Diagenesis of ferriferous phases in the Northampton ironstone in the Cowthick quarry near Corby (England)

Berthierine, siderite and pyrite are the major ferriferous phases in the Northampton ironstone (NIS). Mineralogical and chemical data suggest a formation of these phases in a diagenetic marine environment changing from post-oxic to sulphidic conditions. Berthierine was formed first when the Fe2+ activity in the diagenetic system increased. Later, this phase was partially replaced by siderite and/or pyrite. A second stage of the diagenetic development in the NIS with increasing CO2 partial pressure (PCO2 ) is documented by siderite. The isotopic composition (δ18O mean value: -1.7‰PDB; δ13C mean value: -8.6‰PDB) points to siderite precipitation from a marine porewater environment with a microbial CO2 source. The shift from post-oxic to sulphidic conditions is indicated by the occurrence of pyrite and can be considered as a final stage. The diagenetic processes in the marine environment and the formation of the ferriferous phases were stopped by the influx of brackish or fresh water when the Midland Shelf turned estuarin

EPR evidence for maghemitization of magnetite in a tropical soil

Electron paramagnetic spectroscopy (EPR) was used in combination with standard rock magnetic methods to study magnetic minerals in a tropical soil. The susceptibility and hysteresis measurements showed magnetite grains with a Curie temperature near 850 K as the dominant magnetic remanence carriers in the soil. A minor Ti content in the magnetite was found by energy dispersive X-ray analysis. In order to get insight into the weathering status of the magnetite, different chemical treatments, including oxalate and citrate—bicarbonate—dithionite (CBD) extraction, were applied to the soil samples. The hysteretic properties exhibited no significant differences between the untreated and the CBD or oxalate treated samples. By contrast, the comparison of the EPR spectra revealed a significant broadening of the linewidth (δB) and a shift of the g-values (geff) to lower fields after the CBD treatment. Furthermore, the spectral parameters geff and δB exhibited an angular dependence. At low temperature, the CBD treated samples showed a jump in δB between 120 and 100 K, the temperature range characteristic for the Verwey transition in magnetite. The changes in the spectral properties after the CBD treatment, which dissolves ferric oxides, were attributed to the removal of maghemite formed by the oxidation of magnetite, that is, during the maghemitization of the magnetite grain

Fe-Ti-O exchange at high temperature and thermal hysteresis

In this study, the Fe-Ti-O exchange behaviour between the systems hemo-ilmenite (y)FeTiO3-(1 −y)Fe2O3 and titano-magnetite (x)Fe2TiO4-(1 −x)Fe3O4 was investigated in the temperature range from 900 to 1400 K in an inert Ar atmosphere. Starting from a mixture of hematite and ilmenite with a fixed mol per cent, heat treatment generates a self-adjusting chemical equilibrium between hemo-ilmenite and titano-magnetite solid solution by means of interdiffusion and Fe3+→ Fe2+ reduction. Structural and magnetic characterization reveals that hemo-ilmenite is stable at all temperatures, whereas titano-magnetite shows increasing Ti-content with increasing treatment temperature. Heating-cooling cycles were performed for a sample to mimic slow cooling and study its effects on the two solid solutions. The magnetic properties of that sample exhibit thermal hysteresis during these cycles, as the Ti departs from titano-magnetite and thus leads to a new chemical equilibrium. The experimental data provide insight into the dynamics of the formation of Fe-Ti-O phases formed under varying conditions in geological system

Magnetic anisotropy and Verwey transition of magnetosome chains in Magnetospirillum gryphiswaldense

Magnetotactic bacteria (MTB) are characterized by cellular magnetic dipoles formed by the 1-D assembly of magnetite and/or greigite particles aligned along their magnetic easy axes. This alignment creates strong interaction-induced shape anisotropy. Ferromagnetic resonance (FMR) spectroscopy is applied to study the changes in anisotropy of the MTB Magnetospirillum gryphiswaldense between room temperature and 10 K. The Verwey transition is found at about 100 K. The characteristic FMR signal of the cellular dipole at room temperature vanishes upon cooling to the isotropic point at Ti ≈ 130 K, where the magnetocrystalline anisotropy constant K1 becomes zero. Monitoring of the FMR response of intact MTB as a function of temperature is taken to discuss theoretically the reduction of the interaction-induced shape anisotropy in magnetofossils because of diagenetic processes. It is concluded that there is a similarity in the FMR response between magnetofossils at room temperature and intact MTB near Ti. This is because the critical effect of the magnetocrystalline anisotropy constant K1 and of the alignment of magnetic easy axes on the cellular dipole. Low-temperature FMR results of intact MTB can thus be used as a guideline for detecting magnetofossils in geological environment

