Experimental concentration-time curves for the iron(II) sulphide precipitation process in aqueous solutions and their interpretation     

David Rickard 《Chemical Geology》1989,78(3-4):315-324

The first precipitate formed through the reaction between aqueous Fe(II) salts and dissolved sulphide at ambient temperatures and pH < 9, appears to be a highly disordered gel approaching the composition Fe(HS)₂ on a water-free basis. After 0.4 s this precipitate loses sulphide and amorphous FeS begins to appear. The mackinawite structure begins to develop after several hours.

The rate of formation of the initial precipitate can be approximated by a pseudo first-order reaction, directly dependent on total sulphide concentration and with an apparent pseudo first-order rate constant of 48 ± 9 s⁻¹. Dissolved Fe concentration does not appear to be rate limiting.

The estimated solubility of the initial phase is variable but consistently one to two orders of magnitude greater than the measured solubilities for amorphous FeS. In natural systems this may lead to variable Fe(II) solubilities in sulphidic environments. This initial material may play a more central role in iron sulphide reaction pathways than either mackinawite or amorphous FeS.  相似文献   

Ore lead isotope variations in the Proterozoic massive pyrite deposit at Näsliden,Skellefte district,Sweden     

D. Rickard  Sven-Åke Svensson 《Mineralium Deposita》1984,19(2):145-151

The ore lead isotope ratios from the Näsliden deposit are mantle-like in terms of the idealized plumbotectonics model. They suggest that the Näsliden deposit was formed in the Proterozoic equivalent of a Phanerozoic primitive island arc. The mantle-like nature of the lead and its homogeneity indicate that crustal lead does not constitute the major component of the ore lead. Crustal lead contamination is suggested to be the cause of divergences in ore lead isotopic compositions associated with later veins in the deposit.  相似文献   

Seasonal dynamics of a cyanobacteria-dominated microbial community in surface sediments of a shallow,eutrophic lake   总被引：16，自引：0，他引：16  

Bengt Boström  Anna-Kristina Pettersson  Ingemar Ahlgren 《Aquatic Sciences - Research Across Boundaries》1989,51(2):153-178

The seasonal variation of microbial biomass and activity in the surface sediments (0–10 cm) of the shallow, eutrophic Lake Vallentunasjön was followed during one year. OverwinteringMicrocystis colonies dominated the microbial community during all seasons, constituting 60–90% of the total microbial biomass. Expressed on an areal basis, the benthic biomass was, throughout the year, larger than or similar to the planktonic biomass during the peak of the summer bloom, indicating an ability of the colonies to survive in the sediments for extended periods. Abundance of other, non-photosynthetic bacteria varied in the range 3.0–15.5 · 10¹⁰ cells g^–1 d. w. over the year with minimum values in summer and maximum values in autumn in connection with the sedimentation of theMicrocystis bloom. A substantial part of the non-photosynthetic bacteria, up to circa 40%, was associated with the mucilage of healthyMicrocystis colonies. Bacterial production (³H-thymidine incorporation) appeared to be strongly temperature dependent and less influenced by the seasonal sedimentation pattern. Our data indicate an increasing proportion of non-growing cells in autumn and winter. Biomass-bound phosphorus constituted a significant portion, circa 10%, of the phosphorus content in Lake Vallentunasjön sediments. This pool has normally been overlooked in studies on phosphorus dynamics in lake sediments. Different mechanisms whereby organic phosphorus can be released from the sediments are discussed.  相似文献   

Boundary layer plasmas as a source for high-latitude, early afternoon, auroral arcs     

Rickard Lundin  David S. Evans   《Planetary and Space Science》1985,33(12):1389-1406

Simultaneous measurements of hot boundary layer plasma from PROGNOZ-7 and particle precipitation from the TIROS/NOAA satellite in nearly magnetically conjugate regions have been used to study the dynamo process responsible for the formation of high latitude, early afternoon, auroral arcs.

Characteristic for the PROGNOZ-7 observations in the dayside boundary layer at high latitudes is the frequent occurrence of regions with injected magnetosheath plasma embedded in a “halo” of antisunward flowing magnetosphere plasma. The injected magnetosheath plasma have several features which indicate that it also acts as a local source of EMF in the boundary layer. The process resembles that of a local MHD dynamo driven by the excess drift velocity of the injected magnetosheath plasma relative to the background magnetospheric plasma.

The dynamo region is capable of driving field-aligned currents that couple to the ionosphere, where the upward current is associated with the high latitude auroral arcs.

