bluuebunny/crossref_metadata_2025 huggingface dataset:
Syntheses of Disordered and Al-Rich Hydrotalcite-Like Compounds
Authors: Pausch I. , Lohse H.-H. , Schürmann K. , Allmann R.
Published: September 2006

Hydrotalcite-like compounds, [Mg1-xAlx(OH)2]x+ [xX−·n H2O], where X− = ½CO32− or OH−, were prepared by hydrothermal syntheses at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:c="http://cambridge.org/core/content" xmlns:core="http://cambridge.org/core" xmlns:m="http://cambridge.org/core/metadata" xmlns:cup="http://contentservices.cambridge.org" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest">PH2O=100MPa and T = 100°–350°C. Starting materials were MgO, γ-Al2O3, H2O, and MgC2O4·2H2O. The synthesis depended on temperature, pressure, the Al/(Al + Mg) ratio x, and the CO2 content of the starting material. Previously an Al content of x = 0.33 was thought to be the upper limit in these double-layer compounds, but by using pressure the Al-content was increased to x = 0.44. Up to x = 0.33, a0 decreased linearly to about 3.04 Å, but for x ≥0.33, a0 remained nearly constant at this value. For the synthesized products the layer thickness c’ varied between 7.40 and 7.57 Å in contrast to the natural phases wherein c’ varies from 7.60 to 7.80 Å. At higher temperatures CO2-free syntheses, i.e., those without Mg-oxalate, resulted in a disordered hydrotalcite-like phase. The transition temperature between the ordered and the disordered hydrotalcite-like phase depended on the Al-content, x.
Ion Exchange, Thermal Transformations, and Oxidizing Properties of Birnessite
Authors: Golden D. C. , Dixon J. B. , Chen C. C.
Published: September 2006

Synthetic sodium bimessite, having a cation-exchange capacity (CEC) of 240 meq/100 g (cmol/kg) was transformed into Li, K, Mg, Ca, Sr, Ni, and Mn2+ cationic forms by ion exchange in an aqueous medium. Competitive adsorption studies of Ni and Ba vs. Mg showed a strong preference for Ni and Ba by bimessite. The product of Mg2+-exchange was buserite, which showed a basal spacing of 9.6 Å (22°C, relative humidity (RH) = 54%), which on drying at 105°C under vacuum collapsed to 7 Å. Of the cation- saturated bimessites with 7-Å basal spacing, only Li-, Na-, Mg-, and Ca-bimessites showed cation exchange.

Heating bimessite saturated with cations other than K produced a disordered phase between 200° and 400°C, which transformed to well-crystallized phases at 600°C. K-exchanged bimessite did not transform to a disordered phase; rather a topotactic transformation to cryptomelane was observed. Generally the larger cations, K, Ba, and Sr, gave rise to hollandite-type structures. Mn- and Ni-bimessite transformed to bixbyite-type products, and Mg-bimessite (buserite) transformed to a hausmannite-type product. Li-bimessite transformed to cryptomelane and at higher temperature converted to hausmannite. The hollandite-type products retained the morphology of the parent bimessite. The mineralogy of final products were controlled by the saturating cation. Products obtained by heating natural bimessite were similar to those obtained by heating bimessite saturated with transition elements.
Effect of Ambient Atmosphere on Solid State Reaction of Kaolin-Salt Mixtures
Authors: Gábor M. , Pöppl L. , Körös E.
Published: September 2006

The reaction of kaolin with NaCl was followed by dynamic thermal analysis and mass spectrometry under N2, CO2, and air atmospheres and in a 10−5-torr vacuum. The weight loss was a function of the atmosphere used and, according to mass spectrometry, was due to the evolution of H2O, HCl, and very small amounts of H2. HCl was formed only after the release of 85% of the hydroxyl content of the kaolin. When the clay was pretreated with saturated salt solution, H2O and HCl evolved in more or less the same temperature range, indicating that only some of the OH groups reacted with the chloride ion. High-temperature X-ray powder diffraction patterns showed that the sodium ion reacted with the noncrystalline metakaolin to give NaAlSiO4. Chemical analysis showed that the reaction of kaolinite and sodium chloride started below 400°C. The rate of the reaction increased at higher water vapor concentration. From mass spectrometric data, the NaCl-treated kaolin appeared to adsorb CO2. Desorption at several distinct temperatures suggests that CO2 was adsorbed by different parts of the structure, i.e., holes and channels. X-ray powder diffraction and infrared absorption data indicate that the kaolinite structure persisted even after it had been heated with NaCl in a CO2 atmosphere to as high as 800°C.
Chemistry of Detrital Biotites and Their Phyllosilicate Intergrowths in Sandstones
Authors: Aldahan Ala Adin , Morad Sadoon
Published: September 2006

