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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 [Al.sup.0] flake 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 [Al.sup.0] formation: (1) desilication of pegmatite, resulting in its Al residual enrichment; and (2) serpentinization of an ultramafic body, providing a strongly reduced front ([H.sub.2] and hydrocarbons) toward the serpentinite/pegmatite contact. These processes have presumably led to the reduction of Al to [Al.sup.0] at discrete sites of alumina-rich minerals. Keywords: Native Al, serpentinization, desilication, reduction, pegmatite

During a 1977 cruise of the R/V Bannock, a core was taken from an area of extensive hydrothermal mineralization located on the slope of Eolo Seamount, Aeolian Arc (Tyrrhenian Sea). The Eolo hydrothermal sediments exhibit unusual Pb isotopic compositions that cannot be solely derived from the plausible natural sources in the area. Mixing considerations indicate that a significant part of the Pb in these sediments is anthropogenic in origin. The anthropogenic Pb was most probably scavenged from the Mediterranean seawater by sinking hydrothermal plume particles. Apparently the hydrothermal products entering the seawater column did not carry significant amounts of Pb extracted from the underlying rocks, so their Pb isotopic compositions were swamped by the seawater Pb. This indicates that Pb in low-temperature hydrothermal products is controlled by the seawater Pb, not by hydrothermal leaching from underlying rocks.

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 [Fe.sup.3+], some of them in moderate amounts (up to 17% [Fe.sub.2][O.sub.3]) distributed between the tetrahedral and octahedral sheets (maximum values: [sup.IV]Fe = 0.32, [sup.VI]Fe = 0.68, per [O.sub.10][[OH].sub.2]). Octahedral Fe abundance correlates with the presence of molecular water that is lost in a well-defined dehydration event above 200[degrees]C. This water does not cause layer expansion and is interpreted to be present within the pseudo-hexagonal cavity, next to [Fe.sup.3+] ions that generate a local charge imbalance. The presence of octahedral [Fe.sup.3+] is accompanied by vacancies in the octahedral sheet to balance the excess positive charge. Ah infrared band at ~790 [cm.sup.-1] is 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 sequence tale, 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. Keywords: Crystal chemistry, infrared spectroscopy, kerolite, talc, talc-smectite, thermal analysis, trioctahedral smectite, X-ray diffraction

Abstract: Native Hg droplets, documented here for the first time in mid-ocean ridge metalliferous sediments, were found in the surface sediment of a magmatically robust and ultra-fast spreading mid-ocean ridge, the East Pacific Rise. The surface sediment and Fe–Mn micronodules in it are enriched in Hg. Two different processes can produce Hg0 liq in this environment: hydrothermal and biogenic. Bio-transfer of ionic Hg dissolved in the hydrothermal fluids into Hg0 in the sediment can account for the eventual native Hg occurrences in the close vicinity of the hydrothermal vents. The Hg0 liq occurrences in the distal metalliferous sediments most probably have originated directly from the hydrothermal fluids and been transported by hydrothermal plumes away from the vents. Condensation of Hg0 liq from Hg0 vap in the vent fluids subjected to near critical phase separation seems the most plausible mode of origin of the native Hg occurrences. The high-temperature vents at the global spreading ridge chain introduce an assessed amount of 1.62–4.86×103 t/yr Hg0 liq into the ocean. With time, the deposited Hg0 liq will likely be dissolved in the overlaying seawater and will not survive in the geologic record. [Copyright &y& Elsevier]