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Featured article from karst/cave journal

Journal of Cave and Karst Studies, 2009, Vol 71, Issue 2, p. 136-143
The mineralogy and trace element chemistry of black manganese oxide deposits from caves

Free surface streams in caves and their surface infeeders often contain pebbles and cobbles coated with black manganese oxide minerals. Coating thicknesses vary from fractions of a millimeter to a few millimeters. In addition, a few caves contain loose masses of black oxide material. The results reported here are based on examination of 39 specimens and detailed chemical analyses of 18 of them. Most of the coatings are amorphous to x-rays, with at best, only a few broad diffraction lines. Infrared spectroscopy shows that most of the specimens are birnessite, with evidence for romanechite, ranceite, and pyrolusite in a few specimens. All specimens contain both iron and manganese, but the Mn/Fe ratio varies widely. Many specimens are enriched in Ba but depleted in Sr. The manganese and iron oxides contain the transition metals Co, Cu, Ni, V, and Zn in concentrations greater than 0.5 wt% in some specimens. Minor Cr and Mo also occur. Given the extremely low concentrations of these elements expected in freshwater streams in carbonate terrains, the manganese oxides exert a dramatic amplifying effect over the expected background. Manganese oxides appear to act as a dosimeter for heavy metals in karst waters.