Vladimír Kropáček

The vibrating-sample magnetometer and Preisach diagram

Abstract The results of studying the magnetic properties of rocks with a vibrating-sample magnetometer are presented. The limiting parameters of the measuring system are given and demonstrated on a concrete example of measurements. The possibilities of identifying stochastic models of the distribution of Preisach-type microcoercitive forces are discussed and the result of the identification is presented for a sample of pure haematite.

Transformation of hematite to maghemite as observed by changes in magnetic parameters: Effects of mechanical activation?

A natural hematite of hydrothermal origin shows evidence of time-dependent behavior related to the sample preparation procedure (involving significant crushing and milling). A strongly magnetic phase with a Curie temperature (∼630°C) corresponding to that of maghemite is present after storage for one-and-a-half-years, while being absent in the original sample material. The magnetic properties of the hematite-maghemite in relation to mechanical treatment are compared and possible mechanisms of the hematite transformation are discussed. The hematite-maghemite transformation is suggested to have resulted primarily from recrystallization induced by shear stress resulting from crushing and milling.

Some magnetic properties of synthetic and natural haematite of different grain size

Abstract The behaviour of some magnetic properties of natural and synthetic haematite of different grain size is examined. The natural haematite was obtained from the hydrothermal deposit Kadaň (Czech and Slovak Federal Rep.). Six grain-size fractions ranging from 120 to 40 μm were prepared by means of sieving and two further fractions down to 5 μm by wet ultrasonic sieving. Since the behaviour of the fractions is similar, that of only four representative samples is reported. In addition, the behaviour of one submicron synthetic haematite fraction (0.5 μm) prepared by oxidation of ferrous sulphate (uniform in size and shape) was investigated. The initial remanence value (Jr) seems to increase with decreasing grain size. During alternating field (AF) demagnetization, all fractions behaved similarly, except for the submicron fraction which is considerably softer than the others. Normalized (isothermal remanent magnetization) IRM acquisition curves were similar for all fractions. Parameters of the anisotropy of magnetic susceptibility (AMS) display significant changes, mainly during IRM acquisition. During AF demagnetization, the anisotropy degree P exhibits a slight increase (some %), while the behaviour of the shape factor T is complicated. The anisotropy ellipsoid exhibits a tendency to rotate. Significant changes in the AMS parameters occur during IRM acquisition. Curves of P and T vs. IRM acquisition field, for various grain-size fractions, show no coherent pattern. For all the samples studied, the T vs. H curve exhibits a threshold value at which change in the type of arrangement of easy axis of magnetization occurs. For the IRM acquisition fields higher than some 320 kA m−1, the minimum susceptibility axis parallels the direction of the IRM acquisition field. Hysteresis curves of the fractions are similar to each other. The Preisach distribution function was determined and it indicates that the reversible part of the magnetization process plays an important role comparatively. Based on the coercivity data presented no unambiguous conclusion could be drawn from the single-domain (SD)-multidomain (MD) transition, associated with a coercivity maximum.

Experimental determination of magnetic interactions within a system of synthetic haematite particles

Abstract A remanence-based technique, developed recently in magnetic recording media research with the aim of identifying the nature and intensity of interparticle magnetic-interaction fields, is reported. The technique is based on Wohlfarths linear relation linking isothermal remanent magnetization (IRM) acquisition and saturation IRM (SIRM) d.c. demagnetization with the field as a parameter, deviation from the linearity being attributed to many-body effects. Physical interpretation of an interaction-dependent parameter is discussed. The method is demonstrated on artificial samples of synthetic fine-grained haematite with various grain sizes and shapes and different initial magnetic configurations (i.e. the degree of spin-spin ordering). The observed negative character of interactions, yielding a net demagnetizing effect, is attributed to the specific arrangement of magnetic particles in the specimens and/or to a significant portion of superparamagnetic particles in some specimens.

