and shows no sharp Morin transition.After annealing the haematite fractions at 700 °C, Jrs is considerably reduced relative to the original haematite predominantly due to removal of the defect moment. Both Hcr and Hcr, have increased and their maximal values are shifted to the 2.1-1.0 μm fraction. All fractions show a much sharper Morin transition during low-temperature cycling of Jrs due to crystallite enlargement. TRM decreases with grain size.The different rock-magnetic properties before and after annealing might provide a basis for the discrimination in sediments between a haematite DRM with a high-temperature origin and a haematite CRM which has a low-temperature origin. Two natural red beds, one with a CRM and one with a DRM associated with high-temperature haematite, broadly show the expected differences in coercivity and in annealing behaviour, although a full account of all aspects of the observed behaviour can not yet be given.

Notes: The behaviour of a thermoremanent magnetization (TRM), acquired by cooling from 145°C in a field of 16 kAM-1, is reported for five natural goethites in a grain-size range of 〈5 up to 250n̈m. the TRM intensity increases with isomorphous substitution in the goethite until a certain maximum, to decrease again at higher levels of isomorphous substitution due to lowering of the goethite Nëel temperature. Different grain-size trends are observed for the TRM intensity: in some samples it slightly decreases with grain size, whereas in others a maximum is observed in the intermediate grain-size range. With increasing isomorphous substitution the maximum blocking temperatures during continuous and stepwise thermal demagnetization are decreasing from 90–105°C down to 45–60°C. No grain-size dependence is observed in thermal decay curves.The initial susceptibility measured at room temperature generally decreases some 10 per cent after annealing at 145°C. the decrease after once annealing and its constancy after subsequent annealings at the same temperature is a strong indication that these goethites have never been at that temperature during their geological history. Hence, their NRM is most probably of chemical origin.The goethites are extremely hard with respect to AF demagnetization. Hardly any remanence decrease is observed after application of the maximum alternating fields available (240 kAM-1 or 300 mT), varying from nill to 8 per cent without grain-size dependence. Only in extremely finely crystalline goethite is the decay larger and shows grain-size dependence. For all goethites, the remanence steadily increases with decreasing temperature upon cooling to liquid nitrogen temperature in a field-free space. With increasing isomorphous substitution, the relative remanence increase is larger. Upon rewarming the remanence decreases again, showing virtually reversible behaviour. As goethite is unique in this behaviour, low-temperature treatment can be used for the identification of goethite remanences without the need to heat the sample, avoiding chemical alteration.

Notes: Five goethite concentrates, separated from natural samples of different supergene geological origin, are sieved in 12 well-defined grain-size fractions ranging from 250 μm down to 〈5 μm. the goethites show differences in their isomorphous substitution and in their contents of dispersed non-Fe elements. the crystallite size ranges from 240 to 480 Å which is considerably smaller than the grain size of even the smallest fraction. Nevertheless grain-size dependent trends are present in most observed rockmagnetic parameters. the behaviour in a grain-size dependent framework is reported for the initial susceptibility (Xin), the high field susceptibility (Xhf), the saturation magnetization (Js), the saturation remanence (Jrs; acquired in fields of 16 times 106 Am−1 or 20 T), the coercive force (Hc), ferromagnetic coercive force (Hcferr.), remanent coercive force (Hcr) and the remanent acquisition coercive force (Hcr). Xhf, Hc and Hcr decrease with grain size for each sample. A tendency to decrease with grain size is observed for Jrs. Xin shows a weak maximum in the intermediate grain-size range. Hcferr. is approximately constant in the coarse grain-size fractions and falls in the same grain-size range as Xin rises. No grain-size dependence of Js is observed. Hcr' does not show a consistent grain-size dependence: in three samples it is grain-size independent and the other two it decreases with grain size. There is a great variety between the samples in the absolute figures of Js (30−450 times 10−3 Am2 kg−1), Jrs (15−115 times 10−3 Am2 kg−1), Hc (2−20 times 104 Am −1), Hcferr. (2−70 times 104 Am−1), Hcr (40−320 times 104 Am−1) and Hcr' (100−550 times 104 Am−1). Xin shows smaller differences (0.50−1.50 times 10−6 m3 kg−1) and Xhf is approximately equal for all samples (varying from 0.40 times 10−6 m3 kg−1 down to 0.20 times 10−1 m3 kg−1 with decreasing grain size). Sample differences and some ratios of these parameters are discussed in terms of different goethite chemical composition, crystallite size and the presence of intergrowths. the magnetic properties of mirocrystalline goethite are best understood by taking into account crystallite size and magnetic interaction amongst crystallites.

