Depletion Of Mg Research Articles

Phthalic acid complexation and the dissolution of forsteritic glass studied via in situ FTIR and X-ray scattering

Multiple Internal Reflection Fourier Transform Infra-Red (MIR-FTIR) spectroscopy was developed and used for in situ flow-through experiments designed to study the process of organic acid promoted silicate dissolution. In tandem with the FTIR analysis, ex situ X-ray scattering was used to perform detailed analyses of the changes in the surface structure and chemistry resulting from the dissolution process. Phthalic acid and forsteritic glass that had been Chemically Vapour Deposited (CVD) onto an internal reflection element were used as reactants, and the MIR-FTIR results showed that phthalic acid may promote dissolution by directly binding to exposed Mg metal ion centers on the solid surface. Integrated infrared absorption intensity as a function of time shows that phthalic acid attachment apparently follows a t 1/2 dependence, indicating that attachment is a diffusive process. The diffusion coefficient of phthalic acid was estimated to be approximately 7 × 10 −6 cm 2 s −1 in the solution near the interface with the glass. Shifts in the infrared absorption structure of the phthalate complexed with the surface compared to the solute species indicate that phthalate forms a seven-membered ring chelate complex. This bidentate complex efficiently depletes Mg from the glass surface, such that after reaction as much as 95% of the Mg may be removed. Surface depletion in Mg causes adsorbate density to fall after an initial attachment stage for the organic ligand. In addition, the infrared analysis shows that silica in the near surface polymerizes after Mg removal, presumably to maintain charge balance. X-ray reflectivity shows that the dissolution rate of forsteritic glass at pH 4 based on Mg removal in such flow-through experiments was equal to 4 × 10 −12 mol cm −2 s −1 (geometric surface area normalized). Reflectivity also shows how the surface mass density decreases during reaction from 2.64 g cm −3 to 2.2 g cm −3, consistent with preferential loss of Mg from the surface. Auxiliary batch experiments with forsteritic glass films deposited onto soda glass were also completed to add further constraints to the mechanism of reaction. By combining reflectivity with diffuse scatter measurements it is shown that the primary interface changes little in terms of atomic-scale roughness even after removal of several hundred angstroms of material. These measurements unequivocally show how a dicarboxylic acid bonds to and may chelate the dissolution of a magnesium-bearing silicate. At the molecular level the solid surface retreat may best be described by a depinning model where Mg is preferentially removed and residual silica tetrahedra polymerize and act to episodically “pin” the surface.

Chemical and physical transfers in an ultramafic rock weathering profile: Part 1. Supergene dissolution of Pt-bearing chromite

Chemical weathering and supergene dissolution processes of Pt-bearing chromite have been studied in a lateritic weathering profile developed on ultramafic rocks in New Caledonia (southwest Pacific). The chemical distributions of alkaline earth, transition metals, and precious metals (including Pt and Pd) were determined in a weathering profile varying from bedrock at the base upward through coarse and fine saprolites, and capped by a mottled zone and a lateritic colluvial nodular horizon. Chemi- cal analyses and mass-balance calculations suggest that progressive weathering of the parent rock is characterized by an enrichment of Fe, Co, and Mn, a segregation of Ni at the boundary between the bedrock and the coarse sapolite and in the lower part of the fine saprolite, and a depletion of Mg, Ca, Si, Al, and Cr. The higher concentration of transition metals at the interface between the coarse and fine saprolite is due to vertical transfer and precipitation at the base of the weathering profile. In such a lateritic environment, the Pt-bearing chromite grains are progressively dissolved and the Pt-group minerals (PGM) are released in the weathering mantle with a preferential depletion of Pd with regard to Pt.

