Andreas U. Gehring

INHIBITION OF SINTERING BY Si DURING THE CONVERSION OF Si-RICH FERRIHYDRITE TO HEMATITE

The distribution and chemical state of Si in a synthetic 2-line ferrihydrite with a Si/(Si + Fe) molar ratio of 0.11 was studied. Heat treatment under oxidizing conditions shows that Si-rich ferrihydrite is stable to 400°C. The transformation to hematite and the formation of a polymerized amorphous-silica phase occur at 850°C. At this temperature, the specific surface area decreases greatly and the average pore diameter increases, which is indicative of sintering. Heating under severe reducing conditions causes a segregation of Si from Fe and results in a mixture of elemental Fe and SiO2. Surface and structural data suggest that Si is located near the particle surface where it limits the rearrangement of Fe octahedra to form hematite.

Timing of natural remanent magnetization in ferriferous limestones from the Swiss Jura Mountains

Abstract Goethite and hematite are the main magnetic phases contained in two ferriferous middle Jurassic beds which are exposed within the Mandach structure, the major tectonic structure of the eastern tabular Jura of northern Switzerland. The iron oxidic phases in a reddish brown sparry limestone known as Spatkalk were formed diagenetically, whereas these phases are weathering products in a stratigraphically slightly younger oolitic bed. The hematite in the oolitic bed was magnetized after the Jura main tectonic event in the Miocene whereas the hematite in the Spatkalk was magnetized before the Miocene. The goethites in the oolitic bed carry a pretectonic and in the Spatkalk a syntectonic magnetization. Physical and chemical data show that the goethite which was resistant to remagnetization contains an Al-substitution of 3.4 mol% resulting in a Neel temperature of 90°C. The remagnetized goethite has an Al-content of 10.7 mol% and a Neel temperature of 70°C. The different goethite properties indicate that the tectonic formation of the Mandach structure occurred under low-temperature conditions below 90°C.

Evolution of magnetic anisotropy and thermal stability during nanocrystal-chain growth

We compare measurements and simulations of ferromagnetic resonance spectra of magnetite nanocrystal-chains at different growth-stages. By fitting the spectra, we extracted the cubic magnetocrystalline anisotropy field and the uniaxial dipole field at each stage. During the growth of the nanoparticle-chain assembly, the magnetocrystalline anisotropy grows linearly with increasing particle diameter. Above a threshold average diameter of D ≈ 23 nm, a dipole field is generated, which then increases with particle size and the ensemble becomes thermally stable. These findings demonstrate the anisotropy evolution on going from nano to mesoscopic scales and the dominance of dipole fields over crystalline fields in one-dimensional assemblies.

Paleomagnetism and tectonics of the Jura arcuate mountain belt in France and Switzerland

Abstract Goethite and hematite in ferriferous oolitic beds of Callovian age from the Jura mountains (Switzerland, France) carry either pre- and/or post-tectonic magnetization. The frequent pre-tectonic origin of goethite magnetization indicates a temperature range during formation of the arcuate Jura mountain belt below the goethite Neel temperature of about 100°C. The scatter of the pre-tectonic paleomagnetic directions (D = 11.5°E, I = 55.5°; α95 = 4.7) which reside both in goethite and hematite, provides strong evidence that the arcuate mountain belt was shaped without significant rotation. The paleomagnetic results support tectonic thin-skinned models for the formation of the Jura mountain belt.

Magnetic Decoupling of Surface Fe3+ in Magnetite Nanoparticles upon Nitrocatechol-Anchored Dispersant Binding

Bind and distort: NitroDOPA, an anchor shown to irreversibly couple dispersants and functional groups to superparamagnetic Fe 3O 4 nanoparticles, can magnetically decouple Fe 3+ in a nanoparticle surface layer. The strong binding of nitroDOPA distorts the Fe 3+ ligand field. This ESR finding sheds light on an important binding mechanism of anchors used for nanoparticle surface modification (see graphic). Copyright

The fate of structure-bound Mn2+ during the decomposition of dolomite and in the resulting conversion products: An EPR study

Abstract Dolomite from the Jhamarkotra phosphate mine (India) contains 680 ppm Mn2+ as indicated by electron-spin resonance spectroscopy (EPR). Dipolar broadening of the EPR signal prevents a quantitative assignment of Mn2+ to the Ca and Mg sites. Upon heating to 700 °C, over 99% of the dolomite is decomposed and all the Mn2+ is released from the dolomite structure. Approximately 95% of the original Mn2+ is oxidized and forms Mn-oxide. The remaining Mn2+ preferentially migrates into CaO. This behavior is interpreted by the Goldschmidt rule stating that smaller ions are taken up at the sites of larger ions. During subsequent hydration of CaO into Ca(OH)2, the Mn2+ remains stable. The protection of Mn2+ against oxidation is explained by the topotaxical alteration of the host minerals. Carbonation of the Ca(OH)2 leads to Mn2+ oxidation caused by the dissolution of the hydroxide prior to the formation of CaCO3. Divalent Mn monitored by EPR can be used to unravel the behavior of trace elements on the molecular level during mineral alteration