V. Hoffmann

Magnetic susceptibility mapping of roadside pollution

Abstract The aims of our study are to trace the distribution and concentration of contaminants in soils along roads and highways carrying appreciable traffic by using magnetic proxies. In addition, the concentration gradients and possible accumulation of pollutants in deeper soil layers are investigated. Magnetic susceptibility is used for pollution mapping in the field (Bartington bridge). The distribution of the susceptibility values represents polluted areas strongly influenced by traffic frequency, roadside topography, meteorological conditions (e.g., wind direction) and other factors. A magnetite-like phase was found to be responsible for the enhancement of the magnetic signal in the roadside soil.

Mineral magnetic characterization of anthropogenic magnetic phases in the Danube river sediments (Bulgarian part)

Abstract Magnetic methods have been widely used for delineation of environmental pollution of soils and sediments under certain conditions during the last decade. A number of structure-sensitive magnetic characteristics (coercive force, coercivity of remanence, remanent magnetization) were utilized for deduction of the grain size of the anthropogenic particulate matter which serves as a proxy for the evaluation of the distance from the pollution source, possible health effects, etc. However, up to now there is no detailed study on the influence of the morphology, internal structure and phase composition of anthropogenic particles on the corresponding magnetic characteristics. The aim of the present investigation is to obtain detailed microscopic, magnetic and microchemical data on single grains from polluted river sediments and thus to check the validity of the classical grain size-dependent trends of coercivity and remanence parameters obtained for natural and synthetic magnetites for the anthropogenic magnetic phases. Single anthropogenic particles from magnetic extracts have been separated from several sites along the Bulgarian part of the Danube river, which show the strongest magnetic enhancement. Results from the combined magnetic (thermomagnetic analysis and magnetic hysteresis), microscopic (optical and scanning electron microscopy (SEM)) and microchemical (electron microprobe analysis (EMPA)) study on single anthropogenic particles show that there is a general trend for significant underestimation of the physical size of the anthropogenic particles due to the following reasons: (1) in most cases the spherules possess a complex internal structure, in which the magnetic phase is presented as dendrites exsolved within a glassy non-magnetic aluminosilicate matrix; (2) the surface morphology is characterized by orange-peel or dendritic structures, or smaller strongly magnetic spherules are adhered to the surface. Magnetic mineralogy as deduced from single grain thermomagnetic K ( T ) analysis and further elucidated by microprobe data on the same particles shows that magnetite is the main ferromagnetic phase. In isolated cases significant Al substitution leads to a decrease of T c down to 500°C. Hysteresis parameters for all studied single grains of physical sizes varying from 39 μm up to 1.5 mm fall mainly in the pseudo single domain (PSD) range. On the basis of the results from single grain magnetic measurements, a new approach for semiquantitative estimation of the degree of anthropogenic pollution is proposed.

First domain observations with the magneto-optical Kerr effect on Ti-ferrites in rocks and their synthetic equivalents

Abstract Ti-ferrites of the composition around Fe 2.4 Ti 0.6 O 4 are commonly found in basaltic rocks. They carry most of the information about the ancient earth magnetic field. For the first time, the magneto-optical Kerr effect could be applied on natural and synthetic Ti-ferrites. The experimental success is based on 1. - the achievement of a sufficiently smooth and stressfree surface obtained by polishing with amorphous SiO 2 suspension and 2. - the use of digital image processing. The results support earlier conclusions that, despite the cubic crystallographic structure, the true intrinsic anisotropy is uniaxial showing domain patterns similar to metallic glasses due to dominating internal stress. The application of the Kerr technique offers new perspectives to investigate the magnetic structure and the magnetization processes of the Ti-ferrites.