Paleomagnetism and magnetic fabric of the deformed redbeds of the Cap de la Chèvre formation, Brittany, France

Abstract We present the results of a palaeomagnetism and magnetic fabric study in six sampling sites of the lower Ordovician redbeds of the Cap de la Chevre formation, Crozon Peninsula, France. Measurement of anisotropy of magnetic susceptibility (AMS) shows that these beds are affected by a weak but significant tectonic strain during the Hercynian folding of the Armorican Palaeozoic cover. Samples containing small elliptical reduction spots allowed us to check the control of strain on AMS. Site mean AMS data were then used to provide strain estimates at each site. Analysis of remanent magnetization through thermal and chemical stepwise demagnetization procedures allowed us to isolate three magnetic components. Component A, defined in the lower unblocking temperature range, is a recent overprinting which clusters around the present-day dipole field direction. Components B and C, defined respectively in the intermediate and higher unblocking temperature ranges, appear to be pretectonic in age. The fold test, however, is inconclusive for both, because of the significant dispersion still remaining after the simple tilt correction. Assuming that at least a part of this dispersion is due to strain, we have applied the strain removal technique using AMS-derived strains at each site. This results in a clustering of component C from four sites, allowing recovery of the primary Ordovician paleomagnetic direction (D = 261°, I = 71°, α95 = 11°). In contrast, unstrained component B shows a small circle distribution, containing the formation mean of unstrained component C. The hypothesis that component B and C distributions arise from beds being tilted during a long-term magnetization acquisition in the Ordovician field is discussed, taking into account the extensional geodynamic context during deposition of lower Palaeozoic redbeds in Brittany.

Palaeointensity of the geomagnetic field during upper cainozoic derived from palaeo-slags and porcellanites in north Bohemia

РеэюмеПорчеллaнumы u naлеоченовые щлaкu нa mеррumорuu северноŭ Чехuu nре¶rt;сmaвляюm собоŭ nрuро¶rt;ныŭ мamерuaл, коmорыŭ очень nо¶rt;хо¶rt;um ¶rt;ля выве¶rt;енuя naлеомaгнumных нanрaвленuŭ u naлеоuнmенсuвносmuгеомaгнumного nоля, ¶rt;еŭсmвующего во время кaусmuческого uзмененuя. Эmuгорные nоро¶rt;ы, нaзывaемые эр¶rt;брaн¶rt;ы, вознuклu nо¶rt; влuянuем кaусmuческого uзмененuя nреuмущесmвенно nелumовых осa¶rt;очных nоро¶rt; всле¶rt;сmвuе сaмовозгорaнuя угольных слоев. Кaусmuческое uзмененuе uмело месmо в nерuо¶rt; оm верхнего nлuоценa ¶rt;о чеmверmuчного nерuо¶rt;a. В рaбоmе nре¶rt;ложены mрu uнmерnреmaцuонных nрuемa, nо¶rt;хо¶rt;ящuе ¶rt;ля вы¶rt;еленuя naлеоuнmенсuвносmu, оnuрaющuеся нa меmо¶rt;ы Телье u Нaгama. Новaя рaэрaбоmaннaя annaрamурa MAVACS (Magnetic Vacuum Control System) былa uсnользовaнa ¶rt;ля mермuческого рaзмaгнuчuвaнuя в сре¶rt;е высокого мaгнеmuческого вaкуумa. Был nре¶rt;ложен u nроверен nо¶rt;хо¶rt;, основaнныŭ нa мульmuкомnоненmном aнaлuзе мaгнеmuэaцuu. Кроме nолученuя меmо¶rt;uческuх знaчuŭ, было maкже усmaновлено, чmо uнmенсuвносmьгеомaгнumного nоля в uссле¶rt;уемыŭ nерuо¶rt; менялaсь в nре¶rt;елaх 48%±4%-154%±32% знaченuя uнmенсuвносmu современногогеомaгнumного nоля.SummaryPorcellanites and palaeo-slags from North Bohemia are natural materials which can be used to derive the palaeomagnetic directions and palaeointensity of the geomagnetic field active at the time of caustic alteration. The origin of these rocks, called erdbrands, was due to the caustic alteration of predominantly pelitic sediments as a result of underground fires conditioned by spontaneous ignition of coal seams. The caustic alteration occurred during the Upper Pliocene to the Quaternary. Three procedure based on the methods by Thellier and Nagata are presented in the paper. The newly developed apparatus MAVACS (Magnetic Vacuum Control System) was used for the thermal demagnetization of samples. A procedure based on multi-component analysis was also proposed and tested. Besides some methodic results, it was found that the geomagnetic field intensity varied during the respective period within the limits of 48%±4% to 154%±32% of the present geomagnetic field intensity.

