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•Oxic to euxinic transition of the Black Sea resulted in elemental enrichment or depletion of the sediments.•Diagenetic reactions, basin reservoir effect, adsorption and co-precipitation are the major controls on element enrichment.•Sr-Nd-Pb isotope composition of the terrigenous component is mainly controlled by the riverine suspended matter.

Several papers reporting exotic native elements have been published within the last few decades. The “native” occurrences described are rather dubious in view of the lack of solid proof of their relationships with the host-rock minerals. Consequently, the genetic models proposed ranging from bio-reduction to the influence of deep-mantle, strongly reduced fluids, are somewhat speculative. Here we present data for a unique Al0flake protruding from the phlogopite matrix of a rock specimen collected from a desilicated pegmatite vein. The geologic setting suggests two processes that might have played a key role in the Al0formation: (1) desilication of pegmatite, resulting in its Al residual enrichment; and (2) serpentinization of an ultramafic body, providing a strongly reduced front (H2and hydrocarbons) toward the serpentinite/pegmatite contact. These processes have presumably led to the reduction of Al to Al0at discrete sites of alumina-rich minerals.

Clay samples of hydrothermal origin from several oceanic spreading centers were studied using XRD, microprobe, infrared, and thermal analysis. They are talc, smectite, and mixed-layer talc-smectite (T-S) where the talc layers have several degrees of crystalline order. The smectite is trioctahedral in most cases, but there is also dioctahedral smectite both as mixed-layer and as a separate phase. All specimens contain Fe3+, some of them in moderate amounts (up to 17% Fe2O3) distributed between the tetrahedral and octahedral sheets (maximum values: IVFe = 0.32, VIFe = 0.68, per O10[OH]2). Octahedral Fe abundance correlates with the presence of molecular water that is lost in a well-defined dehydration event above 200 °C. This water does not cause layer expansion and is interpreted to be present within the pseudo-hexagonal cavity, next to Fe3+ions that generate a local charge imbalance. The presence of octahedral Fe3+is accompanied by vacancies in the octahedral sheet to balance the excess positive charge. An infrared band at ~790 cm-1is assigned to OH bending in the group Fe-Mg-□-OH. Analysis of this band suggests a range of short-range Fe-□ distributions, from random to ordered. Our sequece talc, T-S, trioctahedral smectite is defined by an increasing Al for Si substitution in the tetrahedra and increasing crystal disorder. The presence of Fe also causes crystal defects. This mixed-layer series can be considered as a continuum generated by the combination of chemical and crystal defect variability. Kerolite was used to designate disordered, hydrated talc. We find that there is no clear line delimiting talc from kerolite as a single phase or in mixed-layer minerals and that it is better to use a descriptive term for the latter such as “disordered talc.” Dioctahedral smectite is also a possible end-member of the mixed-layer sequence, which implies an Al + □ for Mg substitution in the octahedral sheet. If T-S consists of polar TOT layers, the existence of dioctahedral smectite in T-S raises the question of the actual composition of the octahedral sheets within polar TOT layers.

An endogenous model of the formation of small amounts of > 100 μm metallic aluminium particles from the metalliferous sediments of the East Pacific Rise (21°S) is proposed on the basis of a detailed study of their morphology, chemical composition, X-ray data, spatial and temporal distribution. According to this model native aluminium is formed as an accessory mineral during the process of the pre-chamber evolution of basic and ultrabasic systems in the spreading zones, under high P--T and low &z.hfl;O 2 conditions, as well as during the later serpentinization of basites and ultrabasites from the second and third oceanic layer. As a result of the active tectono-magmatic processes in the rift zones the native aluminium particles are carried upwards into the seafloor water layer by the upwelling vent fluids of the hydrothermal circulation cells. During periods of high hydrothermal activity, the number of Al° particles carried upwards increases. The spatial distribution of the particles depends on the energy of the hydrothermal system and the direction and energy of the seafloor currents. The thin oxyhydroxide, silicate and chloride crusts formed on the surface of the Al° particles, as well as a monoatomic oxide film, prevent further alteration of the particles.