Notes: The field and frequency dependences of the initial susceptibility of pyrrhotite have been analysed as a function of grain size, motivated by a strong field dependence recently observed for large (mm-sized) pyrrhotite crystals (Worm 1991) and smaller field dependences determined on smaller grain sizes by Clark (1984). In the present study, the frequency ranged from 30 Hz to 27 kHz. At 2 kHz, a field range from 0.05 to 1500 μT was investigated. Separate determinations of in-phase (K') and quadrature (k”) susceptibility components allow for the analysis of eddy current effects.Up to 4 kHz the in-phase susceptibility of a pyrrhotite-ore specimen is practically independent of frequency whereafter it decreases while the quadrature component increases linearly with frequency to a value on the order of k' at 20kHz for large grains. k” is proportional to d2μ where d is grain diameter, μ the intrinsic permeability, σ the electrical conductivity and f the frequency. the frequency response of magnetite is essentially flat up to frequencies 〉20 kHz. Both frequency dependences agree well with calculations based on the theory by Wait (1951). the conductivity of the pyrrhotite ore has been determined to be σ= 1.40 (±0.05). 105Ω−1m−1.The susceptibility of pyrrhotite and its field dependence increase strongly with grain size. While the susceptibility of grains smaller than 30 μm is field independent (up to 1.5 mT) it may increase as kα H0.25 for mm-sized crystals in fields 〉10μT. For most samples the Rayleigh law is inadequate to characterize induced magnetizations in weak alternating fields. When susceptibilities are measured for geomagnetic anomaly modelling, laboratory fields should be of similar intensity as the Earth's field and of frequency ≤1 kHz.

Notes: The principle of a Curie balance was changed by using a sinusoidally cycling applied magnetic field instead of a fixed applied field. This was done with a horizontal translation type Curie balance. By cycling between field values Bmin and Bmix, the output signal is amenable to Fourier analysis. Partial Fourier analysis yields the fundamental harmonic and the second harmonic, termed SIG1 and SIG2 respectively. These are related to the saturation magnetization (Ms) by MS=2 SIG1 - 8 SIG2 [(Bmax+Bmin)/(Bmax-Bmin)])/[A″(Bmax-Bmin)] and to the paramagnetic susceptibility (χpar) by χpar= 8 SIG2/[A″(Bmix - Bmix)2], whereby A is a calibration constant. Through the Fourier analysis continuous drift correction is achieved simultaneously. A personal computer takes care of field control, temperature control and data acquisition in real time mode, as well as processing the data, to yield SIG1 and SIG2. After the experiment, SIG1 and SIG2 are processed further with a separate transversal filtering program that improves the signal-to-noise ratio. The working temperature range of the adapted horizontal translation type Curie balance is between room temperature and 900°C. Its noise level corresponds to a magnetic moment of 2 ° 10−-9 Am2, making it a very powerful tool for thermomagnetic analysis of weakly magnetic material. Examples demonstrating this potential of the device are shown.

Notes: Summary Sediment cores with different sub-bottom depths (I: 45 cm and II: 700 cm) from the Peru Basin have been investigated. From the depth profile of the relative amount of Fe(II) a redox zone is obtained which correlates with the organic carbon flux into the sediment (core I). Mössbauer parameters suggest that the iron in the sediments is mainly contained in clay minerals and to varying extent also in goethite.