Mass Changes during Hydrothermal Alteration/Mineralization at the Sar-Cheshmeh Porphyry Copper Deposit, Southeastern Iran

The Sar-Cheshmeh porphyry copper deposit is associated with a Miocene dioritic to granodioritic pluton that intruded Eocene volcano-sedimentary and related rocks. Copper mineralization was accompanied mainly by phyllic, and to a lesser extent potassic, alteration. These hydrothermal alteration episodes were studied in terms of mass transfer and element mobility. Isocon plots illustrate these changes quantitatively. Al, Ti, and Nd were relatively immobile, and mass was essentially conserved during alteration. At all stages in the evolution of the hydrothermal system, the volume change was inappreciable. In the potassic alteration zone, enrichment of K and depletion of Mg, Ca, and Fe took place. These changes attended replacement of plagioclase and amphibole by Na- and/or K-feldspar and biotite, respectively. Potassic alteration was associated with a major addition of Cu, as evident from the occurrence of disseminated chalcopyrite and bornite in this zone. Phyllic alteration was accompanied by depletion of Na, K, Mg, Rb, and Ba and enrichment of Si and Cu. Losses of Na, K, and Fe reflect sericitization of alkali feldspar and destruction of ferromagnesian minerals. The addition of Si is consistent with widespread silicification, which is a major feature of phyllic alteration, as well as the addition of Cu mobilized from the potassic zone.

Regulation of Na +/Mg 2+ antiport in rat erythrocytes

In rat erythrocytes, the regulation of Na +/Mg 2+ antiport by protein kinases (PKs), protein phosphatases (PPs), intracellular Mg 2+, ATP and Cl − was investigated. In untreated erythrocytes, Na +/Mg 2+ antiport was slightly inhibited by the PK inhibitor staurosporine, slightly stimulated by the PP inhibitor calyculin A and strongly stimulated by vanadate. PMA stimulated Na +/Mg 2+ antiport. This effect was completely inhibited by staurosporine and partially inhibited by the PKC inhibitors Ro-31-8425 and BIM I. Participation of other PKs such as PKA, the MAPK cascade, PTK, CK I, CK II, CAM II-K, PI 3-K, and MLCK was excluded by use of inhibitors. Na +/Mg 2+ antiport in rat erythrocytes can thus be stimulated by PKCα. In non-Mg 2+-loaded erythrocytes, ATP depletion reduced Mg 2+ efflux and PMA stimulation in NaCl medium. A drastic activation of Na +/Mg 2+ antiport was induced by Mg 2+ loading which was not further stimulated by PMA. Staurosporine, Ro-31-8425, BIM I and calyculin A did not inhibit Na +/Mg 2+ antiport of Mg 2+-loaded cells. Obviously, at high [Mg 2+] i Na +/Mg 2+ antiport is maximally stimulated. PKC α or PPs are not involved in stimulation by intracellular Mg 2+. ATP depletion of Mg 2+-loaded erythrocytes reduced Mg 2+ efflux and the affinity of Mg 2+ binding sites of the Na +/Mg 2+ antiporter to Mg 2+. In non-Mg 2+-loaded erythrocytes Na +/Mg 2+ antiport essentially depends on Cl −. Mg 2+-loaded erythrocytes were less sensitive to the activation of Na +/Mg 2+ antiport by [Cl −] i.

The distribution of arsenic in soils affected by the Aznalcóllar mine spill, SW Spain

In April 1998, a holding lagoon containing pyrite ore processing waste, failed and released an estimated 5–6 million m 3 of metal rich sludge and acidic waters into the Rio Guadiamar, SW Spain. Over 2700 ha of the internationally important Doñana National and Natural Parks were contaminated. The sludge component of the waste contained up to 0.6% arsenic. This paper presents an extensive set of 0–5 cm soil analyses results from samples taken 6–8 months after the spill. Data are presented on pseudo-total arsenic levels in these samples, and on arsenic removed by both single batch and sequential extraction techniques. Pseudo-total levels of arsenic in the sludge ranged from 1521 to 3510 mg kg −1, and a mean 4.4% of this was found to be extractable using 2.5% acetic acid. Soils in the Guadiamar Valley and Entremuros areas (those worst affected by the spill) were found to contain 85.4–782 mg kg −1 and 7.1–196 mg kg −1 pseudo-total arsenic, respectively. The mean acetic acid extractable component in these areas was 2.5% and 4.9%, respectively. Background pseudo-total arsenic levels in the soils of the area have been found to be 4.2–13.6 mg kg −1. Rapid input of acidic waters, and the acidic nature of the sludge itself, may have caused depletion of Mg, Na and K, and to a lesser extent Mn, Ca and PO 4 in the upper 5 cm of the worst affected soils. Sequential extraction results suggest clear As–Fe associations, and possible As–Mn associations within the more soluble phases.