We demonstrate that the large-scale morphology as well as the detailed data intercomparison between PROGNOZ-7 and TIROS-N both agree well with a local injection of magnetosheath plasma into the dayside boundary layer as the main dynamo process powering the high-latitude, early afternoon auroral arcs.  相似文献   

Hydrogen defect saturation in natural pyroxene   总被引：1，自引：0，他引：1  

Rickard Sundvall  Henrik Skogby 《Physics and Chemistry of Minerals》2011,38(5):335-344

Dehydration via the redox reaction: OH^? + Fe²⁺ ? O^2? + Fe³⁺ + ½H₂ is believed to be a commonly occurring process in pyroxenes and other nominally anhydrous minerals (NAMs) from the upper mantle and appears to be fast enough to allow significant dehydration during magma ascent. Nevertheless, the mobility of hydrogen incorporating defects is controlled by cation diffusion with approximately two orders of magnitude slower reaction kinetics than the iron redox reaction, and host defects have a much higher likelihood to be preserved than the hydrogen itself. Therefore, restoring hydrogen into the structure would be possible by driving the redox reaction backwards, as long as temperature and time are limited so as not to change the defect state of the crystal structure. Here we investigate the re-hydration capacity of megacryst and xenocryst ortho- and clinopyroxene by stepwise thermal annealing of crystallographically oriented samples in 1 atm H₂. H concentration was measured by FTIR spectroscopy after each annealing step. Most samples show only a small increase in water content up to a presumed saturation level, after which further heat treatments in H₂ resulted in a slight decrease in water contents. However, two of the studied samples, both fairly Fe rich megacrysts, are significantly rehydrated. Some samples or crystal sections exhibit a practically inert behavior, with minor fluctuations around initial water concentrations. Present results indicate that most mantle pyroxenes have not been substantially dehydrated during late stage magma processes, and that restoring water is possible in samples which have lost considerable amounts of water.  相似文献   

Book reviews     

G. A. Chapman  Giancarlo Noci  J. J. Rickard 《Solar physics》1992,142(2):415-417

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Hydrogeochemical Processes in the Kafue River upstream from the Copperbelt Mining Area, Zambia     

Ulf T. Pettersson  Johan Ingri  Per S. Andersson 《Aquatic Geochemistry》2000,6(4):385-411

Frequent sampling during an annual cycle of dissolved(<0.45 m) and suspended (>0.45 m) elementshas been conducted in the Kafue River at Raglan'sFarm, upstream from the mining activities within theCopperbelt Province, Zambia. Additional sampling ofsediment and interstitial pore water was conductedduring low water discharge. The presence of carbonateswithin the drainage basin naturally gives rise to highelement concentrations in the dissolved phase(Ca = 626, Mg = 494, Na = 360 and K = 24 mmol l^-1).During the rainy season the relative composition ofthe dissolved elements indicated a wash out ofaccumulated weathering products and mineralisedorganic material from the unsaturated zone of the soilprofile. High concentrations of dissolved Al, Fe andMn were measured during high water discharge. At lowwater discharge the sediment was a major source of Fe,Mn and associated Co and Cu to the water column.Enhanced concentrations of dissolved and suspended S,Co and Cu during the rainy season indicated thatatmospheric deposited particles from the mining areawere washed out into the river. Autochthonousformation of particles rich in Si indicated diatomproduction during low water discharge.  相似文献   

The Origin and Significance of Debris-charged Ridges at the Surface of Storglaciären, Northern Sweden     

Neil F. Glasser  Michael J. Hambrey  James L. Etienne  Peter Jansson  Rickard Pettersson 《Geografiska Annaler: Series A, Physical Geography》2003,85(2):127-147

Storglaciären is a 3.2 km long polythermal valley glacier in northern Sweden. Since 1994 a number of small (1–2 m high) transverse debris‐charged ridges have emerged at the ice surface in the terminal zone of the glacier. This paper presents the results of a combined structural glaciological, isotopic, sedimentological and ground‐penetrating radar (GPR) study of the terminal area of the glacier with the aim of understanding the evolution of these debris‐charged ridges, features which are typical of many polythermal glaciers. The ridges originate from steeply dipping (50–70°) curvilinear fractures on the glacier surface. Here, the fractures contain bands of sediment‐rich ice between 0.2 and 0.4 m thick composed of sandy gravel and diamicton, interpreted as glaciofluvial and basal glacial material, respectively. Structural mapping of the glacier from aerial photography demonstrates that the curvilinear fractures cannot be traced up‐glacier into pre‐existing structures visible at the glacier surface such as crevasses or crevasse traces. These curvilinear fractures are therefore interpreted as new features formed near the glacier snout. Ice adjacent to these fractures shows complex folding, partly defined by variations in ice facies, and partly by disseminated sediment. The isotopic composition (δ¹⁸O) of both coarse‐clear and coarse‐bubbly glacier ice facies is similar to the isotopic composition of the interstitial ice in debris layers that forms the debris‐charged ridges, implying that none of these facies have undergone any significant isotopic fractionation by the incomplete freezing of available water. The GPR survey shows strong internal reflections within the ice beneath the debris‐charged ridges, interpreted as debris layers within the glacier. Overall, the morphology and distribution of the fractures indicate an origin by compressional glaciotectonics near the snout, either at the thermal boundary, where active temperate glacier ice is being thrust over cold stagnant ice near the snout, or as a result of large‐scale recumbent folding in the glacier. Further work is required to elucidate the precise role of each of these mechanisms in elevating the basal glacial and glaciofluvial material to the ice surface.  相似文献   

