Rock magnetic and microstructural investigation of Fe-bearing coatings on sand grains from the Namib Sand Sea

The Namib Desert (Namibia) hosts giant dune fields whose sand color ranges from yellow to deep red. While provenance and grain composition contribute to these variations, a major control is the occurrence of Fe-bearing coatings (“varnish”) made of iron oxides and oxyhydroxides (e.g., magnetite, hematite, goethite) that surround sand grains. In this study, we analyze a set of 10 samples collected across various environments within a ~600 km perimeter in the northern Namib Desert. The mineralogical and textural nature of these coatings was investigated using a rock-magnetic approach. We combine bulk magnetic measurements, including magnetic susceptibility, hysteresis parameters, IRM acquisition, and thermomagnetic experiments, to identify the dominant magnetic carriers and assess the relative contributions of ferrimagnetic versus antiferromagnetic phases. These data are coupled with complementary mineralogical analyses (optical and scanning microscopy, Raman spectroscopy). Preliminary observations suggest that the magnetic phases are embedded in a clay-rich matrix and may be associated with microbial aggregates, raising questions about their origin and formation pathways. Furthermore, dune, terrace, and riverbed samples display distinct magnetic signatures, indicating the role of transport and/or in situ processes. By linking magnetic signatures to colorimetric variability and microstructural observations, this study aims to evaluate sand color as a potential environmental proxy for sediment transport pathways, weathering conditions, and hydroclimatic controls on iron oxide formation in arid environments.