Characterization of secondary minerals formed as the result of weathering of the Anakeesta Formation, Alum Cave, Great Smoky Mountains National Park, Tennessee

Flohr, Marta; Dillenburg, Roberta; Plumlee, Geoffrey

The environmental impact of weathering of both natural and man-made exposures of the Anakeesta Formation in the Great Smoky Mountains National Park (GSMNP), North Carolina and Tennessee (Fig. 1) has been of concern to the National Park Service for a number of years. Weathering of the iron-sulfide-bearing metamorphic rocks that comprise the Precambrian Anakeesta Formation has produced acid drainage in parts of the park. Exposures of Anakeesta rocks at Alum Cave (Fig. 1), which is along the heavily hiked Alum Cave Trail, has raised health concerns of a different nature. Alum Cave is not a real cave, but rather a bluff that has been eroded to produce a cave-like overhang. Within this structure, the Anakeesta phyllite is protected from precipitation, but low volumes of groundwater seep along planes of schistosity and alter the rock. Fine-grained soluble secondary minerals form by the evaporation of this acidic metal-bearing groundwater. This fine-grained material sloughs off the formation and is incorporated into the soil below. As hikers walk through the soft dry soil beneath the Alum Cave overhang, significant amounts of dust are raised that have a bitter "alum" taste (hence, the name "Alum" Cave), which has been attributed to the presence of the secondary minerals derived from the phyllite. Hiker traffic through the area is heavy because Alum Cave is both a popular destination for day hikers and it lies along the trail that leads to Le Conte Lodge, which is located on the west slope of Mount Le Conte and which is only accessible by foot. The concern of possible adverse health effects on hikers that walk through the soil and breathe the dust at Alum Cave has been raised by the National Park Service. The purpose of this study was to identify and chemically characterize the secondary minerals that form from the Anakeesta phyllite at Alum Cave, so that their potential environmental and (or) health impact could be further assessed. This mineralogical study compliments leaching experiments currently in progress by one of the authors (G.S.P.) on samples from Alum Cave. In addition to specifically addressing the formation and stability of the secondary minerals at Alum Cave, these studies will contribute to a better understanding of the processes that lead to alteration of Anakeesta rocks and to the generation of acidic drainage within the GSMNP. A brief summary of the environmental problems caused by the weathering of Anakeesta rocks is given in the following section.

Characterization of secondary minerals formed as the result of weathering of the Anakeesta Formation, Alum Cave, Great Smoky Mountains National Park, Tennessee

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