Comparison of the hysteresis characteristics of synthetic samples with different magnetite and haematite contents

The values of the initial magnetic susceptibility κ were measured with different instruments. The values of saturated magnetic polarization Js, the coercive force Hc, the alternating current demagnetization curves JrsJrso = F(H) and the hysteresis curves J = J(H) were determined for synthetic samples containing synthetic magnetite with grain sizes of < 40 μm, 40–80 μm, 80–120 μm and 120–160 μm, and concentrations of 0.1–5.0% by volume. Analogous parameters were also determined for synthetic samples containing natural and synthetic haematite with a grain size of 40–80 μm and concentration of 1.0–10.0% by volume. The results indicate a significant effect of the magnetic interactions between the grains of the magnetic minerals in the samples and the ways of determining the grain size and concentration of magnetite or haematite in rock samples.

Magnetism of natural pyrrhotite, haematite and ilmenite

SummaryThe paper deals with the magnetic properties of the natural minerals haematite, ilmenite and pyrrhotite. The natural remanent magnetization Jn, the volume susceptibility ℵ, the specific magnetization of saturation σso, the Curie temperature Θ, the A.C. demagnetization of natural remanent magnetization, the thermal demagnetization Jn, the temperature dependence of the specific magnetization of saturation, and several other parameters were determined for the individual sets of minerals. Since natural minerals can and do contain various heterogeneous and isomorphic admixtures, their magnetic properties may change depending on the content and type of these admixtures. Therefore, all the investigations of the magnetic properties described in this paper were carried out with extensive sets of minerals, representing various Czechoslovak and world localities. A number of interesting results were obtained from the executed experiments, e.g., the different stability of the natural remanent magnetization of haematite with respect to A.C. demagnetization, the presence and type of heterogeneous inclusions in ilmenite, the phase changes connected with the γ-transition in natural pyrrhotites, etc. The types of distributions of the investigated values of the magnetic properties were also determined.

Changes of the magnetic susceptibility and its anisotropy of basalts by oxidation of titanomagnetites

SummaryThe changes of the anisotropy of the magnetic susceptibility of alkaline Tertiary basalts under titanomagnetite oxidation are investigated. The oxidation of the titanomagnetites was achieved by the following thermal processing of rock samples: 60 mins at a temperature of 400°C, 300 mins at T=400°C and 60 mins at T=500°C. It is shown that considerable changes of the values of the magnetic susceptibility, as well as of the degree of the preferred orientation of the titanomagnetites and of the ellipsoid of anisotropy in the sample occur as a result of the oxidation of titanomagnetites, contained in the samples, due to the said thermal processing. Moreover, the micro-structure of the grains, created by the oxidation of the titanomagnetites, causes the mostly linearly parallel configuration to change to a planar-parallel configuration.

Changes of electric conductivity in the neighbourhood of the curie temperature of basalts

SummaryIn investigating the electric conductivity of rocks σ as a function of the temperature it was found that rocks containing ferrimagnetic minerals display a change in the slope of the functionlg σ=f(1/T) in the neighbourhood of the Curie temperature Θ. In order to explain these changes the curveslg σ=f(1/T) and the Curie temperatures Θ, obtained from the temperature dependence of the saturated magnetization Js=f(T), were compared. Eight samples of basalts, 2 samples of haematite and magnetite were used to demonstrate the relation between Θ and the changes in the pattern of the electric conductivity, caused by the variations of the exchangeable energy of ferrimagnetic minerals.