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SUMMARY In complex sedimentary environments with different sources of the magnetic particles, it is vital that all factors contributing to and affecting the magnetic signals be well understood for a better constrained interpretation of palaeomagnetic and climatic data. Palaeoclimatically driven signals may be convolved with volcanic and cosmogenic sources, and diagenesis may alter the primary magnetic mineral assemblage. Rock magnetic methods and electron microscopy provide suitable means to characterize magnetic assemblages. In this study of core-retrieved sediments from the last 150 kyr off the coast of Senegal (NW Africa), the occurrence of magnetic spherules of volcanic origin and cosmogenic particles appeared to be insignificant. In glacial stages, ferrimagnetic iron and iron-titanium oxides magnetically dominate with few signs of a diagenetic overprint. Low- and room temperature magnetic results reveal evidence for haematite and goethite while haemoilmenite and titanomagnetite were identified from low-temperature AC susceptibility measurements and SEM observations. In contrast, the interglacial samples appeared to have been severely affected by diagenesis, with pyrite making up over 50 per cent of the iron-bearing phases. The Verwey transition observed at 110 K confirms the presence of magnetite. The glacial and interglacial contrast reveals the close link between northwest African climate and the magnetic mineralogy of the sediments.

SUMMARY Here we report a detailed mineral magnetic study of Neolithic burnt levels in the Mirador Cave (Sierra de Atapuerca, Burgos, Spain) to reconstruct the burning history and to investigate their suitability for archaeomagnetic purposes. As a consequence of the ancient burning, a characteristic facies sequence was developed along the Holocene stratigraphy. From top to bottom it includes: (i) 2–10 cm ash layers, (ii) ∼2 cm underlying rubefied layers and (iii) clay, mainly unburnt and of variable thickness. In some cases a thermally altered facies (2–6 cm) with a heterogeneous texture was identified (facies TF), usually between rubefactions and the unburnt levels. 126 oriented samples from 4 units (MIR12, 15, 18 and 21) and a 2 m section, all comprised between units MIR21 (6380 ± 40 14C BP) and MIR9 (5090 ± 40 14C BP) were analysed with rock magnetic methods. In addition, bulk sediment from each facies that comprise the Neolithic sequence was investigated. Measurements included: stepwise alternating field and thermal demagnetization of natural remanent magnetization (NRM), viscosity experiments, determination of the anisotropy of the magnetic susceptibility (AMS), the susceptibility frequency dependence at room temperature and determination of the temperature dependence of the susceptibility. Additional experiments consisted in the determination of the behaviour of anhysteretic and isothermal (IRM) remanences, magnetic hysteresis loops, first-order-reversal-curve diagrams, and thermal demagnetization of three-axial IRM. It appeared that the facies all show a fairly similar magnetic mineralogy and grain size dominated by low-titanium magnetite that is often partially maghaemitized. Main differences constitute the amount of superparamagnetic particles that is higher in unburnt strata concurring with a less well-defined NRM behaviour. The magnetic mineral concentration is notably higher in ashes. This homogeneity strongly suggests that similar sources and burial conditions prevailed during Neolithic times. Agreeing with archaeological observations and favoured by rapid burial conditions, very limited alterations have been deduced. AMS data revealed the absence of fluid flow in the ash lenses sampled. Demagnetization revealed a stable single NRM component in ashes, a single or two-component NRM in rubefactions and less stable multicomponent behaviour in clays. In ashes, magnetic minerals are likely secondary in origin formed by low-temperature oxidization soon after burning. Although this thermochemical nature of the NRM invalidates the use of these sediments for palaeointensity studies, archaeomagnetic (directional) data can be successfully obtained because the burning and oxidation are closely confined in time.

