Minimum Grain Size Limit for Paleomagnetically Stable Goethite

Abstract

Goethite can be a major magnetic mineral in several sediment types on Earth, for example, soils, and speleothems, while it is also reported on Mars. It is important in paleomagnetic and paleoclimatic studies. However, its minimum grain size limit for paleomagnetic stability, the superparamagnetic to single domain (SP/SD) threshold size, is poorly constrained. Here, we investigate the magnetic properties of unsubstituted synthetic goethite samples with variable grain size. Goethite is often acicular, which makes grain length an important variable. With decreasing goethite rod length, the remanent coercive force and maximum blocking temperature (Tb) decrease gradually; Tb drops abruptly from 400 K to 300 K at grain lengths of ∼100–120 nm and widths of ∼30–40 nm. This defines the SP/SD threshold size for goethite at room temperature and demonstrates that rod width and length are both important for defining the SP/SD threshold size. This means that substantial goethite fractions in natural samples will likely have SP properties and that the exceptionally hard coercivities associated with SD goethite only apply to grains larger than this threshold size. The large SP/SD threshold size for goethite is important for goethite quantification and paleomagnetic studies.