Chemistry of grain boundaries in mantle rocks

The compositions of olivine grain boundaries have been analyzed with scanning transmission electron microscopy (STEM) via energy dispersive X-ray (EDX) spectrum profiling in three specimens: a peridotite ultramylonite, olivine phenocrysts in a basaltic rock, and synthesized compacts of olivine + diopside. Composition profiles across grain boundaries in both natural and synthetic samples exhibit a characteristic width of 5 nm and a depletion of Mg and concomitant enrichments of Ca, Al, Ti, and Cr. Chemical segregation is known to affect grain boundary processes such as grain boundary diffusion, sliding, fracture, and migration, all of which influence the rheological properties of polycrystalline aggregates. Also, because grain boundaries are enriched in trace elements, the boundaries can be important storage sites for such elements in mantle rocks. Mantlederived melts with unusual compositions, such as those rich in Ca and/or Ti, might be explained by preferential melting of olivine grain boundaries enriched in these elements. The common chemical signatures at grain boundaries in all samples indicate that chemical segregation is an energetically favorable phenomenon and thus should occur elsewhere in Earth’s mantle. Segregation of trace elements to grain boundaries may play an important role in dynamical and geochemical processes in Earth’s mantle.

Abundance Anomalies in the X‐Ray Spectra of Planetary Nebulae NGC 7027 and BD +30o3639

We revisit Chandra observations of planetary nebulae NGC 7027 and BD +30°3639 in order to address the question of abundance anomalies in the X-ray-emitting gas. Enhanced abundances relative to solar of magnesium (Mg) for NGC 7027 and neon (Ne) for BD +30°3639 are required to fit their X-ray spectra, whereas observations at optical and infrared wavelengths show depleted Mg and Ne in these systems. We attribute the enhancement of Mg in NGC 7027 in the X-ray, relative to the optical, to the depletion of Mg onto dust grains within the optical nebula. For BD +30°3639, we speculate that the highly enhanced Ne comes from a white dwarf companion, which accreted a fraction of the wind blown by the asymptotic giant branch progenitor and went through a novalike outburst that enriched the X-ray-emitting gas with Ne.

Characterization of oxide layers on amorphous Mg-based alloys by Auger electron spectroscopy with sputter depth profiling.

Amorphous ribbons of Mg-Y-TM-[Ag] (TM: Cu, Ni), prepared by melt spinning, were subjected to electrochemical investigations. Oxide layers formed anodically under potentiostatic control in different electrolytes were investigated by AES and sputter depth profiling. Problems and specific features of characterization of the composition of oxide layers and amorphous ternary or quaternary Mg-based alloys have been investigated. In the alloys the Mg(KL(23)L(23)) peak exhibits a different shape compared to that in the pure element. Analysis of the peak of elastically scattered electrons proved the absence of plasmon loss features, characteristic of pure Mg, in the alloy. A different loss feature emerges in Mg(KL(23)L(23)) and Cu(L(23)VV). The system Mg-Y-TM-[Ag] suffers preferential sputtering. Depletion of Mg and enrichment of TM and Y are found. This is attributed mainly to the preferential sputtering of Mg. Thickness and composition of the formed oxide layer depend on the electrochemical treatment. After removing the oxide by sputtering the concentration of the underlying alloy was found to be affected by the treatment.