SUMMARY The reliability of primary natural remanent magnetization (NRM) signals in greigite-bearing sediments has been frequently questioned. Here, we show that the stable NRM in the deep marine Middle to Late Miocene sediments at Monte dei Corvi, northern Italy, is mainly carried by greigite. Combined rock magnetic experiments and scanning electron microscopy successfully enabled discrimination between two greigite populations. One fine-grained and relatively well-dispersed greigite population (grain size between 60 and 200 nm) is most likely of magnetotactic origin. The second greigite population with larger grain sizes (typically 700 nm to 1 μm) is most likely of authigenic (bacterially mediated) origin, and is related to post-depositional sulphidization processes. Greigite is the main magnetic remanence carrier in the older part of the section (12.8 to 8.7 Ma), whereas greigite and fine-grained (presumably magnetotactic) magnetite are present in the younger part of the section (8.7 to 6.9 Ma). Similar remanent magnetization directions of the magnetite and greigite components, and the likelihood of a magnetotactic origin, suggests that the NRM is of syn-depositional age. We suggest that moderate methane seepage from the underlying sediments may have occurred that resulted in the preservation of pristine greigite. This corroborates the reliability of the previously established magnetostratigraphy at Monte dei Corvi.

SUMMARY To identify remagnetization is essential for palaeomagnetic studies and their geodynamic implications. The traditional approach is often based on directional analysis of palaeomagnetic data and field tests, which may be inconclusive if the apparent polar wander path (APWP) is poorly constrained or if the remagnetization predates folding. In several cases, rock magnetic work, particularly, the measurement of hysteresis loops allows identification of the so-called ‘remagnetized’ and ‘non-remagnetized’ trends. However, for weakly magnetic samples, this approach can be equivocal. Here, to improve the diagnosis of remagnetization, we investigated 192 isothermal remanent magnetization (IRM) acquisition curves (up to 700 mT) of remagnetized and non-remagnetized limestones from the Organy` a Basin, northern Spain. Also, 96 IRM acquisition curves from non-remagnetized marls were studied as a cross-check for the non-remagnetized limestones. A non-parametric end-member modelling approach is used to analyse the IRM acquisition curve data sets. First, remagnetized and non-remagnetized groups were treated separately. Two or three end-members were found to adequately describe the data variability: one end-member represents the high-coercivity contribution, whereas the low-coercivity part can be described by either one end-member or two reasonably similar endmembers. In the remagnetized limestones, the low-coercivity end-members tend to saturate at higher field values than in the non-remagnetized limestones. When the entire data set was processed together, a three-end-member model was judged optimal. This model consists of a high-coercivity end-member, a low-coercivity end-member that saturates at ∼300–400 mT and a low-coercivity end-member that approximately saturates at 700 mT. Higher contributions of the latter end-member appear to occur dominantly in the remagnetized limestones, whereas the reverse is true for the non-remagnetized limestones, so they plot in clearly distinguishable areas. Meanwhile, the IRM curves from non-remagnetized marls show a behaviour similar to that of the non-remagnetized end-member in the limestones. Therefore, this new approach can be a very useful tool to diagnose remagnetization in weakly magnetic limestones and marls. We recommend applying it to other areas of potentially remagnetized low-intensity sediments.

SUMMARY Here, we report an increased cooling coupled with an intensified aridification for the Xiantai fluvio-lacustrine sequence of Pleistocene age (1.9–0.1 Ma) in the Nihewan Basin (North China), known for the occurrence of several hominin sites. The cooling is related to an up-section decrease in propensity to chemical weathering as inferred from an increase in lowfield susceptibility after cycling to 700 ◦ C. Close to 700 ◦ C, reacting chlorite is providing the iron source for newly formed very fine-grained ferrimagnetic minerals which enhances the susceptibility signal. The reactivity of chlorite after annealing at temperatures above 600 ◦ C is documented with X-ray diffraction and the up-section decrease in propensity to chemical weathering is confirmed by the up-section declining illite/chlorite ratios and increasing chlorite and illite contents based on X-ray diffraction analysis on selected samples.

eve basin (southern France), also deposited at low latitudes. Additionally, we critically review published data from low latitude Permian red beds from the same basins. We focus on records with a sufficient number of samples, and acquire, whenever possible, the original data, or—as a second choice—parametrically sampled published site means. We test the distributions of these directions using a statistical secular variation field model (TK03.GAD). The angular standard deviation (ASD) of the virtual geomagnetic pole (VGP) distribution is a measure of (palaeo)secular variation, and we compare our values of ASD from the Permian red beds to those from lavas erupted in the last 5 Myr. Contrary to our previous studies, we now conclude that secular variation during the Permo-Carboniferous Reversed Superchron is slightly, but significantly, reduced at low latitudes compared with the recent field, as based on a latest compilation that supercedes earlier compilations. It also agrees with a new study, which found that PSV, at low palaeolatitudes during the Cretaceous Normal Superchron, was also significantly lower than in periods with frequent reversals.

