Investigation on forensic soil analysis using quartz shapes and surface microtextures

Abstract

Soil can provide important information as forensic trace evidence in criminal investigations, and various analyses, mainly based on geologic methods, are applied to discriminate between soil evidence and control samples. To improve the discriminating power of soil, it is necessary to investigate new analyses based on other characters. Quartz grains in soil are highly resistant to weathering and preserve various surface microtextures that reflect the sedimentary history of the grains. The microtextures of quartz are observed using scanning electron microscopy (SEM). Previous research has demonstrated the utility of quartz surface analyses in forensic examinations. However, quartz microtextures observed in soil from Japan may differ from those reported in previous foreign studies owing to their different geological history and geomorphological setting. To apply quartz analysis to forensic examination in Japan, we first need to survey the morphological diversity of quartz grains from Japan. To this end, we collected 38 sediment samples from rivers and beaches of Japan. Fifteen types of microtextures on the quartz surfaces were identified using SEM and the data were statistically processed. The quartz shapes were evaluated in terms of roundness and fractal dimension using image analysis. Roundness and fractal dimension were correlated and provide a quantitative measure of grain angularity. Samples from similar depositional environments were grouped by principal component analysis (PCA), and the surface textures and shapes of quartz showed diversity across the study areas. Image analysis, SEM observations, and PCA showed good repeatability and therefore may be applied in forensic discrimination. Most types of surface microtextures reported by previous foreign studies were also observed in Japan, but some types were rarely observed compared with previous studies. Therefore, forensic investigations involving quartz analysis need to assess grain provenance with reference to the geological and geomorphological history of grains from Japan.