WHEN A FORENSIC MINERALOGIST IS WRONG

In 1997 David Moeller was convicted of the 1990 rape and murder of a nine old girl near Sioux Falls, South Dakota. Critical for his conviction was DNA evidence (contested by the defense attorney), and evidence presented by a mineralogist. The mineralogist hired by the prosecuting attorney said he found a 1/2 cm diameter pebble in an exposure of glacial till at the crime scene, a dirt road along the Big Sioux River. He visually identified the pebble as gahnite (ZnAl2O4), an extremely rare mineral. He also maintained that he found “gahnite” in sand from the fender of Moeller's pickup truck, thereby confirming that Moeller had been at the crime scene. I was asked by the defense attorney to review the mineralogical evidence. At the trial l told the jury that eastern South Dakota was extensively glaciated, and much of the till was transported over vast distances by continental ice sheets. I emphasized that this gahnite specimen did not originate as a component of a residual soil that would reflect a unique locality such as the crime scene. I think the jury members viewed all this “soils” testimony as a difference of opinion between two geologists. The jury found the defendant guilty. In 2001 the 1/2 cm specimen identified as gahnite by the mineralogist was chemically analyzed by the Engineering and Mining Experiment Station at South Dakota School of Mines and Technology. It is a silicate mineral. There is no zinc present. The chemical formula is SiO2. It is simply quartz (variety onyx), not gahnite. The defense attorney submitted a formal appeal, but in 2004 the South Dakota Supreme Court upheld the guilty verdict. Moeller is soon to be executed by lethal injection. Forensic mineralogists make valuable contributions to the justice system. But when they err, the consequences can be deadly.