Abiotic and Biotic Links Work Two Ways: Effects on the Deposit at the Cliff Foot Induced by Mechanical Action of Date Mussel Harvesting (<Emphasis Type="Italic">Lithophaga lithophaga</Emphasis>)

The existence of linkages between ecological and geomorphological processes and the influence of human activities both on the biota and on geomorphology is well known for terrestrial environments, while it has been seldom reported for submerged ones. A survey in the Noli-Bergeggi area (NW Mediterranean Sea) has been carried out to test the hypothesis that the harvesting of Lithophaga lithophaga has an impact not only on the hammered rock, but also on the sediments at the cliff foot. Size and roundness of pebbles have been measured at different stations and statistical analyses have been used to test the differences between impact and control situations. Results show a dependence of the roundness of pebbles on the date mussel harvesting, indicating a decrease both of the roundness index and of its variability at the cliff foot in impacted sites. Conclusions stress the biogeomorphological nature of this activity, not limited to the present location.     

Re-examination of the crystal structure of weberite

Summary The crystal structure of weberite, Na₂MgAlF₇, was redetermined from 3-dim. X-ray data and refined by least squares methods in space groupImma to a conventional reliability factorR=0.036. Mg and Al have octahedral 6-coordination. Two crystallographically independent kinds of Na atoms form different NaF₈-polyhedra with the F atoms at the corners of a flattened hexagonal bipyramid and of a square prism, respectively.

---

Neubestimmung der Kristallstruktur des Weberits

Zusammenfassung Die Kristallstruktur des Weberits, Na₂MgAlF₇, wurde aus 3-dim. Röntgendaten neu hestimmt und mit der Methode der kleinsten Quadrate in RaumgruppeImma auf den konventionellen ZuverlässigkeitsfaktorR=0,036 verfeinert. Mg und Al haben oktaedrische 6-Koordination. Zwei kristallographisch unabhängige Arten von Na-Atomen bilden mit den F-Atomen zwei verschiedene NaF₈-Polyeder; in einem sitzen die F-Atome an den Ecken einer abgeflachten hexagonalen Bipyramide, im anderen an den Ecken eines quadratischen Prismas.

---

---

With 2 Figures     

The crystal structure of sarcolite

Summary The crystal structure of sarcolite from Monte Somma (Vesuvius), Na(Na, K, Fe, Mg)<1 Ca₆[Al₄Si₆O₂₃](OH, H₂O)<2 [(Si,P)O₄]_0.5[(CO₃, Cl)]_0.5, space groupI4/m witha=12,343(5)Å,c=15,463(5)Å andZ=4, has been determined from X-ray data collected on an automatic diffractometer. The 1637 independent reflections withI>2 (I) converged to a conventionalR value of 0.054 with partially anisotropic factors.The tetrahedral framework in sarcolite has a sharing coefficient of 1.85. Mean Si–O and Al–O distances are 1.616 and 1.763 Å, respectively. Isolated (Si, P)O₄, CO₃, OH, H₂O and Cl species occupy cavities in the tetrahedral framework in a partially disordered way. The two crystallographically different Ca atoms coordinate respectively with 5 and 6 framework oxygens; further contacts occur with available anions. Ca–O distances range from 2.34 to 2.69 Å. Na atoms coordinate with 4 oxygens of the tetrahedral frame and one from the CO₃ groups.A structure analysis of a sarcolite crystal baked out at 1100°C confirmed some structural details involving atoms occupying cavities in the tetrahedral framework.

