The origin of loess microstructure: 3D insights from initial aeolian dust packing

• The initial loess deposit has an extremely loose particle packing structure. • The loose packing governs by strong inter-particle forces during aeolian deposition. • Initially loose aggregates and overhead pores crucially shaped today’s porous loess. Aeolian deposition is fundamental to the formation of the porous structure of present-day loess. However, the true initial packing structure of loess dust remains poorly understood. In this study, we reconstructed initial loess deposits by simulating the dust deposition process, using Malan loess, which is compositionally analogous to ancient dust, as the raw material. The microstructure of the simulated deposits was characterised using micro-computed tomography (μ-CT) scanning and compared with that of natural Malan loess. The results indicate that the initial loess deposits possessed an exceptionally loose particle packing, with void ratios ranging from 2.79 to 3.75. Loose clay or silt–clay aggregates formed extensively due to inter-particle forces (such as van der Waals force and electrostatic attraction) during deposition. These aggregates, along with isolated detrital particles, acted as the primary skeletal components. Surface clays on skeletal particles play a critical role in bonding adjacent particles, which is essential for establishing and stabilising the loose framework. Notably, the initial deposits with an intermediate clay content (23%) among the samples studied exhibited the loosest microstructure, featuring chain-like particle packing and abundant large overhead pores. The initially loose aggregates and open packings compact over geological time while retaining structural inheritance, resulting in the reduced but partially preserved pore space that shapes the microstructure of present-day loess. This study provides the first direct 3D visualisation and quantification of the initial particle-scale microstructure, offering a key reference for understanding subsequent post-depositional processes and associated geomechanical properties.