Ultra-thin section study of the mineralogy and geochemistry of Mn micronodules from the South Pacific

Abstract

An embedding technique has been used to make ultra-thin sections (~ 500 Å thickness) in order to study Mn micronodules with the transmission electron microscope. The micronodules exhibit a concentric structure, comprising two alternating facies. One facies is inflated, with crystals forming several unit-layers which are small and thin. The second facies is more massive, the crystals are thicker and consist of 20–30 unit-layers. In both cases, electronic diffraction patterns are those of a 10-Å manganate or todorokite. Microprobe analysis shows that Mn, Ni, Cu, Mg and Ca contents are higher in the massive facies, Al and Si values being similar. By analogy with the behaviour of water—clay system, the authors consider that these two facies are formed under different environmental conditions. Water content, controlled by pressure, and exchangeable cations content influence the ordering of particles, although diagenesis may also be important, especially in the case of the poorly ordered particles.