SUMMARY The San Quintin volcanic field (SQVF) in Baja California (Mexico) consists of at least 13 volcanic complexes with about 30 lava flows. 40Ar/39Ar step-heating and 3He surface exposure dating experiments suggest eruption ages between 22 and 165 ka. Paleodirections of 14 flows reported here are consistent with normal secular variation. Because of low Curie temperatures and thermal alteration starting at ∼350 °C, classical Thellier–Thellier paleointensity (PI) experiments were deemed unsuccessful. Microwave PI experiments were carried out on 139 samples with 97 being interpreted to be successful, yielding values smaller than 20 μT for all sites and averaging 16.5 μT. Another sample set was studied with the multispecimen parallel differential pTRM method; temperatures for acquisition of pTRM were chosen between 200—and 225 °C. In two cases of higher unblocking temperatures, it was set at 480 °C. The experiments were successful for 104 out of 120 specimens with PI values ranging between 14 and 21 μT and clustering around 18 μT. Three noticeably higher values between 28 and 35.7 μT were obtained. Both methods yield in general well comparable PI data with the microwave PI showing a tendency to be slightly smaller than those from the multispecimen PI. The reason for it is presently not known with certainty. Possible causes include 1) incipient thermal alteration, anticipated to be slightly more prominent in the multispecimen method. 2) Cooling time effects: the microwave method involves heating during only 10–30 s compared to 2400 or more seconds in the multispecimen method. This may account for a relative underestimate of the field intensity by about 4 per cent. 3) A general PI underestimate (by ∼4 per cent) inherent to the microwave perpendicular field method applied to multidomain particles in comparison to parallel field methods. 4) A PI overestimate for small multidomain particles in the multispecimen method. However, both PI methods produced very similar results even for difficult materials as the SQVF lavas giving credit to the veracity of the obtained PI values. The multispecimen parallel differential pTRM paleointensity method thus presents a valuable alternative for traditional thermal PI determinations, as it can be applied using standard laboratory equipment. The microwave paleointensity method may yield better results but requires specialized equipment not yet available commercially.

SUMMARY Here we report on the sources of magnetic anisotropy in pyrrhotite, an iron sulphide present in many rocks as an important carrier of the Natural Remanent Magnetization. While the magnetic hysteresis parameters of pyrrhotite are well known, the existing database concerning its anisotropy behaviour is patchy and ambiguous. Therefore, a collection of 11 seemingly single crystals of natural pyrrhotite was scrutinized. Before embarking on the anisotropy determinations the set of single crystals was extensively characterized rock magnetically by measuring Curie temperatures, hysteresis loops, IRM acquisition curves, and FORC diagrams (the latter three all at room temperature). First the variation of the low-field susceptibility as function of applied field and grain size was evaluated for fields ranging from 1 to 450 A m −1 . Existing grain size dependent data and the present larger crystals show a logarithmic grain size dependence. This enables estimating the grain size for unimodal pyrrhotite distributions in rocks. Measured trends are better fitted with an exponential function than with a Rayleigh Law style function. Based on the rock magnetic characterization and the behaviour of the anisotropy of magnetic susceptibility six samples (of the original 11) were selected for the high-field anisotropy determinations within the basal plane. Those data were acquired with a torque cantilever-type magnetometer. As expected, most single crystals showed a pure 6‐ θ curve within their basal plane because of the easy axis configuration. In some crystals, however, lower harmonic terms overlapped the 6‐θ term. This may be the dominant source of the observed variation in magnetic anisotropy properties. Torque data of three of the six samples were of sufficient quality to allow evaluation of K 1. Re-evaluation of existing torque data and including the present newly derived determinations, yields for the anisotropy constant of pyrrhotite within the basal plane K 1: (2.7 ± 0.2) 10 4 Jm −3 . This is over an order of magnitude more precise than the sparse existing K 1 data; only the value reported by Mikami and co-authors in 1959 agrees with the new determination. With this firmly established K 1 value meaningful anisotropy models are now possible for pyrrhotite-bearing rocks.