Magnetic Susceptibility Screening of Anthropogenic Impact on the Danube River Sediments in Northwestern Bulgaria - Preliminary Results

Several events of anthropogenic impacts on the floodplain river sediments of the Danube along a section near the town of Oryakhovo (NW Bulgaria) have been detected using magnetic technique. In the field, magnetic susceptibility was measured using three susceptibility meters – MS2D, KT-5 and SM30. The differences in the surface susceptibility values measured with the different instruments are ascribed to the various penetration depths which depend on several parameters like sensor diameter, frequency and field strength. This is supported by the detailed laboratory study on penetration depth and sensitivity of the new SM30 susceptibility meter and the comparison with the already existing data for the other two sensors. Boundaries between different flooding events are clearly linked with significant variability of the measured susceptibility values corresponding to one and the same level. Sediment cores, taken at different distances from the water level, show the history of depositional events and corresponding degree of anthropogenic pollution. Identification of the magnetic phases responsible for the signal was carried out by means of thermomagnetic κ(T) curves. The main carrier is magnetite with Tc of 580°C or oxidized magnetite with Tc of 600°C. A second kink at about 300°C is better expressed for samples with lower susceptibilities. It may correspond to several minerals, e.g. maghemite, pyrrothite, titanomagnetite, as well as different mineral transformations of paramagnetic minerals (decomposition, dehydroxilation, etc. for example siderite, lepidocrocite). The samples showing strong magnetic enhancement are characterized by the predominance of magnetite. Optical microscopy on magnetic extracts shows the presence of small spherical particles, typical for the anthropogenic magnetic phases from high-temperature technological processes. Well expressed susceptibility variations along the cores suggest the presence of several stages of different degree of pollution, covering an estimated period of about 30 years.

Magneto-optical Kerr effect on magnetite crystals with externally applied magnetic fields

Abstract Magnetic domain structures of a multidomain magnetite crystal were observed as a function of applied magnetic field with the magneto-optical Kerr effect. Normal and reverse magnetic fields were applied from zero to 165 mT until the grain was nearly saturated. The magnetic hysteresis, determined from the domain patterns on the surface of a single crystal, was inconsistent with bulk hysteresis measurements on magnetite of equivalent grain size. A nearly single-domain configuration at zero field for the remanence state was interpreted as a surface phenomenon concealing magnetic domains in the interior of the grain. A model is presented in which magnetization lies in the observed plane and domain walls are dipping at a shallow angle.

The dam breakage of Baia Mare––a pilot study of magnetic screening

Magnetic screening in the area of Baia Mare (Romania) was carried out in June 2000 in order to detect the degree of environmental pollution and to test the applicability of this method in this area. With a long tradition of mining activities, a gradual pollution of soil, air and rivers took place continuously in addition to smaller accidents in this area until the dam breakage on the 30.1.2000. During this accident, about 100,000 m3 of mud containing cyanide and heavy metals leaked out and moved over fields and through a village into the river system of Lapus, Somes, Tisza and Danube. Initial magnetic monitoring carried out during the translocation of the polluted waters along the Bulgarian part of the Danube revealed the effectiveness of the method for a proper and fast identification of pollution both in time and space even at remote distances from the source. For the later pilot project magnetic (χ) screening was performed using a portable Bartington MS2 kappameter with a D-loop sensor. In addition fine-grained material (<0.5 mm) was sampled from the fields and river sediments and measured in the laboratory using a MS2 B-sensor. The results of the collected samples show a clear decrease in χ with increasing distance from the dams and mining areas in a regional and local scale, while the results gained by the MS2-D strongly depend on the nature of the ground. The dynamic geological setting around Baia Mare (fluvial sediments and valley fills), produces a heterogeneous background signal. A continuous monitoring system controlling the basic conditions could overcome these limitations.

