Practical Use of Heavy Minerals in Forensic Studies

Heavy minerals are a ubiquitous component in sediments. Their abundance ranges from trace amounts (usually less than 0.1%) in carbonates, to 1%, or more, in clastic sediments. These minerals have proven useful in both civil and criminal investigations. Their utility often is a direct reflection of Edmond Locard's (1877-1966) Exchange Principle. Simply stated, “any time two objects touch, there is a transfer of material.” The principle, consequently, is directly applicable to soil investigations where heavy minerals in soils found at one site may be directly associated with similar materials subsequently found on items (shoes, clothing, automobile floorboards, etc.) far removed from the site. Further, whereas a deeply weathered soil may consist essentially of quartz, and possibly one or two clay mineral phases, its heavy mineral fraction may contain a rich suite that includes ilmenite, leucoxene, rutile, kyanite, staurolite, zircon, garnet, hornblende, etc. in quantities that may be diagnostic for the unit's identification. Additionally, while two soils from a local area may be indistinguishable based on their major mineral content (e.g., “quartz plus kaolinite”), they may be quite distinguishable, even if they contain the same heavy mineral suite. This arises from the fact that if the sediments on which the soils developed were deposited by different river systems, or formed at different times (i.e., are separated stratigraphically by even a diastem), then it is unlikely that they will contain the same relative proportions of heavy minerals. Hence, ratios of certain heavy minerals (e.g., zircon + tourmaline/rutile, the classic “ZTR index,” or the Ilmenite/Leucoxene ratio) can often be used to distinguish, or associate, samples from two sites.

Examples are given illustrating heavy mineral use in a murder investigation, a case involving industrial sabotage, and a case involving the cause of premature failure of aircraft engines built for a major aircraft company.