SUMMARY During the Miocene‐Pliocene, the Carpathian region represented the westernmost part of the so-called Eastern Paratethys, a palaeobioprovince that covered central and eastern Europe as well as parts of southwest Asia. Previous palaeomagnetic investigations provide a highresolution magnetochronology for the sedimentary sequences of the Carpathian foredeep and indicate a marked transition in magnetic carriers from iron oxides to iron sulphides, in Chron C3r. Here, we demonstrate using detailed rock magnetic investigations and scanning electron microscope (SEM) analyses that the major magnetic iron sulphide mineral is greigite (Fe3S4). Thermomagnetic runs indicate an irreversible decrease in magnetization with increasing temperature up to 400 ◦ C and SEM observations indicate octahedral grain morphologies and Fe:S ratios that are indicative of greigite. Hysteresis loops have ‘rectangular’ shapes, which are typical of single domain behaviour, with coercivity and coercivity of remanence at values of Bc = 35‐45 mT and Bcr = 52‐67 mT, respectively. First-order reversal curve diagrams have contours that close around single domain peaks with Bc values of 45‐90 mT. Isothermal remanent magnetization component analysis reveals a small dispersion for the greigite component (dispersion parameter, DP ∼ 0.15 log units) indicating a narrow grain size distribution. A positive fold test, inclination shallowing and two positive reversal tests are additional arguments for the syn-depositional formation of this greigite. We thus argue that (most of) the greigite was formed under early diagenetic conditions, that is, within 1000 yr of deposition of the sediment in this setting, and that it thus can be considered as a reliable recorder of the palaeomagnetic signal in this situation. The appearance of greigite during Chron C3r (between 6.0 and 5.5 Ma) in the Carpathian foredeep is most likely related to regional tectonic and/or climatic events that reshaped the basin configuration and changed the palaeoenvironmental conditions.

SUMMARY Low-temperature rock magnetic measurements have distinct diagnostic value. However, in most bulk marine sediments the concentration of ferrimagnetic and antiferromagnetic minerals is extremely low, so even sensitive instrumentation often responds to the paramagnetic contribution of the silicate matrix in the residual field of the magnetometer. Analysis of magnetic extracts is usually performed to solve the problems raised by low magnetic concentrations. Additionally magnetic extracts can be used for several other analyses, for example electron microscopy or X-ray diffraction. The magnetic extraction technique is generally sufficient for sediments dominated by magnetite. In this study however, we show that high-coercivity components are rather underrepresented in magnetic extracts of sediments with a more complex magnetic mineralogy. We test heavy liquid separation, using hydrophilic sodium polytungstenate solution Na6[H2W12O40], to demonstrate the efficiencies of both concentration techniques. Low-temperature cycling of zero-field-cooled, field-cooled and saturation isothermal remanent magnetization acquired at room temperature was performed on dry bulk sediments, magnetic extracts, and heavy liquid separates of clay-rich pelagic sediments originating from the Equatorial Atlantic. The results of the thermomagnetic measurements clarify that magnetic extraction favours components with high spontaneous magnetization, such as magnetite and titanomagnetite. The heavy liquid separation is unbiased with respect to high- and low-coercive minerals, thus it represents the entire magnetic assemblage.

In paleomagnetic and environmental magnetic studies the magnetomineralogical identification is usually based on a set of rock magnetic parameters, complemented by crystallographic and chemical information retrieved from X-ray diffraction (XRD), (electron) microscopy or energy dispersive spectroscopy (EDS) of selected samples. While very useful, each of these supplementary techniques has its limitations when applied to natural sample material which are related to low particle concentrations (down to the ppm range in marine sediments) and very fine grain sizes (down to the nm scale). Therefore, meaningful application of such techniques depends on sample quality. Electron backscatter diffraction (EBSD) of individual grains in scanning electron microscopy (SEM) enables mineralogical identification of grains down to ∼0.2 micrometer and is particularly powerful when combined with EDS. In this study, we show the merits of EBSD for rock magnetic investigations by analyzing titanomagnetites and hemoilmenites of various compositions and submicron lamella of titanomagnetite-hemoilmenite intergrowths. Such particles often occur in natural marine sediments where EDS often has a semi-quantitative character and compositionally similar intergrowths may be difficult to distinguish. With the mineralogical information provided by EBSD unambiguous identification of spinel-type and trigonal oxides is obtained. Optimal EBSD patterns are gathered from smooth, polished surfaces, but here we show that interpretable EBSD patterns can be obtained directly from the surface of unconsolidated, so called 'non-embedded' particles from marine sediments. This information enhances the interpretative value of rock magnetic parameters. © 2007 The Authors Journal compilation © 2007 RAS.