Microprobe analyses of optically homogeneous detrital biotites from sandstones of the Visingsö Group and the Dala Sandstone, Sweden, revealed a consistently low K content (generally <0.75 atom/formula unit) and variable amounts of Fe, Mg, Al, and Si. Electron probe profiles of some biotite grains indicated two major types of interstratification, one consisting of mainly illite layers and the other apparently consisting of chlorite layers. The layer thicknesses commonly ranged between 0.5 and 3 μm. Microprobe analyses of some thick (∼5 μm) illitic layers indicated a phengitic composition, wherein the mica was relatively rich in octahederal Fe and Mg. The chloritic layers appeared to be Fe-Mg-rich and generally had octahederal totals of <6 atom/formula unit. Variations in the chemical composition of the biotite and some of the illite and chlorite were probably due to an uneven distribution of small amounts of the interstratified phases. The illite and chlorite layers were apparently formed by pseudomorphic replacement of detrital biotites, i.e., gradual replacement of one biotite layer by one layer(s) of illite and/ or chlorite during diagenesis.
Factors That Govern the Formation of Multi-Domainic Goethites
Authors: Cornell R. M. , Giovanoli R.
Published: September 2006

Transmission electron microscopic (TEM) examination has shown that multi-domainic crystals of synthetic goethite consist of almost parallel intergrowths, each of which is slightly misoriented with respect to its neighbors. These intergrowths emanate from a central nucleus within the crystal. They can nucleate along both the x and y crystal axes, but subsequent growth is mainly in the z direction.

The formation of multi-domainic goethites from ferrihydrite was favored by high pH (≥ 13) and, at lower pHs, by the addition of NaNO3 to the system. Decreasing the temperature of synthesis from 70° to 20°C also enhanced domain formation. The nucléation of domains was confined to the initial stage of goethite formation. Domains probably formed when crystal growth was very rapid or when adsorbed species blocked the appropriate sites on the nucleus material.
Clay-Organic Complexes as Adsorbents for Phenol and Chlorophenols
Authors: Mortland Max M. , Shaobai Sun , Boyd Stephen A.
Published: September 2006

Several clay-organic complexes were synthesized by placing quaternary ammonium cations on smectite by cation exchange. They were then examined for their ability to adsorb phenol and several of its chlorinated congeners. The organic cations used were: hexadecylpyridinium (HDPY+), hexadecyltri-methyl ammonium (HDTMA+), trimethylphenyl ammonium (TMPA), and tetramethylammonium (TMA+). The complexes containing long-chain alkyl (hexadecyl) groups were the most hydrophobic and adsorbed the phenols from water in proportion to their hydrophobicities, which increase with chlorine addition (phenol < chlorophenol < dichloropohenol < trichlorophenol). With n-hexane as the solvent, different adsorption was found which depended on the type and degree of solvent interactions with the compound and the clay-organic complex. Thus, the amount of adsorption of these phenols on clay-organic complexes was dependent on the relative energies of adsorbent-adsorbate and adsorbate-solvent interactions.
Apparent and Partial Specific Sorption of Oxine by Montmorillonite and Silica Gel in Binary Mixtures
Authors: Helmy A. K. , Ferreiro E. A. , De Bussetti S. G.
Published: September 2006

Oxine sorption by binary mixtures of montmorillonite and silica gel was studied at pH 5 as a function of oxine concentration and mixture composition. The investigation was made to resolve the problem of the determination of the oxine sorbed by each substance, inasmuch as the total amount sorbed by the mixture is the only value accessible experimentally. By the application of the thermodynamic method of partial quantities, the apparent and partial specific sorption by the clay and by the gel were calculated. These sorption values were found to increase with oxine concentration and to decrease as the proportion of the clay increased in the mixtures.
Methylation of Sepiolite and Palygorskite with Diazomethane
Authors: Hermosín María C. , Cornejo Juan
Published: September 2006

The methylation of the fibrous clays, sepiolite and palygorskite, should be facilitated by their large content of surface silanol groups and the low acidity and inaccessibility of coordinated water molecules. Infrared spectroscopy showed that the reaction of diazomethane with these clays resulted in the methylation of their silanol surface groups. The grafting of CH3 groups on the surfaces of these clays produced a decrease in the surface area due primarily to the lowering of the microporosity.
Methyl t-Butyl Ether (MTBE) Production: A Comparison of Montmorillonite-Derived Catalysts with an Ion-Exchange Resin
Authors: Adams J. M. , Martin K. , McCabe R. W. , Murray S.
Published: September 2006

