Microaggregation of goethite and illite evaluated by mechanistic modeling

Abstract Clay minerals and Fe- and Al-oxyhydroxides are considered as the most important materials for the formation of microaggregate building units in soils. The mode of aggregation and the resulting structure and stability are largely determined by their size, aspect ratio (AR), differences in surface charge (SC), and mixing ratio. Here, the impact of these parameters on the formation of microaggregate building units (BUs) was elucidated with a mechanistic model, i.e. using a twophase cellular automaton method, in which prototypes of illite and goethite were implemented. The simulations allowed assessing the formation of particle arrangements during aggregation, moreover building unit diameters, and the number of discrete particles. The contact area between particles was evaluated as a quantitative measure for stability. Homoaggregation of illite under the sole influence of attracting van der Waals forces enabling face-to-face as well as edge-to-face contacts, resulted in compact structures with a small mean diameter and specific surface area. In contrast, electrostatic attraction between negatively charged faces and positively charged edges favored formation of larger card-house structures, where low contact areas indicated the low stability of the resulting BUs. At constant AR, homoaggregation of illite of different particle size led to BUs of different stability, more precisely smaller particle sizes led to more stable BUs. Variation of the AR of illite, while keeping the particle size constant, revealed that most stable BUs arose from illite with smallest AR. Shielding of coarse goethite particles due to attachment of fine illite with negatively charged edges increased the number of discrete fine illite particles not aggregated. Aggregation of illite with positively charged edges was in fundamental difference to results from simulations with negative edges because of electrostatic attraction between illite particles. The simulation setting, without using fitting parameters resulted in an appropriate approximation of laboratory findings. The simulation clearly facilitated the understanding of microaggregate building unit formation and stability by illuminating turnover dynamics and provision of quantitative data.