The spectral characteristics of hematite are critical for its remote sensing identification and inversion, but these characteristics are significantly influenced by particle size. Previous studies have primarily focused on particle size ranges (>40 µm) that have already been investigated and generally concluded that spectral reflectance in the near-infrared (NIR) band increases as particle size decreases. However, the potential “reversal” of this trend—specifically, a decrease in reflectance with decreasing particle size due to agglomeration effects—and its underlying mechanism at the micron and sub-micron scales remain unclear. To address this issue, six distinct particle size grades targeting the ultrafine scale were systematically prepared from high-purity hematite, with average diameters ranging from 37.5 µm down to 0.76 µm. Reflectance spectroscopy measurements were conducted to analyze spectral variations across the 350~2500 nm wavelength range. The experimental results showed that particle size had little influence on reflectance within the 350~1175 nm wavelength range. In contrast, significant dependence on particle size was observed in the 1175~2500 nm range, where a reversal of the reflectance trend occurred at a critical particle size of 15.41 µm. Specifically, reflectance increased with decreasing particle size above 15.41 µm. However, reflectance decreases dramatically when particle size falls below 15.41 µm due to increased agglomeration. This contrasts with the trend reported in previous studies. Mechanism analysis revealed that, within the 350~1175 nm range, the high complex refractive index of hematite resulted in minimal influence of particle size on reflectance. In the range of 1175~2500 nm, reflectance increased with decreasing particle size when the particle size exceeded 15.41 µm, a behavior primarily governed by particle scattering effects. Conversely, when the particle size decreased below 15.41 µm, the reflectance declined significantly with a further reduction in particle size, demonstrating a distinct trend reversal. This phenomenon is attributed to the low complex refractive index of hematite combined with a dramatic increase in particle aggregation effects as particle size decreases. These factors collectively increase the equivalent optical path length and intensify multiple absorption, leading to the observed decrease in reflectance. This study establishes the key control of agglomeration effects on the spectral behavior of fine hematite particles, providing crucial theoretical and experimental foundations for advancing high-precision, quantitative remote sensing inversion.
Ding et al. (Tue,) studied this question.