SUMMARY We report on the magnetic properties and the acquisition of a chemical remanent magnetization (CRM) in a field of 100 μT as a function of temperature and time during the lepidocrocite–maghemite–haematite reaction chain. The development of CRM was monitored at a series of 13 temperatures ranging from 175 to 550 °C; data acquisition was done at the specific formation temperatures for durations of up to 500 hr. Up to acquisition temperatures of 200 °C it takes a considerable time (up to 7 hr) before the CRM is measurable. This time decreases with increasing temperature, reflecting the activation energy of the reaction to form the first maghemite. During the lepidocrocite conversion, formation of two types of maghemite is suggested by two peaks in the CRM versus time curves. Magnetic properties were analysed after various stages in the reaction. They indicate a mixture of superparamagnetic and single-domain maghemite. The first reaction product (obtained after annealing at 200 °C) is a fine-grained yet crystalline maghemite (labelled type A). Before massive maghemite formation occurs, the coercive and remanent coercive forces go through a minimum at intermediate temperatures of 250–300 °C (annealing for 2.5 hr). This minimum lowers to 200–250 °C with increasing annealing time (500 hr). This is probably the result of two processes acting simultaneously—formation of superparamagnetic maghemite particles of a second less crystalline maghemite type (labelled type B) and removal of stacking faults in type A maghemite. The second process is suggested by analogy to the behaviour of natural magnetite/maghemite systems on annealing. Removal of stacking faults is reported to result in a magnetic softening of the grain assemblage. Annealing at 300–350 °C removes most of the lepidocrocite and the second maghemite type, type B, becomes prominent. Haematite formation sets in at slightly higher temperatures, yet the type B maghemite is in part thermally stable up to 600 °C enabling Thellier–Thellier experiments. This stability is also inferred from Arrhenius fitting that shows a comparatively high activation energy for the maghemite to haematite reaction. In Thellier–Thellier experiments the CRM showed a markedly downward convex Arai–Nagata plot while a second thermoremanent magnetization (TRM) showed perfect linear behaviour as expected. This feature may be used to recognize CRM in natural rocks.

SUMMARY A number of recent studies have applied the cumulative log Gaussian (CLG) modelling tech- nique to bulk isothermal remanent magnetization (IRM) curves to investigate the coercivity contributions of different minerals contained within a natural sample. Here, we present a series of Preisach-Neel models used to investigate how robust the assumption of fitting lognormal coercivity distributions is in the presence of local interaction fields and thermal relaxation. Our models indicate that the starting state of the magnetic system, magnetic interaction and thermal relaxation have a strong influence on the form of IRM curve, meaning that in a sub- stantial number of cases the lognormal assumption fails and the CLG procedure can produce misleading interpretations. In some cases the failure of the CLG assumptions produces models which introduce spurious additional coercivity components into the fitted curve.

SUMMARY An earlier study of the cyclic Miocene red bed sequence of La Gloria (Spain) by Krijgsman et al. indicated complicated behaviour of the natural remanent magnetization (NRM) in parts of the section, particularly close to reversal boundaries. We resampled part of the section with high resolution and used extensive rock-magnetic analyses to characterize the magnetic remanence carriers. Below a conspicuous hydromorphous layer, the magnetic contributions of haematite and magnetite are approximately equal, while in the brown layers on top of the hydromorphous layer the contribution of magnetite is drastically reduced. This is probably related to a change in hydrological conditions. The NRM is characterized by: (1) a low-temperature (200‐360 ◦ C) overprint of unknown age, (2) a medium-temperature (360‐580 ◦ C) component, interpreted as the characteristic remanent magnetization (ChRM); and (3) a high-temperature haematite component (600‐ 680 ◦ C), carrying the same directions as the medium-temperature component. There is no perceivable delay in NRM acquisition between the medium- and the high-temperature components; they are both regarded as primary. The behaviour of the NRM seems to be similar in the cyclic lithologies. The acquisition of NRM thus seems to be independent of lithology in the cyclic part of the section. The higher sampling resolution yielded the detection of a new polarity zone, which probably represents a geomagnetic feature according to rock-magnetic properties. However, the more detailed magnetostratigraphy of the resampled part of the section indicates that the earlier correlation to the geomagnetic polarity timescale is no longer tenable. Therefore, the La Gloria section should no longer be used as a magnetostratigraphic dating tool of mammal biochronology.

