Magnetic properties of natural greigite flower-like microspheres
Abstract
Greigite (Fe3S4) is a ferrimagnetic mineral widespread in sedimentary environments, where it is a precursor of pyrite, especially of pyrite framboids. A critical question is, if spherical greigite can form a template for the framboids? Recent experimental studies showed that organo-sulfur compounds can critically affect the precipitation of sulphur minerals and their morphology (e.g., the formation of microspheres).
Here we present a magnetic study of the Lower Lake Williams tailings in Ontario Canada, where pyrite framboids were found within a marsh area. X-ray diffraction and scanning electron microscopy of magnetic separates revealed that the framboids occur associated with flower-like microspheres formed by inter-grown flakes of greigite with sizes of a few hundreds of nanometers that were precipitated on root debris. For magnetically separated samples, the magnetic hysteresis parameters Bc= 33.0 mT and Mr/Ms= 0.37 were found for the greigite. The FORC diagram displays complex features with multiple coercivity clusters that suggest inhomogeneous spin rotation. Such features were reported from micromagnetic simulations of FORC diagrams for greigite in a vortex-like state. The AC susceptibility (χ) versus temperature, however, shows neither frequency nor amplitude dependence, and this in turn rules out the contribution of multi-domain or super-paramagnetic particles to the FORC diagram. Moreover, χ(T) exhibits a continuous decrease down to about 60 K followed by a slightly enhanced decline down to 10 K. For the greigite sample, the ferromagnetic resonance spectra between 300 and 10 K can be fit with one Gaussian, and this in turn suggests a relatively homogeneous grain-size population. The resonance field of the spectra decreases upon cooling and reveals a minor discontinuity at about 60 K similar to χ. This discontinuity is also reflected in a more pronounced increase of Bc, whereas Mr/Msvaries only little and shows a ratio of 0.43 at 10 K. In summary, the greigite microspheres are formed by inter-grown flakes that can act as templates for pyrite framboids. The flakes are in a single-domain magnetic state that is associated with magnetic vortices. The results of this research can open a door to study the effect of the morphology on the magnetic properties of greigite formed in natural environments.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP31B0732L
- Keywords:
-
- 0419 Biomineralization;
- BIOGEOSCIENCES;
- 1512 Environmental magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1522 Paleomagnetic secular variation;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1540 Rock and mineral magnetism;
- GEOMAGNETISM AND PALEOMAGNETISM