Journal of Cave and Karst Studies - ISSN 0146-9517
Volume 59 Number 3: 128-131 -December 1997

A publication of the National Speleological Society

Presence of Rare-Earth Elements in Black Ferromanganese Coatings from Vāntului Cave
Bogdan P. Onac, Rolf B. Pedersen and Magne Tysseland

Abstract

This study examines the rare-earth elements (REEs) found in ferromanganese coatings covering both sandy alluvium and submerged boulders in an underground stream from Vāntului Cave, Romania. The black ferromanganese sediments are mainly composed of birnessite and other poorly-crystallized manganese oxide and hydroxides (pyrolusite, romanechite, todorokite, rhodochrosite) as well as goethite and kaolinite. Scanning electron microscope and EDX analyses performed on the black ferromanganese sediments show the material to have concentrated considerable amounts of REEs (La, Ce, Sm, Nd) in iron-rich spheres that build up botryoidal-like aggregates. The correlation of 143Nd/144Nd ratio for 6 different samples indicates that the REEs were concentrated in the cave environment after being leached from bauxitic and red residual clays from above the cave. Based on our observations, we conclude that an increase in pH resulted in adsorption of REE onto the surface of ferromanganese minerals. This study demonstrates the potential of using Nd isotopes as a tool for paleochemistry studies of the cave environment.

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