The dynamics of magnetic ordering in a natural hemo-ilmenite solid solution

We investigated the micromagnetic properties of hemo-ilmenite particles in an alluvial soil. All magnetic accessory minerals except the weathering resistant hemo-ilmenite grains were removed from the soil matrix by chemical treatment with concentrated acid followed by magnetic separation. X-ray diffraction revealed hemo-ilmenite grains with single crystal properties and an ilmenite mole fraction of y = 0.86. Magnetization versus applied magnetic field plots in a temperature range between 6 and 300 K were recorded in order to study the hysteresis and the exchange properties. In addition, field and frequency-dependent AC susceptibility measurements were performed with and without a DC bias field in order to analyse the dynamic magnetization of the sample down to 3 K. The hemo-ilmenite particles are considered as a mixed system with nano-sized cation-ordered areas (COA) and cation-disordered areas (CDA), which differ in their local Fe(III) concentration. Ferrimagnetic single-domain order in the Fe(III)-enriched CDA started at about 220 K. Upon cooling gradual transdomain transformation generates multidomain order. A maximum in the blocking distribution was reached at 44 K, followed by the onset of spin-glass dynamics. At lower temperature, blocking of superparamagnetic clusters in the COA created antiferromagnetic (AFM) ordering, which became more prominent with decreasing temperature. The interaction between the spin-glass like CDA and the AFM areas was documented by the onset of exchange bias at T < 20 K. The occurrence of exchange bias as well as spin-glass dynamics in the hemo-ilmenite grains is probably an effect of the inhomogeneity of the local Fe(III) concentration. This effect leaves a magnetically competitive regime with areas showing ilmenite-like magnetic properties, and Fe(III) rich disordered areas with magnetic long-range ordering up to 220 K and frustration near the ordering temperature of ilmenit

High temperature stability of natural maghemite: a magnetic and spectroscopic study

A combined magneto-mineralogical approach is used to diagnose maghemitization in magnetic grains of basaltic rock fragments from sand dunes in the Namibian desert in SW Africa. Data were obtained from static magnetic analysis, ferromagnetic resonance (FMR) spectroscopy, micro-Raman spectroscopy and electron microscopy. Micro-Raman spectroscopy showed that the magnetic grains in the lithic fragments form oxidative solid solution series with magnetite and maghemite as end-members. The five active Raman modes at 712, 665, 507, 380 and 344 cm−1 indicate that maghemite in the magnetic grains has well-defined structural properties. The FMR spectral analysis provides evidence for long-range dipolar coupling, which suggests intergrowth of the magnetic phases of the oxidative solid solution series. Thermomagnetic experiments and hysteresis measurements reveal a Curie temperature of about 890 K for this maghemite. Upon heating to 970 K part of the maghemite is altered to thermodynamically more stable hematite. After selective thermal decomposition of the maghemite in a protected atmosphere, the remaining magnetic phase has a Curie temperature of 850 K, characteristic for magnetite. The unique thermal stability of this natural maghemite above its Curie temperature is explained by the well-defined mineral structure, which formed during slow oxidative alteration of magnetite under arid climate condition

Rock magnetic techniques complemented by ferromagnetic resonance spectroscopy to analyse a sediment record

Environmental magnetism uses the spatial and temporal occurrence of magnetic carriers as diagnostic tools to detect environmental changes. Concentration, composition, grain size and configuration of the carriers inferred from magnetic properties are key parameters, because they are indicative of the formation conditions of magnetic phases, and/or of processes such as diagenesis and weathering. We present a detailed ferromagnetic resonance (FMR) spectroscopy analysis in concert with routinely used rock magnetic measurements to determine these parameters in a sediment record that documents the development of Lake Soppensee (Central Switzerland) since latest Pleistocene. FMR spectroscopy monitors varying concentration of the predominant magnetite/maghemite by the spectral signal intensity, whereas the stable single domain and superparamagnetic states are determined by the signal shape at room and low temperature. Fitting and simulation of FMR spectra are successfully applied to samples with well-defined magnetite components in the sediment matrix. Clear evidence for the colonization of magnetotactic bacteria (MTB) in Lake Soppensee was possible by applying empirical spectral separation to measured FMR signals that yield two magnetite populations differing in their configuration, that is, dispersed and aligned in chains. Low temperature measurements showed that these MTB can be preserved as pure or oxidized magnetite. The FMR data set confirms and completes rock magnetic information obtained from the lacustrine sedimentary record. The advanced application of FMR spectroscopy in the presented study critically highlights the benefit of this rapid and non-destructive method for future analysis of magnetic properties in environmental studie

The Besnus transition in 4C pyrrhotite revisited

Ferrimagnetic, monoclinic 4C pyrrhotite (Fe7S8) is the only iron sulphide with high relevance for palaeomagnetism and rock magnetism that can be identified in rock materials by its characteristic low-temperature anomaly. Despite its relevance in natural magnetism and the many magnetic studies over the last decades, the physics and the crystallography behind this anomaly, also denoted Besnus transition, is a matter of debate. In this study, we analyse the static and dynamic magnetization associated with the Besnus transition in conjunction with low-temperature structural data of 4C pyrrhotite reported in the literature. The correlation between the Fe-Fe bonds causing spin-orbit coupling and the dynamic magnetic properties show that the magnetic characteristics of the Besnus transition stem from the interaction of two magnetocrystalline anisotropy systems triggered by thermally induced structural changes on an atomic level in monoclinic 4C pyrrhotite. This refutes the widespread view that the Besnus transition is caused by a crystallographic change from monoclinic to triclinic.ISSN:0956-540XISSN:1365-246