Magnetic domain structure of multidomain magnetite as a function of temperature: observation by Kerr microscopy

Abstract This study presents magnetic domain observations of the crystallographic (110)- and (111)-planes of various monocrystalline multidomain (MD) magnetites (Fe 3 O 4 ) using the magneto-optical Kerr effect (MOKE). At room temperature (RT), lamellar domains separated by 180°-, and closure domains with 71°-, and 109°-walls have been observed on the (110)-plane due to the two easy directions of magnetization within this plane. Domain structure observations on the (111)-plane reveal more complex domain patterns, which are formed by arrays of closure domains. Furthermore, in this study, the behaviour of the domain structure as a function of temperature has been observed. For high temperature domain observations, a special furnace has been developed which allows domain observations using the MOKE between room temperature and Curie-point (575°C). During observation, the furnace is evacuated to 10 −3 Pa in order to prevent chemical alteration effects of the sample. The temperature dependence of the domain structure on the (110)-plane has been observed for various (MD) magnetites from Minais Gerais (Brazil) and the Zillertal (Austria). Magneto-optical domain contrasts could be observed up to 555°C. During heating and cooling, the domain structure proved to be reversible. The mean domain width at each temperature step was determined by two methods in order to obtain an improved approximation. At RT, the mean domain width for different crystals varies between 40 and 90 μm. The heating experiments on Brazilian magnetites show that the mean domain width is relatively stable from RT up to 350–400°C; it then increases significantly at higher temperatures up to the Curie-point of magnetite. The experimental results are in good agreement with theoretical calculations of the absolute domain width in MD magnetite for a large range of grain sizes and varying temperatures.

Mineralogical, magnetic and mössbauer data of symthite (Fe9S11)

SummaryThe mineralogical and magnetic properties of the magnetic iron-sulfide symthite (Fe9S11 from the Sokolov Basin, Czech Republic, were studied. Our experimental results show that smythite is hexagonal with cell edges a0=3.468 Å and c0=34.470 Å. Thermomagnetic measurements Ms(T) (Ms: saturation magnetization) and χ(T) (χ: susceptibility) were performed in order to study the thermal alteration of the smythite. Its Curie point could not be determined. The hysteresis loops had a shape typical for SD - grains with relatively low values of saturation magnetization (Ms=10.3 Am2/kg) and coercive forces of ≈ 37 mT. Similarities in the magnetic, crystallographic, and the Mössbauer parameters to the structure of pyrrhotite (Fe7S8) suggest that smythite could be antiferromagnetic with vacancy ordering resulting in a small net magnetic moment similar to that in pyrrhotite.

Magnetic responses to acidification in Lake Yangzonghai, SW China

Abstract A short core of 33 cm length was obtained from lacustrine sediments of Lake Yangzonghai, Yunnan Province, China. A power plant, built in 1960 is located close to the northeast corner of the lake, and black burned coal ashes can be found within the uppermost 12 cm of the sediments. The age frame of the core was determined based on 137 Cs and 210 Pb analyses. Magnetic, biological and geochemical analyses were performed on the sediments and magnetic parameters were also determined from the source material of the power plant. The burned coal ash, collected directly from the power plant, is strongly magnetic, and its main magnetic content is magnetite. However, magnetic susceptibility and anhysteretic remanence of the sediment core is much lower and shows some evident fluctuations above 12 cm, corresponding to the time when the power plant was built. Ostracoda show a similar pattern, with a lower concentration at the top and a higher one below. However, some certain diatom species, which only can survive in acid water environment, show the reverse pattern. Element contents such as Al, a sensitive indicator for acidification, decrease above the 10 cm level. Based on all these data, it can be concluded that the water of the lake was acidified after the power plant was built, which caused dissolution of magnetic particles, and resulted in the decrease of magnetic concentration at the top of the core. This work demonstrates that magnetic response towards environmental pollution can be in different ways, i.e. not only by an enhancement of magnetic concentration.

Magneto-optical Kerr effect in (titano)magnetite, pyrrhotite and hematite

Abstract The successful application of the magneto-optical Kerr effect in rock magnetism is demonstrated. Domain structures of the most frequently occurring ferromagnetic minerals in rocks—(titano)magnetite, pyrrhotite and hematite—are shown. Surprisingly, the method works with hematite, where the Bitter pattern technique is not appropriate. Skin depth and sublattice interactions are discussed to explain the effect.