A CASE STUDY OF MEDICAL GEOLOGY: INVESTIGATION OF THE OCCURRENCE OF ERIONITE AND POTENTIAL ENRICHMENT ZONES AT REVA GAP IN HARDING COUNTY, SD

Erionite is a naturally occurring zeolite mineral and is classified as a Group 1 known respiratory carcinogen by the International Agency for Research on Cancer (IARC). Since its initial discovery in 1898, numerous deposits of erionite have been identified around the world, including many in the western United States. In South Dakota, erionite has been found within Tertiary strata at several locations, including at Reva Gap in the Slim Buttes Unit of Custer National Forest in Harding County. However, until now no research has been conducted to try and identify if or where an erionite enrichment zone occurs at this locality. Additionally, no data regarding the morphology of the erionite crystals and their ability to become airborne has been gathered. To address this issue, rock samples from the Arikaree, Brule, and Chadron formations were gathered and subsequently examined using an optical microscope and Scanning Electron Microscope with Energy Dispersive X-ray Microanalysis (SEM/EDS) to determine erionite content and crystal morphology. Results of the study indicate that the Arikaree formation contains the highest concentration of zeolites (~50%), followed by the Brule Formation (~15%), and finally the Chadron Formation (<1%). The morphological data suggests that 50% or more of the crystals in each of the samples analyzed were <2µm in width, and are therefore small enough to become airborne. Samples were also gathered from the Brule and Chadron formations in Buffalo Gap National Grasslands in Shannon County, SD, as there is a greater risk of erionite exposure at this location due to its proximity to Badlands National Park, a popular tourist destination. Analysis confirmed the presence of zeolites in each sample at concentrations <1%.