SUMMARY Chemical extraction techniques are being introduced into environmental magnetism studies to aid in the interpretation of magnetic climate proxies. Previous studies have shown that the acid-ammonium-oxalate/ferrous-iron (AAO‐Fe 2+ ) extraction technique can be used to selectively dissolve very fine-grained magnetite and maghemite from synthetic samples. Here, we present the results of a study of this extraction technique to serve as a tool for selective dissolution of pedogenic magnetic minerals from a loess‐palaeosol transect. Before and after extraction, the samples were subjected to classic mineral‐magnetic methods (measurement of low-field susceptibility and hysteresis parameters), as well as first-order reversal-curve analysis. In addition, acquisition curves of the isothermal remnant magnetization (IRM) were fitted with logarithmic distributions. These analyses showed the magnetic dominance of lowcoercivity magnetic minerals. By subtracting the IRM remaining after extraction from that before extraction, the magnetic fraction that was dissolved could be characterized as well, supplying extra information concerning the performance of the extraction technique. The AAO‐Fe 2+ method successfully dissolved the superparamagnetic and part of the singledomain material from the palaeosol samples in one extraction step, repeating the extraction resulted in hardly any further changes to the magnetic content of the samples. The magnetic characteristics of the loess samples remained stable throughout the extraction experiment. The combination of the AAO‐Fe 2+ extraction with mineral‐magnetic analysis has successfully identified the pedogenic contribution in our samples. Therefore, variations in the lithogenic fraction potentially present in pedogenically enhanced intervals can be assessed, improving the merit of mineral‐magnetic climate proxy parameters.

SUMMARY The acquisition of isothermal remanent magnetization (IRM) curves through the application of stepwise-increasing uniaxial fields to a rock-magnetic sample provides an important non-destructive tool for the investigation of coercivity spectra (Dunlop

SUMMARY It has been realized previously (e.g. Borradaile 1994) that cycling through the Morin transition (Tm, occurring in ideal a-Fe2O3 at x10 uC) may have implications for the NRM of some haematite-bearing rocks. We investigate the behaviour of the low-field susceptibility (xlf), several magnetizations (in fields of 5, 25, 100 and 1600 mT) and SIRM on cycling through Tm of several well-characterized haematite types of varying crystallinity and particle shape. Before low-temperature treatment, xlf of the haematites varied between y40 and y235r10 x8 m 3 kg x1 . Below Tm, where only haematite’s defect moment resides, xlf was much more uniform at y19 to y28r10 x8 m 3 kg x1 . After return to room temperature, increases in xlf of up to y50 per cent were observed (when cycling in the Earth’s magnetic field as well as in a field-free space), inferred to be a function of the domain state of the haematite. This was shown for one of the haematites (LH2 which occurs as small platelets and is particularly well crystalline) where a relation y=(8.60t1.01) ln(x)x2.98 was obtained, where x is the grain size (mm) and y is the percentage susceptibility increase. We suggest that transdomain changes induce the change in xlf. The nucleation of (additional) domain walls in ‘metastable’ single-domain (SD) to pseudo-single-domain (PSD) grains is made possible by the low anisotropy at the Morin transition. In view of this mechanism, small stable SD haematite particles would not be affected and the grain size corresponding to y= 0( y1.5 m mf or LH2) would represent the ‘real’ SD threshold size. Thermal cycling to over the Curie temperature (680 uC) is needed to return to the original domain state before the LT treatment, as expressed by a return to the original xlf values. Measuring xlf between alternating field (AF) demagnetization steps shows that AF demagnetization gradually removes the xlf increase, which appears to be soft; 30 mT is sufficient to erase 90 per cent. Thermal cycling in a 5 mT field between temperatures above Tm showed that irreversible changes in domain structure are noticeable before the isotropic point is passed. After cycling, magnetization is added to PSD and multidomain (MD) grains that intriguingly appears to be remanence, probably induced by the broadening and subsequent irreversible displacement of loosely pinned domain walls. Complete cycling through the isotropic point considerably enhances the new remanence component in ‘metastable’ SD to MD particles by an increase in the number of domains. If this behaviour can be extrapolated to the intensity of the Earth’s magnetic field, this would imply that large ‘metastable’ SD to MD specularite crystals with a well-developed Morin transition are susceptible to acquiring geologically irrelevant remanence components when subjected to low ambient temperatures. Fine-grained haematite pigment, on the other hand, would not be affected. Thermal demagnetization alone would not be able to separate these two remanences as the new domain structure persists up to close to the Curie temperature. Our findings indicate that a cleaning procedure consisting of an initial AF step followed by stepwise thermal demagnetization is preferable in order to isolate the original remanence component properly in haematite-bearing rocks.

