GEOCHEMICAL ANALYSIS OF OXIDE GRAINS AS TRACE EVIDENCE IN A CAPITAL MURDER CASE: NORTH CAROLINA V. MARIO MCNEILL, CUMBERLAND COUNTY, NC

Soil evidence in a capital murder case was useful in determining if the suspect was present at the body recovery site. Individual oxide grains collected from soil evidence were analyzed for major element compositions using a JEOL JXA 8530F Hyperprobe at Fayetteville State University in Fayetteville, North Carolina. Three groups of samples were analyzed: 1) soil collected from the gas pedal of the suspect’s car; 2) two samples from the body recovery site; and 3) three “alibi” samples collected from areas the suspect was known to frequent.

Chemical analysis indicates samples from the gas pedal and body recovery site contain a significantly larger range in values for FeO, TiO₂, and Al₂O₃ than the “alibi” samples. Additionally, samples from the gas pedal and body recovery site lack measurable MnO in all but two grains; whereas oxide grains from the “alibi” locations contain measureable MnO up to 5 weight percent. Compositional fields for oxides from the gas pedal are most consistent with oxides from the body recovery site.

Additionally, all soil samples were viewed and photographed under a binocular microscope to determine mineralogy. Soil samples from the gas pedal and body recovery site both contained garnet grains, whereas the “alibi” samples did not contain garnet. This is consistent with the mapped geology and drainage patterns of the area. The body recovery site is located near crystalline bedrock with garnet-bearing source rocks present upstream. The three “alibi” locations are further into the Coastal Plain away from potential garnet sources, and therefore would be less likely to contain garnet. Soil from the gas pedal and body recovery site also contained submillimeter-sized metallic fibers. The fibers were analyzed by a metals expert at NC State University who concluded they were consistent with each other.

The geochemical data and presence of garnet and metallic fibers in samples from the gas pedal and the body recovery site indicate a physical link exists between the suspect and the body recovery site.