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】 绝大多数土壤法医比对研究均聚焦于黏土与粉粒组分，却忽视了较粗颗粒。本研究旨在探究亚热带土壤砂粒组分的元素与物理分析潜力，以区分采集自不同母质（黏土岩与大理岩）区域的样本，以及同一母质区域内模拟犯罪现场的样本。本研究采用扫描电子显微镜-能量色散X射线能谱仪（SEM-EDS）对粒径0.05~0.25 mm的细砂粒进行分析，并对全部砂粒组分开展X射线衍射（XRD）与粒度分布（PSD）检测。结果显示，上述方法无法清晰区分亚热带环境下采集的土壤砂粒样本。这一现象可归因于砂粒组分中广泛存在的均质性，该均质性与其化学（EDS）、物理（粒度分布）、形貌（SEM）及矿物学（XRD）特征相关。在热带与亚热带环境中，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，并使石英在砂粒组分中富集。基于此，我们建议在法医土壤研究中优先选取更细的土壤组分开展无机与有机相关分析。