ALBERT

Description: 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.

Description: The accretion rate of manganese nodules and the sedimentation rate of the underlying sediment has been studied with 10Be and 14C on a nodule covered hill in the Madeira Abyssal Plain. The accretion rate of (3.2 +/- 0.6) mm/Ma for the nodules is considerably smaller than the average sedimentation rate of 12 mm/ka for the last 50-100 ka. The mechanism that kept the nodules at the surface in this specific environment during the last 40 ka is likely to be flotation. The discovery of a large manganese-covered concretion in a piston core points to a long period of sediment winnowing. The 14C analyses also seem to point to an increase in the sedimentation rate round 6500 yrs B.P.

Description: Three manganese nodules from the Pacific Ocean have been analysed for 35 elements by using mainly spectrophotometric and spectrographic methods. Cu, Co, Ni, Zn, and Pb were found in amounts approaching 1 %, which far exceeds their average concentrations in igneous rocks. On the other hand, elements having readily hydrolysable ions, such as Ga, Sc, Zr, Y, La and Ti, are present only in amounts comparable with their concentrations in igneous rocks. Sb, Bit Be, and Cr were not detected. The hydrochloric acid-insoluble fraction of nodules is practically free of the heavy metals that are characteristic of the acid-soluble fraction; it consists principally of clay minerals, together with lesser amounts of quartz, apatite, biotite and sodium and potassium felspars.

Description: The porewater and sediment composition of two boxcores and of a small gravity core, taken on a manganese-nodule-covered hill and in the Madeira Abyssal Plain proper respectively, are compared. The pore-water study of the two boxcores indicates that oxic conditions prevail in both cores. In addition, it indicates that no detectable fluxes of Mn or Fe occur from the porewater to the ocean bottom water. Variations in the geochemical composition of the sediments can be explained by fluctuations in the amount of carbonate, which acts as a diluting agent. A clear carbonate minimum is observed at 20-22 cm depth in the two cores. This minimum is likely to be associated with the last glacial period (10-20 kyr B.P.). This association is supported by the sediment accumulation rate of 15 mm/kyr as found by extrapolation from the rate for pelagic sediments in the Madeira Abyssal Plain. The bulk composition of the manganese nodules recovered from the submarine hill is chemically almost identical to the average composition of Atlantic nodules. The trace metal and Rare Earth Elements composition indicate a hydrogenous origin for the manganese nodules of this study. On the basis of the chemical composition, and that of nodules relative to that of the adjacent sediments, an average nodule accretian rate of 2.8-3.3 mm/myr has been calculated. Although the analyses of the entire ferromanganese nodules that have been studied seem to indicate a homogenous composition, internal structures of the nodules reveal great inhomogeneity, both visually and chemically. These fluctuations may be related to variations in the fluxes of Mn and Fe, which in turn could be climate-related.

Description: Thanks to the courtesy of the British Museum of Natural History the author obtained from their Challenger collections two small nodules, and through a similar courtesy of the Mineralogical Department of the Riksmuseum in Stockholm one half of a much larger nodule, also from the Challenger Expedition. Results from his initial measurements of the radium contents of these samples convinced the author that the radium in the nodules is accumulated from the surrounding sediment. In the present paper the author conducted a much more thorough investigation on nodules obtained during the U.S. Albatross cruises of Dr. Agassiz. Detailed measurements of radium were conducted on individual layers and spots inside each nodule.

Description: Horizons of several types of Upper Jurassic to Lower Cretaceous manganese nodules occur locally in sequences of radiolarian cherts within the Nicoya Ophiolite Complex (NW Costa Rica). Field studies, X-ray diffraction analysis, petrographic, chemical and experimental studies give evidence of a sedimentary, early diagenetic origin of the nodules, in contrast to earlier suggestions. Smooth, discoidal, compact and very dense nodules with diameters of some mm to 9 cm dominate. They are characterized by braunite, hollandite, pyrolusite and quartz as well as 39-61% Mn, 0.9-1.6% Fe, 5-26% SiO2, 1.3-1.9% Al2O3, 1.5-3.0% Ba, 460-5400 ppm Cu, 85-340 ppm Ni and 40-130 ppm Co, among others. It is suggested that the original mineralogy (todorokite?) was altered during thermometamorphic (braunite) and hydrothermal (hollandite. pyrolusite) events. Petrographic similarities between the fossil nodules and modern deep-sea nodules are striking. Using standard hydrothermal techniques in an experimental study it is shown that under special conditions, braunite can be produced from modern nodule material.