---

Die Kristallstruktur des Sarkoliths

Zusammenfassung Die Kristallstruktur des Sarkoliths vom Monte Somma (Vesuv), Na(Na, K, Fe, Mg)<1 Ca₆[Al₄Si₆O₂₃](OH, H₂O)<2[(Si, P)O₄]_0,5[(CO₃, Cl)]_0,5, RaumgruppeI4/m,a ₀=12,343(5)Å,c ₀=15,463(5)Å,Z=4, wurde aus Röntgendaten, die auf einem automatischen Diffraktometer gesammelt worden waren, bestimmt. Der konventionelleR-Wert für 1637 kristallographisch unabhängige Reflexe mitI>2 (I) konvergierte mit partiell anisotropen Temperaturfaktoren auf 0.054.Der Verknüpfungskoeffizient des Tetraedergerüstes in Sarkolith ist 1,85. Die mittleren Si–O-bzw. Al–O-Abstände sind 1,616Å und 1,763 Å. Isolierte Strukturbestandteile (Si, P)O₄, CO₃, OH, H₂O und Cl besetzen zum Teil ungeordnet die Hohlräume des Tetraedergerüstes. Die beiden kristallographisch verschiedenen Ca-Atome werden von funf bzw. sechs Sauerstoffen des Gerüstes koordiniert, weitere Kontakte bestehen zu verfügbaren Anionen. Die Ca–O-Abstände variieren von 2,34 bis 2,69 Å. Die Na–Atome sind von vier Sauerstoffen des Tetraedergerüstes und von einem weiteren der CO₃-Gruppen koordiniert. Die Strukturanalyse eines bei 1100°C getemperten Sarkolithkristalls bestätigte einige Details über die Atome, welche die Hohlräume des Tetraedergerüstes besetzen.

---

---

With 4 Figures     

Uranium-lead geochronology of the Archean Imataca Series,Venezuelan Guayana Shield

Uranium and lead isotopic analyses of total-rock and feldspar samples from the Imataca Series indicate an early Archean (3.5⁺ b.y.) age for the basement gneisses. Lead isotopic evolution in most gneiss samples may approximately be described by a three-stage model. The start of the third stage is marked by a pervasive metamorphic event, locally up to pyroxene-granulite grade, at 2 b.y. ago, which commonly resulted in extensive depletion of uranium relative to lead.     

Micropores in plagioclase

Plagioclase feldspar is often surprisingly porous on a small scale. Pores range in size from less than 1 μm to about 40 μm, and occupy up to 2.3 % by volume. In general, the highest plagioclase porosities occur in rocks which would have had a comparatively “wet” history: normal granites and pegmatites. Rocks which were drier—gabbro, a diabase and an exceptionally dry granite—have much lower plagioclase porosities, as do two lowgrade metamorphic samples. It is suggested that the nearly equant pores represent sites of former fluid inclusions, while the planar cavities of cracklike cross-section may be partially annealed fractures formed early in the history of the rock.     

Rare Earth Element Geochemistry of the Zigzag - Klondyke Sedimentary Rock Formations: Clues to the Evolution of the Baguio Mineral District (Luzon), Philippines

Abstract. Petrographic analysis of the Oligocene-Miocene Zigzag Formation and Miocene Klondyke Formation of the Baguio Mineral District reveals a transitional arc source for the former and an undissected arc source for the latter. Whole rock geochemistry of these sedimentary rock formations show affiliation of the Zigzag Formation to active continental margin whereas the Klondyke Formation appears to have been derived from an oceanic island arc source. A different aspect of the geology of the district is further gleaned from the whole rock rare earth element geochemistry of these rocks. Samples from the Zigzag Formation are characterized by higher REE concentrations compared to the Klondyke Formation samples. The REE data indicate that the sedimentary rocks of the Klondyke Formation had geochemically-less fractionated igneous lithologies as their progenitor whereas the older Zigzag Formation was derived from a more differentiated source. This new set of information contributes to the understanding of the evolution of this district from a marginal basin to an island arc setting.     

Thermobarometry, Sm/Nd Ages and Geophysical Evidence for the Location of the Suture Zone Between Cuyania and the Western Proto-Andean Margin of Gondwana

Basement rocks comprising ortho- and paragneisses and schists whose tectono-metamorphic evolution is poorly known, are exposed in the Sierras de Umango, Maz-Espinal and Las Ramaditas, in the northwest of the La Rioja Province, Argentina. These units were included in the Maz, El Taco, El Zaino complexes, as well as the Tambillos Metamorphics that would be part of the northern end of the Cuyania terrane, a microcontinent derived from Laurentia that collided with Western Gondwana during the early Paleozoic, or belong to the active margin of the continent. To recognize rocks belonging to each of these tectonic units and to understand the history and physical conditions of accretion were some of the main goals of the multidisciplinary investigation whose preliminary results are presented here. Geochemical studies, trace and REE elements and Sm-Nd model ages allowed the recognition of several episodes of crustal accretion in these rocks. The oldest one occurred at ca. 2.2 Ga in an arc/back-arc environment along the eastern segment of the Sierra de Maz, and was possibly coeval with development of a early Proterozoic continental crust that acted as source to sediments of Maz Complex. The following episode of crustal accretion that formed rocks in this region was at ca. 1.4 Ga and is registered by tonalites emplaced in an extensional environment cropping out in the western flank of the Sierra del Espinal.

