FELDSPAR FORENSICS: A CASE STUDY IN NORTHERN IDAHO

As forensic techniques evolve, we should take advantage of opportunities for using geology to enhance forensic investigations. In the search for suitable benchmark materials, we find that the feldspar group consists of more than twenty separate mineral species and are the most abundant mineral group on the earth's surface. This ubiquity might discourage forensic analysts from using feldspars for correlation between questioned samples and control material, considering that the presence of feldspars is hardly unique. However, the subtle differences between individual species and their intermediate compositions provide opportunities for characterizing samples derived from different geological localities. While feldspars are commonly formed in igneous environments, they are often present in sedimentary and metamorphic rocks; they are relatively durable, especially in arid and semi-arid landscapes. Their widespread distribution and relative durability increase the probability there will be some feldspars in soils or other unconsolidated evidence materials. If we can create reliable and repeatable protocols for characterizing feldspars individually, we have a powerful tool for investigations.

This study began with a survey of feldspars collected from eleven localities in granite near Moscow, Idaho. Electron microprobe examination of grains from each site revealed a range of compositions from nearly end member albite to orthoclase, with various intermediate compositions represented. Several grains contained plagioclase inclusions; these were examined as well. Preliminary results suggest enough variety in composition that an analyst should be able to correlate, if not the exact site of origin with certainty, one of a few possible sites. Samples contained negligible percentages of substituent barium and iron, but further analysis with laser ablation mass spectrometry may reveal characteristic trace elements or isotopes unique to each site. This study will continue with a similar analysis of regional basalts and unconsolidated samples. The expectation is that there should be a high probability of connecting a suspect with a crime scene by virtue of analyzing any feldspars present among trace evidence.