Collaborative impact of complex refractive indices and particle sizes on the reflectance spectra of minerals

The influence of particle size on the spectral characteristics of minerals has received significant attention. However, the significant impact of mineral complex refractive indices on the spectral characteristics of granular minerals has been overlooked. This study investigates the visible-near infrared spectra of quartz, hematite, and magnetite minerals at various particle sizes to explore the mechanisms and influence patterns of particle size on the spectral characteristics of minerals with different complex refractive indices. The results indicate that particle size has a significant impact on the spectral characteristics of minerals with different complex refractive indices. These effects are primarily associated with shadow effects, volume scattering, surface scattering, and aggregation effects. (1) For quartz, within the particle size range of 0.07-3 mm, the enhancement of volume scattering is the dominant factor responsible for the increase in spectral reflectance as the particle size decreases. When the particle size is less than 0.07 mm, the aggregation effect causes a decreasing trend in reflectance. (2) For magnetite, reflectance shows no significant correlation with particle size due to the dominance of shadow effects. (3) For hematite, when the particle size ranges from 3 to 0.07 mm, the reflectance exhibits irregular characteristics similar to those of high-absorption minerals. When the particle size is smaller than 0.07 mm, the negative correlation between reflectance and particle size is primarily due to the combined effect of volume and surface scattering. This study provides valuable theoretical insights to improve the accuracy of the classification, spectral unmixing, and compositional inversion of particulate minerals.