Sand fraction is not suitable for forensic investigations in subtropical soils

ABSTRACT Most of the forensic comparison of soils has focused on the clay and silt fractions at the expense of the coarser particles. This study aimed to test the potential of elemental and physical analyses in the sand fraction from subtropical soils to discriminate samples collected in areas under different parent material (claystone and marble) and in areas with the same parent material at a simulated crime scene. Scanning electron microscopy coupled to an energy dispersive X-ray spectrometer (SEM-EDS) analysis was used on the finer sized sand particles (0.05-0.25 mm). X-ray diffraction (XRD) and particle size distribution (PSD) analyses were performed on the whole sand fraction. These methods did not provide clear discrimination of the sand of the soils sampled in the subtropical environment. This can be explained by the large homogenization observed in the sand fraction related to its chemical (EDS), physical (particle size distribution), morphological (SEM), and mineralogical (XRD). Under tropical and subtropical conditions, the chemical weathering processes dissolve most of the primary minerals, such as the feldspars, biotite, and Fe-bearing particles, and concentrates quartz in the sand fraction. In these environments, we recommend the prioritization of the finer soil fractions for forensic studies, both inorganic and organic.【摘要】目前绝大多数法医土壤比对研究均聚焦于黏土级与粉砂级组分，却忽视了较粗颗粒的分析价值。本研究旨在探究亚热带土壤砂级组分的元素与物理分析潜力，以区分来自不同母质（黏土岩与大理岩）区域的土壤样本，以及同一母质区域内模拟犯罪场景下采集的样本。针对粒径0.05~0.25 mm的细砂颗粒，本研究采用耦合能量色散X射线能谱的扫描电子显微镜（Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectrometer, SEM-EDS）进行分析；对完整砂级组分则开展X射线衍射（X-ray diffraction, XRD）与粒度分布（Particle Size Distribution, PSD）分析。上述分析方法未能对亚热带环境下采集的土壤砂级组分实现清晰的区分甄别。这一结果可归因于砂级组分呈现出显著的均一化特征，其化学组成（对应EDS分析）、物理属性（对应粒度分布分析）、形貌特征（对应SEM分析）与矿物组成（对应XRD分析）均趋于一致。在热带与亚热带气候条件下，化学风化作用会溶解绝大多数原生矿物，例如长石、黑云母与含铁颗粒，并使石英在砂级组分中富集。因此，在这类环境中开展法医土壤研究时，建议优先选用更细粒的土壤组分（包括无机与有机组分）进行分析。