Pan-African Magmatism: Geochemical Evolution and Uranium Mineralization of Granitoid Rocks, Southeastern Desert, Egypt

Granitoid rocks of the Wadi Um Geir-Wadi Allaqi (WUGA) area represent a Neoproterozoic calc-alkaline magmatic episode in the Southeastern Desert of Egypt. The rocks range in composition from high-Ca (>1 wt%) metaluminous to low-Ca (<1 wt%) peraluminous granites. The former are compatible with a volcanic-arc environment, whereas the latter are related to within-plate A-type granites. The WUGA granites are highly fractionated and enriched in Rb, Nb, Y, and REE except for Eu, with corresponding depletions in Mg, Fe, Ti, Ga, Sr, and Ba. REE fractionation patterns and Eu/Eu* values decrease from high-Ca metaluminous granite (La/Ybn = 2.46 - 4.68; Eu/Eu* = 0.32 - 0.73) to low-Ca peraluminous granite (La/Ybn = 0.37 - 1.23; Eu/Eu* = 0.012 - 0.032, respectively). Major-element modeling suggests that the granitic types are products of a single stage of crystal fractionation. The low-Ca peraluminous granite formed by extreme fractionation of the high-Ca metaluminous granite liquid, with plagioclase, K-feldspar, and biotite as the dominant fractionating phases. Radioactivity in the WUGA, attributed to the presence of thorium and uranium, is associated with the younger granites, especially within shear zones, fault planes, and pegmatitic bodies. The accessory assemblage of peraluminous granites consists of monazite, xenotime (in low-Ca varieties), fluorite, garnet, apatite, zircon, tantalite, columbite, Th-orthosilicate (huttonite), umohoite, uranophane, β-uranophane, and betafite-pyrochlore minerals. Metalumious granites contain allanite, titanite, apatite, zircon, minor monazite, and Th-orthosilicate (thorite). The genesis of secondary U minerals is mainly attributed to the action of oxidized groundwater on previously corroded primarly U minerals. These secondary uranium minerals were deposited near the surface by evaporation.

An attempt to model globular cluster red giant abundance anomalies with a simulated hydrogen shell instability

It has been suggested that the anomalous abundances of Na, Mg and Al observed in globular cluster red giant stars could be the result of a thermally unstable hydrogen shell. Currently accepted reaction rates indicate that temperatures of approximately 70-75 × 10 6 K are required to produce the observed enhancements in Na and Al along with depletions in Mg. The work presented here attempts to model the H shell instability by a simple mechanism of altering the energy production in the region of the H shell. We show that even extreme cases only give rise to small intermittent temperature increases that have minimal effect on the surface abundances. Full evolutionary modelling incorporating this technique simply accelerates the evolution of the red giant branch (RGB) phase, producing the same surface abundances as other models but at an earlier time. We conclude that, unless hydrogen shell instabilities manifest themselves quite differently, they are unlikely to lead to the required temperatures and alternative explanations of the abundance anomalies are more promising.

A STUDY OF THE EFFECT OF CHRYSOTILE FIBER SURFACE COMPOSITION ON GENOTOXICITY IN VITRO

Chrysotile fibers (NIEHS intermediate length) were treated with ultrapure HCl to alter the fiber surface chemistry without substantially changing fiber morphology or dimensions. The objective of the study was to determine whether fiber surface chemistry is an important variable in fiber genotoxicity in vitro. The modified fibers, along with native chrysotile fibers, were used to challenge Chinese hamster lung fibroblasts (V79) in vitro using the micronucleus induction genotoxicity assay. Fiber dimensions were assessed using scanning electron microscopy by measuring the distribution of fiber lengths in 3 length ranges: less than 3 mum, 3-10 mum, and greater than 10 mum. For both treated and native fiber samples, 500 fibers were examined. Results indicate that acid-treated fibers were about 20% shorter than untreated chrysotile. Surface chemistry alterations were verified by zeta-potential reversal, x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy/energy-dispersive x-ray spectroscopy (SEM-EDS) elemental analysis. Scanning Auger spectrometry indicated the presence of Mg, O, and Si in both treated and native chrysotile samples, which confirmed the surface purity of both fiber samples. Both XPS and SEM-EDS analysis demonstrated substantial depletion of Mg from fiber surfaces. Results of the micronucleus assay showed a positive concentration-related response for both samples, with toxicity evident only at the highest concentration. No significant difference was found for the treated and untreated chrysotile samples. These results indicate that the surface chemistry is not an important variable in the in vitro genotoxicity of chrysotile asbestos in V79 cells as detected by the micronucleus assay under the conditions used in this study, and support a model of chemically nonspecific chromosomal and spindle damage effects.