A tree-based solver for adaptive ocean modelling   总被引：1，自引：0，他引：1  

Stphane Popinet  Graham Rickard 《Ocean Modelling》2007,16(3-4):224-249

The development of an adaptive (in space and time) ocean model from an existing adaptive finite-volume Navier–Stokes model is described. A flexible and efficient quadtree spatial discretisation is used which requires collocation of all variables (i.e. an A-grid discretisation). We demonstrate that the use of an approximate projection method allows for implicit damping of instabilities generally associated with the A-grid, at the expense of a relatively small amount of numerical energy dissipation, while accurately preserving dispersive properties and geostrophic balance. The finite-volume formulation also maintains second-order spatial accuracy at all solid boundaries. Test cases demonstrate the efficacy of the adaptive ocean model, and the advantages it has in terms of efficient representation of multi-scale behaviour within a single model. The model is freely available as open-source code.  相似文献   

Covellite formation in low temperature aqueous solutions     

D. T. Rickard 《Mineralium Deposita》1972,7(2):180-188

Experimentation with cupric salts and aqueous sulphide solutions at room temperature and pressure resulted in the formation of normal and blaubleibender covellite. Blaubleibender covellite is formed at higher pH and lower Eh values than normal covellite. The experimental pH/Eh values for normal covellite formation fall within the theoretical CuS stability field. Blaubleibender was produced at pH/Eh values outside this area. Variations in pH and Eh during the course of the experimental runs showed that normal covellite formed by the simple reaction: Cu²⁺+HS^–=CuS+H⁺; blaubleibender covellite formation on the other hand involves solid state reduction of an initial normal covellite precipitate. Cell volume and formation pH show a straight line relationship in normal covellite which is not observed with blaubleibender. Blaubleibender covellite can be formed from aqueous solution at low temperatures and pressures. The experimental results indicate that it is a metastable intermediary in the reduction of normal covellite to more reduced, stable, copper sulphides. The copper sulphide formed from sedimentary processes in a normal marine environment should initially be normal covellite, or transitory blaubleibender covellite which may be reduced during diagenesis.

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Zusammenfassung Synthetische Versuche mit Kupfersalzen und wässrigen Sulfidlösungen bei Zimmertemperatur und atmosphärischem Druck ergaben die Bildung von normalem und blaubleibendem Covellin. Blaubleibender Covellin wird bei höheren pH- und niedereren Eh-Werten gebildet als normaler Covellin. Die pH/Eh-Werte für normalen Covellin fallen in das theoretische Stabilitätsfeld von CuS. Die Bildung von blaubleibendem Covellin erfolgte bei pH/Eh-Werten außerhalb dieses Feldes. Systematische Veränderungen der pH- und Eh-Werte während der Syntheseversuche ließen erkennen, daß die Bildung von normalem Covellin nach folgender einfacher Reaktion verläuft: Cu²⁺+HS^–=CuS+H⁺; die Bildung von blaubleibendem Covellin dagegen erfolgt unter teilweiser Reduktion eines vorher gefällten Niederschlags von normalem Covellin. Im Gegensatz zum blaubleibenden Covellin zeigt der normale Covellin eine geradlinige Beziehung zwischen Zellvolumen und Formations-pH-Wert. Blaubleibender Covellin kann aus wässrigen Lösungen bei niederen Temperaturen und niederem Druck gebildet werden. Die Untersuchungsergebnisse zeigen, daß er eine metastabile Zwischenform zu normalem Covellin und stärker reduzierten stabilen Kupfersulfiden bildet. Die Kupfersulfide, die sich während eines Sedimentationsprozesses in einer normalen marinen Facies bilden, werden anfänglich aus normalem Covellin bestehen oder vorübergehend aus blaubleibendem Covellin, mit einer möglichen Reduktion während der Diagenese.

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