Micromagnetic Modeling of Framboidal Grains of Greigite

Greigite (Fe3S4) is a strongly magnetic mineral, which has been increasingly recognized as an important contributor to the paleomagnetic record. It can remain chemically stable over geological timescales, and thus has the possibility to be a dominant recorder of the geomagnetic field, particularly in reducing diagenetic environments. Greigite often forms as closely packed clusters of grains, creating framboidal structures that can be several microns in size. Despite the increasing attention that greigite has received in environmental and peleomagnetic studies in recent years, there has not yet been a detailed study of its fundamental magnetic properties and it’s paleomagentic recording fidelity. We will report the first attempt at building a predictive numerical model to examine the magnetic properties of framboidal aggregates of cubo octahedral greigites. A finite element/boundary element micromagnetic model is used to examine realistic geometries of clusters nano-scale greigite crystallites which are touching or near-touching. In this initial study each crystallite is chosen to be within the single-domain grain size range, although significant inhomogenuties are still able to nucleate as super-domain structures are formed across several crystallites within the framboid. The recording fidelity of greigite is determined in terms of its magnetic remanence and simulated hysteresis properties. The magnetic stability of the system stems the balance of exchange and magnetostatic interactions between neighboring grains, and is seen to be largely independent of the number of grains within the framboid.