The origin and evolution of Canadian Shield brines: evaporation or freezing of seawater? New lithium isotope and geochemical evidence from the Slave craton

New chemical and isotopic data for deep seated calcium chloride brine from the Miramar Con gold mine, Yellowknife N.W.T., strongly suggest that the brine salinity is of marine origin. Diagnostic marine properties include uniformly elevated Br/Cl ratios typical of seawater concentrated beyond halite saturation, and Li/Br ratios (0.0254–0.0325) and δ 6 Li compositions (−32.1 to −36.3‰) similar to seawater (−32.3‰). The mean δ 6 Li for all mine water samples of −35.1‰ may reflect minor uptake of Li by secondary minerals. This interpretation is supported by analyses of altered metabasalt from fault zones which is enriched in Li but depleted in δ 6 Li (−14.7 to −15.6‰) relative to the unaltered metabasalt (−5.4‰). The mechanism responsible for concentrating the hypersaline brine end member is not unequivocal as evidence exists to support both evaporative and cryogenic processes. On the one hand, the Devonian sedimentary record in the Great Slave Lake region, in conjunction with Yellowknife brine isotopic compositions ( δ 2 H and δ 34 S SO 4 ) that are similar to various Devonian fluids, support an evaporative origin. On the other hand, the Na/Cl–Br/Cl relationship in the brine strongly suggests a cryogenic mechanism. Regardless of the concentrative mechanism, the chemical data indicate that the Yellowknife parent brine was concentrated 28- to 30-fold relative to seawater. The extreme depletion of Mg and enrichment of Ca in the brines, accompanied by Sr/Ca ratios similar to that of seawater, are accounted for by dolomitization of an aragonite-rich marine sediment by the brine before infiltration into the crystalline basement rocks. Subsequent alteration of silicate minerals in the shield added additional Ca and Sr to the brine as indicated by their radiogenic 87 Sr/ 86 Sr ratios (up to 0.7147). Based on mineral balance calculations, the major mineral products of the cryogenic and evaporitic concentration and evolution paths are significantly different. The cryogenic evolution results in some 15% mirabilite, 60% hydrohalite, and 18% dolomite whereas the major minerals formed from the evaporitic evolutionary sequence are 36% halite, 8% gypsum, 17% dolomite, and 30% albite. The great similarity between the calcium chloride brine from Yellowknife and other such Canadian Shield brines indicates that they may share a common marine origin.

Boron isotope and geochemical evidence for the origin of Urania and Bannock brines at the eastern Mediterranean: effect of water-rock interactions

The origin of hypersaline brines from Urania and Bannock deep anoxic basins in the eastern Mediterranean Sea has been investigated by integrating geochemical data and boron isotopic ratios. Bottom brines from Urania basin have chloride contents up to 4200 mmole/kg H 2O and a marine Na/Cl ratio (0.87). All the other ionic ratios are different from the marine ratios and show a relative enrichment in Ca, K, Br, and B and depletion in Mg and SO 4, as normalized to the chloride ion. The δ 11B values of the Urania brines (δ 11B = 29.8 ± 2.9‰; n = 7) are lower than that of Mediterranean seawater (39‰). The concentrations of Cl and Na, which make up 95% of the total dissolved ions (in molal units), suggest that the Urania brines were derived from eightfold evaporated seawater. The relative enrichment of Ca and depletion of Mg and SO 4 reflect dolomitization, gypsum precipitation, and sulfate reduction processes which modified the original evaporated seawater while the brines were entrapped as interstitial waters in the sedimentary section of the Mediterranean. The relative enrichments of Br, B, and K, and the low δ 11B value of the Urania brines suggest high-temperatures interactions of the evaporated sea water with sediments. Mass-balance calculations suggest that desorption of exchangeable B from the sediments (δ 11B ∼ 20‰) modified the marine B isotopic composition of the original eightfold evaporated seawater. Potassium was also leached from clay minerals whereas Br was contributed from degradation of organic matter in the sediments. This is consistent with a thermal anomaly (up to 45°C) recorded at depth in the region of Urania basin. In contrast, bottom brines and shallow interstitial fluids from Bannock basin with low temperatures (15°C) show marine δ 11B (δ 11B = 39.6 ± 2.8‰; n = 5; 38.5 ± 2.2‰; n = 5, respectively) and B/Cl ratios (7 × 10 −4). The B isotope data confirm that the Bannock brines were derived from twelvefold evaporated seawater. We argue that the brines from both basins are relics of fossil evaporated seawater that was entrapped in Late-Miocene sediments and accumulated in the deep basins of the Mediterranean seafloor.

