Major elements geochemistry of manganese nodules and ferromanganese crusts from the Pacific Ocean

Experimental substitutions of transition and alkaline earth cations into synthetic 10angstrom(Na-)-manganate show that cation uptake and the stability of the cation-substituted mineral increase with stability of the hydroxide of the cation. Hydrothermal treatment of synthetic 10angström-manganates with different metal contents as well as marine diagenetic and hydrothermal 10angstrom-manganates shows that the stabilities of their structures are enhanced with increasing temperature. The stabilization is due to reinforcement of the "tunnel" walls supporting the [Mn4+O62-] octaheral layers. The diagenetic 10angström-manganates have initially unstable buserite-like structures with each interlayer wall composed of two [Mn2+O3+x2-(OH-)3-x] octahedra (0 less-than-or-equal-to x less-than-or-equal-to 3) with either a [Na+O2x2-(OH-)n-2x] unit (n = 6 and/or 8) or less frequently a [Mn2+O2x2-(OH-)6-2x] octahedron in between. Some of these cations in the walls are post-depositionally substituted by highly hydrated divalent metal cations, particularly Cu2+ and Ni2+, while some of the Mn2+ ions are slowly oxidized to Mn4+. These interlayer changes result in higher crystal field stabilization energy and shifts from interlayer Van der Waal's forces and weak coordination links to strong coordination links which stabilize the mineral structures. Low-temperature hydrothermal 10angstrom-manganates have todorokite-like structures with "tunne"' walls constructed predominantly of [Mn2+O3+x2-(OH-)3-x] and [Mn2+O2x2-(OH-)6.2x] octahedra. High-temperature hydrothermal 10angstrom-manganates have stable todorokite structures with the walls constructed of [Mn4+O62-] octahedra. The positive correlation between the formation or post-depositional alteration temperatures and the mineral stability is due to the increase in oxidation rate of interlayer Mn2+ ions with increasing temperature of the hydrothermal fluids. Marine 10angstrom-manganates can be used as genetic indicators for manganese concretions and the sediments in which they occur and as a geothermometer in the search of ancient and modern hydrothermal vents, where massive sulphide deposits are often found.