SUMMARY In soil magnetism, the magnetic parameters alone are not always sufficient to distinguish the lithogenic from the pedogenic magnetic fractions. Sequential extraction techniques have therefore been incorporated into magnetic studies to constrain the environmental interpretation. Here we report on the dissolution behaviour of magnetite and maghemite in the acid‐ammonium oxalate method to see whether the method is suitable for specific dissolution of magnetic minerals from soils and sediments. To prevent changes in the extraction mechanism during the experiments (see Appendix A), we used an adapted version of the acid‐ammonium oxalate (AAO) method, in which Fe 2+ is added to the extraction solution prior to the experiment [the AAO-Fe(II) method]. The procedure was divided into several 30 min extraction steps to check the dissolution progress. Synthetic samples containing a quartz matrix with 0.1 wt per cent of iron oxides were extracted with the AAO-Fe(II) method. The iron oxides consisted of either magnetite or maghemite with grain sizes of

The formation of traces of a magnetic phase with a Curie point of 470–475 °C is detected during routine thermomagnetic analysis of various haematite types without significant isomorphous substitution. Using heating and cooling rates of 10° min⁻¹, the formation temperature can be as low as 400 °C for synthetic haematite samples, whereas higher temperatures, 700–800 °C, are required for natural samples. The new phase appears to be persistent to prolonged heating at 1000 °C and has a cubic spinel structure with a unit cell length a₀ = 0.8350 ± 0.0005 nm, similar to pure maghemite. This suggests that the reverse reaction of the γ-Fe₂O₃→α-Fe₂O₃ transformation can occur under appropriate conditions. The low Tc of this particular maghemite variety suggests that the vacancy (and/or cation) ordering over the magnetic sublattices is different from usually occurring maghemite. In accordance with Takei & Chiba (1966), who also reported a pure maghemite variety with identical Tc, a cation-deficient spinel structure with part of the vacancies on tetrahedral sites is suggested. Thermally activated release of incorporated hydroxyl groups would trigger the formation of maghemite traces on the surface of well-crystalline haematite planes. Citrate–bicarbonate–dithionite extraction results support the idea that the maghemite is fine-grained and surficial on the haematite because low-field susceptibility values decrease according to the behaviour of fine-grained maghemite particles. The formation of traces of this highly magnetic mineral during routine stepwise thermal demagnetization or during annealing haematite at high temperatures may seriously affect NRM measurements or may be erroneously taken as haematite's defect moment.

SUMMARY The magnetic behaviour of hydrothermally synthesized greigite was analysed in the temperature range from 4 K to 700°C. Below room temperature, hysteresis parameters were determined as a function of temperature, with emphasis on the temperature range below 50 K. Saturation magnetization and initial susceptibility were studied above room temperature, along with X-ray diVraction analysis of material heated to various temperatures. The magnetic behaviour of synthetic greigite on heating is determined by chemical alteration rather than by magnetic unblocking. Heating in air yields more discriminative behaviour than heating in argon. When heated in air, the amount of oxygen available for reaction with greigite determines the products and magnetic behaviour. In systems open to contact with air, haematite is the final reaction product. When the contact with air is restricted, magnetite is the final reaction product. When air is excluded, pyrrhotite and magnetite are the final reaction products; the amount of magnetite formed is determined by the purity of the starting greigite and the degree of its surficial oxidation. The saturation magnetization of synthetic greigite is virtually independent of temperature from room temperature down to 4 K. The saturation remanent magnetization increases slowly by 20‐30 per cent on cooling from room temperature to 4 K. A broad maximum is observed at ~10 K which may be diagnostic of greigite. The coercive and remanent coercive force both increase smoothly with decreasing temperature to 4 K. The coercive force increases from ~50 mT at room temperature to approximately 100‐120 mT at 4 K, and the remanent coercive force increases from approximately 50‐80 mT at room temperature to approximately 110‐180 mT at 4 K.