Comparison of the Electromigration Behavior of Al(MgCu) with Al(Cu) and Al(SiCu)

ABSTRACTMg is one of the elements that are regarded as having a beneficial effect on the Al electromigration (EM) lifetime. In this paper, we compare the EM behavior of 0.4 μm wide passivated Al-1%Mg-0.5%Cu, Al-0.5%Cu and Al-l%Si-0.5%Cu lines. Plan-view transmission electron microscopy and focused ion beam imaging reveal that Al(MgCu) film undergoes bimodal grain growth. Auger electron spectroscopy and secondary ion mass spectrometry show a strong surface segregation and a severe bulk depletion of Mg. Additionally the line-width dependence of the rate of the resistivity decay during aging shows also a different behavior for Al(MgCu) compared to Al(Cu) and Al(SiCu). All these findings are consistent with the EM results that Al(MgCu), processed with our experimental conditions, has both the lowest median time to failure and deviation in time to failure. The results are discussed in the light of the effect of Mg addition on the microstructure and of the great surface activity of Mg.

Post-collision, A-type granites of Homrit Waggat Complex, Egypt: petrological and geochemical constraints on its origin

The Homrit Waggat Complex (HWC) is a ring complex which belongs to the younger granite province of the Nubian Shield. It comprises a central biotite granite encircled by an intermediate ring of mylonitized pink granite. both of which have been emplaced by cauldron subsidence in a granodiorite body. The mylonitized pink granite is locally cut across by hypersolvus coarse-grained red granite. The mylonitized pink granite is characterized by fully-developed mylonitic fabrics and suffered inflation and flattening due to a ‘ballooning effect’ which suggests that it has experienced stronger ductile deformation than the granodiorite and biotite granite. The HW granites are mildly peraluminous, highly fractionated and enriched in Rb, Nb. Y, REE (except Eu) and F with corresponding depletion in Mg, Fe, Ti, Ca, Sr and Ba. REE fractionation patterns and Eu Eu∗ values decrease from granodiorite ( La Yb n = 6.24−6.69 ; Eu Eu∗ = 0.95−1.56 ) to biotite granite ( La Yb n = 2.48−4.68 ; Eu Eu∗ = 0.25−0.75 ). The mylonitized pink and red granites display flat patterns and large negative Eu anomalies ( La Yb n = 0.33−1.09 ; Eu Eu∗ = 0.01−0.07 , respectively). Although accessory minerals (e.g. apatite, allanite and fluorite) exerted some control on REE concentrations, volatile transfer largely controls REE abundances, particularly HREE (heavy rare-earth element). Geochemical data indicate that the granites of HWC are post-collision, A-types. The less fractionated granodiorite has an I-type and volcanic arc geochemical signature. Least-squares analyses of major elements and modelling of REEs indicate that the HW A-type granites were formed through two stages of differentiation. A first stage, crystalmelt fractionation, was dominated by the removal of plagioclase, biotite, quartz and apatite from the granodiorite magma to yield the biotite granite. Further crystal fractionation of the biotite granite liquid with K-feldspar as a dominant separating phase in addition to plagioclase, biotite and quartz gave rise to the mylonitized pink granite (second stage). Volatile phase transfer occurred during the second stage of fractionation in the magma chamber. The red granite represents the last crystallizing granite type from a residual, volatile-rich fluids which accumulated and emplaced as ring dykes in the solidified part of the mylonitized pink granite.