In the Sierra de Umango, an arc/back-arc sequence registered an episode of crustal accretion during the Grenvillian Cycle (ca. 1.3 Ga). The last episode of crustal accretion detected in this area (800 Ma) is represented by an old alkaline volcanism in the Sierra de Umango. This episode could be representing the first stage of break-up of the Rodinia supercontinent during the Neoproterozoic.

The metamorphic grade reached by these rocks is mostly represented by fabrics with mineral assemblages of intermediate to high pressures and high temperatures, typical of collisional environments. The oldest rock-forming fabrics tectono-metamorphic episode recognized is of middle Proterozoic age (ca. 1.04 to 0.969 Ga, garnet-whole-rock Sm/Nd age) being registered by metapelites from Maz Complex that attained temperatures of 650°C-6.3 kbar. A younger metamorphic event (463 Ma, garnet - whole-rock Sm/Nd age) is verified in metatonalites intrusive in these metapelites. Another metamorphic event at ca. 301 Ma (garnet-WR Sm/Nd age) was recognized in metasediments from El Taco Complex. Peak metamorphic conditions of this event, probably registering the last major tectonic episode that affected rocks of this area was established in 868°C-9.8 kbar. It is impossible to distinguish fabrics belonging to totally different tectonic episodes based on structural or metamorphic data. Therefore, distinction between major tectono-thermal events of totally different ages such as the high-T middle Proterozoic deformation and with N-NWestwards tectonic transport direction registered in the Sierras de Maz-Espinal and Umango from the youngest one (ca. 301 Ma) that attained the highest-P/T conditions, recognized in the Sierra de Las Ramaditas, had to be done on the basis of Sm/Nd ages.

Geophysical evidence indicates the presence of extensive WNW-oriented lineaments that separate basements blocks of different magnetic and gravimetric signatures that are thought to represent ancient Grenvillian age suture zones. On the other hand, the northern segment of the Valle Fértil lineament that runs between Sierras de Umango and Maz-Espinal is at present interpreted as marking the eastern boundary of the Cuyania terrane. This is supported by isotopic data as well as the contrasting history of tectono-metamorphic events as determined for both of these segments of the NW Sierras Pampeanas.     

Benthic microbial abundance and activities in an intensively trawled ecosystem (Thermaikos Gulf, Aegean Sea)

Abundance of benthic bacteria, heterotrophic nanoflagellates and ciliates, extracellular enzymatic activities, bacterial C production, C mineralisation and sediment community oxygen consumption rates were measured in the Thermaikos Gulf (Northeastern Mediterranean), before (September 2001), and during intense trawling activities (October 2001 and February 2002). The biochemical composition of sedimentary organic matter has revealed that bottom trawling had an effect on the trophic state of Thermaikos Gulf. Changes on the benthic microbial food web were also recorded, during the three sampling seasons. Even though trawling-induced sediment resuspension did not alter significantly the abundance of the microbial components, with the exception of the most impacted station, it determined changes regarding their relative importance. Thus, the ratios of bacterium to nanoflagellates and ciliate to nanoflagellates abundance increased in the trawled stations, causing a sudden increase in bacterial C production, in comparison to the non-trawled station. Four months later, the effects of trawling on the microbial food web were less evident, masked possibly by the drastic decrease in the water temperature. The results of the present work suggest that bottom trawling induces alteration of the sedimentological variables and can be considered as a factor affecting the function of the microbial food web in marine coastal ecosystems. These alterations cause faster mobilisation of organic C buried in the sediment and increase nutrient concentrations and availability in the system, thus inducing an effect that could lead to coastal eutrophication.     