SUMMARY When interpreting thermomagnetic curves of non-saturated magnetic minerals, irreversible heating and cooling curves need not necessarily imply chemical or structural changes. Increased aligning of magnetic moments on heating in an applied magnetic field can also induce an irreversible cooling curve. The two processes can be distinguished by stirring the sample between subsequent thermomagnetic runs. Sample redispersion considerably enhances the interpretative value of thermomagnetic analysis and is therefore strongly recommended, in particular when analysing non-saturated magnetic minerals. Stirring between subsequent runs was extensively used in the analysis of the thermomagnetic behaviour of haematite and goethite as a function of grain size (i.e. coercivity) in various non-saturating magnetic fields (10‐350 mT). The shape of the thermomagnetic heating curves of haematite is shown to be dependent on the competitive interplay between the temperature dependence of the exchange energy and that of the coercive force with respect to the applied field. On heating, pure defectpoor haematite, which is magnetically dominated by the canted moment, has an initially increasing thermomagnetic heating curve. Further heating causes the magnetization to increase smoothly up to a certain temperature which depends critically on the applied field and the coercivity of the sample. The irreversible block-shaped thermomagnetic cooling curve lies above the heating curve, and shows hardly any dependence on applied field and grain size. In contrast to the heating curve, the shape of the cooling curve depends only on the temperature variation of the exchange energy. Our data seem to indicate that for defect-poor haematites the domain configuration acquired at the maximum heating temperature is retained on cooling to room temperature. More defect-rich haematite has a gently decreasing thermomagnetic heating curve. On heating to increasingly elevated temperatures (800°C) the defects are annealed out oV the lattice, because the thermomagnetic curves approach those of defect-poor haematite. The defect moment due to lattice defects seems to be additive to, but softer than, the

SUMMARY Greigite of variable purity has been synthesized hydrothermally at - 140 "C by injecting a Mohr's salt solution into a sodium sulphide solution containing dissolved zerovalent sulphur, ageing for 30-960 min and rapidly quenching. Oxygen was rigorously excluded during the synthesis. Scanning electron microscope (SEM) observations yield a grainsize range up to 400nm. The variable purity was deduced from a varying specific saturation magnetization (3-29 Am2 kg-I). The MJM, or ors/os ratio varies between 0.3 and 0.4, which is indicative of fine-grained greigite, concurring with SEM observations. Hysteresis measurements at low temperatures indicate the presence of grains that are superparamagnetic at room temperature. Coercivity parameters [(B,), 13-43 mT; (Bo)cr 37-73 mT] are the highest obtained to date for synthetic greigite and compare well with those measured for natural greigite. Ranges for other rock magnetic parameters are discussed. Greigite hysteresis parameters overlap those of magnetite to a large degree. A diagnostic interparametric ratio for greigite is its high o,,/xin ratio; the range of the present greigite is 40-75 kAm-'.

SUMMARY 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 B,,, and B,,,, the output signal is amenable to Fourier analysis. Partial Fourier analysis yields the fundamental harmonic and the second harmonic, termed SIG, and SIGz respectively. These are related to the saturation magnetization (Ms) by M, = (2 SIG, 8 SIG2 [(Bmax + Bmin)/(Bmax - Bm,,,)]}/[A”(Bmd x - Bm,,,)] and to the paramagnetic susceptibility (xpdr) by xpar = 8 SIGz/[A”(B,,, - B,,,J2], 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 SIG, and SIGz. After the experiment, SIG, and SIG, 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 x lop9 Am2, making it a very powerful tool for thermomagnetic analysis of weakly magnetic material. Examples demonstrating this potential of the device are shown.

Summary 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.

SUMMARY Grain-size separation of the

SUMMARY 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 C02/H20 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 11.2MAm-' or 14T), 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

SUMMARY 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

SUMMARY Five goethite concentrates, separated from natural samples of different supergene geological origin, are sieved in 12 well-defined grain-size fractions ranging from 250 pm down to

Summary 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