THE MINERALOGY AND TRACE ELEMENT CHEMISTRY OF BLACK MANGANESE OXIDE DEPOSITS FROM CAVES

摘要

Free surface streams in caves and their surface infeeders often contain pebblesand cobbles coated with black manganese oxide minerals. Coating thicknesses vary fromfractions of a millimeter to a few millimeters. In addition, a few caves contain loosemasses of black oxide material. The results reported here are based on examination of 39specimens and detailed chemical analyses of 18 of them. Most of the coatings areamorphous to x-rays, with at best, only a few broad diffraction lines. Infraredspectroscopy shows that most of the specimens are birnessite, with evidence forromanechite, ranceite, and pyrolusite in a few specimens. All specimens contain both ironand manganese, but the Mn/Fe ratio varies widely. Many specimens are enriched in Babut 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 andMo also occur. Given the extremely low concentrations of these elements expected infreshwater streams in carbonate terrains, the manganese oxides exert a dramaticamplifying effect over the expected background. Manganese oxides appear to act as adosimeter for heavy metals in karst waters.