The application of lithogeochemistry in the search for unconformity-type uranium deposits, Northern Saskatchewan, Canada

Lithogeochemical-mineralogical haloes around unconformity-type uranium deposits in northern Saskatchewan can expand the size of drill targets up to fifteen times. The deposits are located at or near the unconformity between Aphebian metamorphosed basement rocks in close proximity to Archean granitoids and overlying unmetamorphosed sandstones of the Helikian Athabasca Group. Deposits studied include Key Lake (Deilmann), Midwest Lake and Eagle Point.Unconformity-type deposits are associated with broad alteration haloes in the overlying sandstones and more restricted haloes in the basement rocks. The haloes in the sandstone are localized around steeply dipping fault structures and are characterized by zones of intense leaching of specular hematite and detrital heavy mineral layers and by changes in chemistry related to clay mineral alteration and tourmalinization.Clay mineral alteration haloes are common in the sandstone and basement host rocks. Interstitial clays consist mainly of kaolinite and illite with lesser amounts of chlorite. The ratio K₂O/AI₂O₃ proved useful in delineating illite-kaolinite patterns in the sandstone and in the uppermost portions of the paleoweathering profile in basement rocks. The Midwest deposit is characterized by a broad bell-shaped zone (500 m across strike) of high K₂O/AI₂O₃ ratios (>0.18) in which illite is the dominant clay mineral. The sandstone above the Deilmann deposit, on the other hand, is characterized by silicification and kaolinitization with low K₂O/AI₂O₃ ratios (<0.04). This kaolinite cap overprints a preexisting illite zone.Anomalously high boron values are characteristic of the three deposits considered in this study. Boron anomalies are similar in extent to the anomalous clay mineral alteration haloes. Altered sandstones commonly contain aggregates of radiating magnesium-rich dravite needles within the clay matrix. The interpretation of boron patterns is problema tical however, mainly because of the detrital tourmaline component in sandstones and metamorphic tourmalines present in the Aphebian metasediments.Trace elements such as U, Ni, As and Co are generally of limited use in expanding targets in sandstone because their haloes are restricted to a few tens of metres horizon-anomalies along the steeply dipping zones of mineralization related to fault structures. Uranium (>3 ppm) does form an anomaly more than 200 metres laterally across the Midwest deposit. At the Deilmano deposit anomalous uranium dispersion is restricted to within a few metres of high grade ore.In the basement rocks, the various layers of the paleoweathering profile are geochemically overprinted up to 250 m from mineralization. Bleaching related to illitization and chloritization is associated with enrichment in K₂O, MgO, B, S, U, As, Ni and P₂O₃. The ratio Fe₂O₃/MgO is useful in delineating chloritization in the upper portion of the paleo weathering profile.Deposits with large root extensions in the Aphebian metasediments such as Eagle Point, have intense dravite-chlorite-illite alteration zones which are restricted to within a few metres of mineralization across strike. ‘Quartzite” units are alteration related. The complexity of the basement lithology inhibits the use of individual elements as alteration guides other than in the intensely altered zone. The application of multivariant techniques, element ratios and clay mineralogical work prove useful in identifying the mineralogical changes at Eagle Point.     

Quantum-mechanical calculation of the vibrational spectrum of beryl (Al4Be6Si12O36) at the Γ point

A quantum-mechanical calculation of the zone-centre phonon spectrum of beryl has been performed, by using an hybrid HF/DFT Hamiltonian (B3LYP). An excellent agreement with the experiment has been obtained, being the difference between the calculated and the experimental vibrational frequencies (Raman, IR-TO and IR-LO) less than 5 cm^?1 on average. In the few cases where a relatively large disagreement between calculated and experimental data is observed, an explanation can be found which attributes the reason of the discrepancies to the experimental data rather than to the calculated ones. The calculation (i) allows the identification, in the experimental spectra, of the peaks corresponding to fundamental modes, overtones, combination bands and leakage; (ii) solves problems of band assignements due to the presence of LO–TO splitting in the IR spectra; (iii) provides the frequencies of silent modes; (iv) permits a full analysis of the atomic motion corresponding to each normal mode.