Supergene alteration since the upper cretaceous on alkaline igneous and metasomatic rocks of the Poços de Caldas ring complex, Minas Gerais, Brazil

This contribution presents field evidence, mineralogical observations and geochemical data of different modes of lateritic weathering which affected the alkaline magmatic rocks and their metasomatic alteration products since the Upper Cretaceous. Field relations demonstrate that ferralitic (bauxitic) weathering is confined to well-drained hill crest and upper slope settings. In the Poços de Caldas area bauxite normally rests directly on parent rock. Stagnant drainage and reducing conditions beneath tropical peat deposits, along valley floors and in lower slope situations favour sialitic (saprolitic) weathering. The additional importance of high porosity and permeability is demonstrated by co-occurring parent rocks with strongly contrasting response on weathering attack. In normal nepheline syenites and phonolites nepheline breaks down first and nearly instantly upon weathering. This results in high porosity, giving water and atmospheric gases easy access to the more resistant minerals. Thus in well-drained morphological positions bauxite is generated. If the same original magmatic parent rocks are “shielded” against pervasive weathering attack by pre-lateritic metasomatic alteration, no comparable early permeability arises. A fine-grained and dense metasomatic assemblage of kaolinite, illite, microcline and smectite, combined with the annealing of fractures and joints, is able to prevent bauxite generation, and saprolite is formed instead. This metasomatically induced saprolite is significantly richer in illite than “normal” saprolite. Bauxite formation results in a strong depletion of Mg, Ca, Na, K, Si, Ni, Co, Cu, Zn, Rb, Ba, Sr, Y and REE. Sialitic (saprolitic) weathering leads to a generally less pronounced depletion of essentially the same elements. Silicon and Ni are more efficiently retained than in bauxite. Mobile elements are Mn, Ca, Na, K, Mg, Co, Zn, Rb, Ba, Sr and REE. Cerium can be re-precipitated under oxidizing local conditions. Closely associated positive and negative Ce-anomalies are due to inhomogenously concentrated precipitates in small-scale spatial alternation with zones of nearly total depletion. A general correlation between the type of laterite (saprolite or bauxite) and Ce-retainment or depletion does not exist. Host phases for most of the immobile trace elements are goethite and related plasmas in bauxites and saprolites. Immobility is exhibited by V, Pb, Ga, Nb, Zr, Th and U. Relict zircon is too rare to explain a seemingly retarded HREE-loss in saprolites.

Hubble Space Telescope Observations of Interstellar Lines in Three High-Latitude Stars

view Abstract Citations (17) References (68) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hubble Space Telescope Observations of Interstellar Lines in Three High-Latitude Stars Jenkins, Edward B. ; Wallerstein, George Abstract To study changes in the gas-phase abundances of elements in the Galactic halo, we have used the Goddard High Resolution Spectrograph instrument on the Hubble Space Telescope to observe interstellar absorption lines of O I, Mg II, Al II, Al III, Si II, S II, Fe II, and Ni II in the spectra of three stars at high Galactic latitudes: HD 22586, HD 49798, and HD 120086. Recognizing that we would probably observe a decrease in the amount of depletions of certain heavy elements onto dust grains, we also made comparison observations of HD 72089 behind the Vela supernova remnant. This star has a number of high-velocity gas components that have been subjected to strong shocks, allowing us to determine whether the patterns of abundances, in a medium where the grains have been partially destroyed, had any resemblance to abundances in the halo. When we compare various element abundances relative to that of sulfur, a normally undepleted element, we find that as a rule the element depletions of gas toward the halo stars are intermediate between high- and low-velocity gas in the spectrum of HD 72089. Exceptions to this are the extraordinarily light depletions of Si toward HD 22586 and HD 120086 (-0.1 and 0.0 dex, respectively), and a very mild depletion of Mg in HD 120086 (-0.2 dex). We see no evidence that iron-peak elements are enhanced relative to the others (beyond the effects of changes in depletion), as one might expect if the ejecta from Type Ia supernovae were enriching the gas in the halo. It therefore seems apparent that either there is not enough change in the Type II supernovae to Type Ia supernovae ratio from the plane to the halo to produce a noticeable enrichment, or else one or more gas transport processes between the disk and halo, such as mass exchange in a Galactic fountain or the general turbulence of the interstellar medium, are sufficiently strong to cycle material between the disk and halo at a rate that can stifle any abundance contrasts. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177190 Bibcode: 1996ApJ...462..758J Keywords: ISM: DUST; EXTINCTION; GALAXY: HALO; ISM: ABUNDANCES full text sources ADS | data products SIMBAD (11) MAST (1) ESA (1)

