Micromorphological characteristics of a marine ferromanganese crust

Abstract

Marine ferromanganese oxide crust fragments sampled from a massive equatorial Pacific Ocean crust deposit exhibit distinct micromorphological features and a microchemical distribution which have not been previous reported. Black, shiny upper layers of the crust display a prismoidal microstructure and a unique crystallite habit expressed by a mosaic-like design of hexagonal and pentagonal columns. This columnar arrangement, which has been previously reported on a scale of a few millimeters in other marine Fe-Mn deposits, extends vertically for several centimeters and is observed throughout the entire horizontal plane of growth of the crust, and is the largest scale of morphological regularity observed in marine Fe-Mn deposits to date. The polygonal columnar structures which constitute the crust are mainly composed of Mn, Fe, and Ti oxides. Aluminium- and Si-rich intercolumnar space (lamellae) surrounds individual columns and sharply defines each polyhedron. Additionally, the chemical composition of the crust fragments exhibits an unusual uniformity over several centimeters which, to our knowledge, has not been previously observed in marine Fe-Mn minerals.