MANGANESE OXIDE MINERALS IN CAVES: MICROBIALLY DRIVEN HEAVY METAL SCAVENGERS

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 40 specimens and detailed chemical analyses of 20 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 some evidence for romanechite and hollandite in a few specimens. The minerals are extremely fine-grained and reveal little detail in scanning electron microscope images. The extremely insoluble trivalent and tetravalent oxides of manganese are precipitated by the microbial catalyzed oxidation of very dilute concentrations of Mn2+ combined with the increase of pH when streams flow onto carbonate rocks. 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 occurs. Given the extremely low concentrations of these elements expected in fresh water streams in carbonate terrain, the manganese oxides exert a dramatic amplifying effect over the expected background. All specimens contain both iron and manganese by the Mn/Fe ratio varies widely. Many specimens are enriched in Ba but depleted in Sr. Manganese oxides appear to act as a dosimeter for heavy metals in karstic waters.