Protein phosphorylation and magnesium status regulate the degradation of the D1 reaction centre protein of Photosystem II

Illumination of higher plant leaves (pumpkin, wheat and pea) at a photon flux density of 1200 μmol m −2 s −1 induced strong phosphorylation of the D1 reaction centre protein of photosystem II. Phosphorylation of D1 protein was not observed in a moss Ceratodon purpureus. To study the effect of protein phosphorylation on D1 degradation, thylakoids isolated from pumpkin leaves and Ceratodon were phosphorylated in vitro in the presence of ATP before being subjected to photoinhibitory illumination. The rate of photoinhibition of Photosystem II oxygen evolution was not influenced by phosphorylation conditions in pumpkin or Ceratodon thylakoids. D1 protein in its phosphorylated form in pumpkin thylakoids was degraded significantly more slowly whereas in Ceratodon the degradation of D1 protein occurred rapidly with or without the presence of ATP in the photoinhibitory medium. Depletion of Mg 2+ ions also blocked the degradation of unphosphorylated D1 protein in pumpkin thylakoid membranes. We conclude that degradation of the D1 protein in higher plants is regulated by phosphorylation on the substrate level and its optimal rate requires the presence of Mg 2+.

INTERFACE STRUCTURE AND MECHANICAL PROPERTIES OF POWDER METALLURGICAL 6061 ALUMINUM MATRIX COMPOSITES REINFORCED WITH VARIOUS KINDS OF OXIDE WHISKERS

The compatibility of oxide whiskers with 6061Al alloy and the age hardening behavior of composites prepared from powder metallurgy were investigated. Interface reactants containing Mg were shown to be formed during hot pressing and hot extruding for every composite. The composites did not reveal age hardening behavior due to the depletion of Mg in the matrix at all. To keep whiskers from the interfacial reaction, electroless nickel plating onto whiskers was carried out. Although the composite with Ni plated whiskers revealed the age hardening, its hot workability was too poor to apply practical fabrication because of the formation of brittle intermetallic compound Al3Ni in the matrix.

Alcohols induce rapid depletion of intracellular free Mg 2+ in cerebral vascular muscle cells: relation to chain length and partition coefficient

Acute effects of a series of alcohols (methanol, ethanol, n-butanol) on intracellular free magnesium concentration ([Mg 2+] i in canine cerebral vascular smooth muscle cells was studied using mag-fura-2 and digital imaging microscopy. In 1.2 mM [Mg 2+] o, basal [Mg 2+] i was 500 ± 30 μM. Exposure of cells to a low concentration (25 mM) of ethanol, but not methanol, for only 30 s resulted in significant loss of [Mg2 2] i. Exposure to 100 mM methanol, ethanol, and butanol for 30 s resulted in a relative order of potency for [Mg 2+] i depletion, where butanol ⪢ ethanol > methanol. The heterogeneous and relative subcellular compartmented concentrations of [Mg 2+] i, where perinuclear > nuclear ⪢ peripheral (cytosolic) region, was not significantly altered by the alcohols. The degree of cellular depletion of [Mg 2+] i; was directly a function of each alcohol's partition coefficient and chain length. The latter is suggestive of the probability that alcohols promote intracellular depletion of Mg 2+ by partitioning in membranes and disordering lipid bilayers.