Montmorillonite-based catalysts were compared with an acidic ion-exchange resin of the type used industrially for the production of methyl t-butyl ether (MTBE) from methanol and isobutene or t-butanol. When 1,4-dioxan was used as solvent, Al3+-exchanged montmorillonites had about half the efficiency of the resin Amberlyst 15 at 60°C; they were, however, about twice as efficient at this temperature at Ti3+-montmorillonite or K10, a commercially available acid-treated bentonite. Montmorillonite exchanged with Chlorhydrol solutions to give interlayer [Al13O4(OH)2(H2O)12]7+ ions and pillared clays derived from such materials were poor catalysts, as was K306, a more drastically acid-treated bentonite- based commercial catalyst. Freeze-drying of the Al3+-clay before reaction to produce a more open, porous structure had no effect on its catalytic efficiency. The activation energy for the reaction of isobutene and methanol in dioxan was 44 kj/mole for an Al3+-clay catalyst compared with 25 kJ/mole for reactions catalyzed by Amberlyst 15. With no solvent (as in industrial processes), the rates of reaction were considerably slower for both the clay- and resin-catalyzed reactions. As has been found previously for resin-catalyzed reactions using stoichiometric amounts or an excess of methanol, the rate was proportional to the isobutene concentration, and the rate-determining step appeared to be protonation of the alkene. The performance of the Al3+-clay catalyst was increased by reducing the water content of the clay. In most reactions the clay catalysts were equilibrated at 12% relative humidity. Exposure of the clay to a low vacuum (10−1 torr) before use increased its catalytic activity from 50 to 60% of that of Amberlyst 15.
Dioctahedral Corrensite from Permian Red Beds, Lisbon Valley, Utah
Authors: Morrison S. J. , Parry W. T.
Published: September 2006

Regularly interstratified chlorite/smectite (corrensite) occurs as grain coatings in marine and eolian sandstones of the Permian Cutler Formation in Lisbon Valley, Utah. Corrensite dominates the <2-μm clay size fraction along with lesser amounts of regularly interstratified illite/smectite in bleached, permeable sandstones that are interbedded with smectite-dominated arkosic, red, laterally discontinuous fluvial sandstones. Chemical and X-ray powder diffraction analyses of the corrensites show them to be di/dioctahedral with 060 spacings of 1.501 to 1.508 Å and to contain significantly higher Al:Mg ratios than more common trioctahedral types. Calculated structural formulae are: corrensite, M0.6(Fe3+0.2Mg1.6Al4.6)(Al0.2Si7.8)O20(OH)10; illite/smectite, M1.5(Fe3+0.1Mg1.7Al2.8)(Al1.6Si6.4)O20(OH)4; and smectite, M0.9(Fe3+0.3Mg1.2Al2.8)(Al0.6Si7.4)O20(OH)4.

Chemical similarity between the smectite and the corrensite and pervasive distribution of smectite in low-permeability shales and siltstones suggest that the smectite was a precursor of the corrensite. Three stages of mineral precipitation in the Cutler Formation have been recognized. Quartz precipitated early as grain overgrowths, followed by the formation of authigenic clay minerals, and later calcite cementation which destroyed much of the original rock fabric. Calculations show that aluminous corrensite was favored by elevated temperature (∼ 100°C), low pH, and low dissolved silica. Local hydrothermal fluids rising along the Lisbon fault apparently permeated the Cutler red bed section and precipitated the clay minerals. The assemblage corrensite + illite/smectite in the sandstones probably formed by interaction of formation fluids with smectite and an Al-bearing phase, such as K-feldspar or kaolinite.
Relationship Among Derivative Spectroscopy, Color, Crystallite Dimensions, and Al Substitution of Synthetic Goethites and Hematites
Authors: Kosmas C. S. , Franzmeier D. P. , Schulze D. G.
Published: September 2006

Nine hematites and 22 goethites were synthesized by a variety of methods to obtain monomineralic samples having a range of Al substitutions and particle sizes. The second derivative of absorbance and Munsell color designations were calculated from visible reflectance spectra obtained from the dry powders. Unit-cell dimensions, Al substitution, infrared band positions, mean crystallite dimensions (MCD) from X-ray powder diffraction, and particle size from fiber-optic Doppler anemometry (FODA) were determined. Previously reported correlations between Al substitution, goethite unit-cell dimensions, and OH-stretching and -bending band positions were confirmed. For hematite, the position of the second derivative peak at ∼600 nm was negatively correlated with Al substitution (r = −.86). Munsell value and chroma were positively correlated with Al substitution (r =.94 for both), but hue was not related to Al substitution. Hue appeared to become redder, however, as particle size measured either by FODA or MCD increased. For goethite, the position of the second derivative minimum at ∼485 nm was negatively correlated with Al substitution (r = −.99). Munsell hue appeared to be related to both Al substitution and MCD perpendicular to (110), MCD110, with hues becoming redder with increasing Al substitution and yellower with increasing MCD110. Correlations between Munsell value and chroma and parameters such as Al substitution, particle size, and OH-stretching and -bending band positions were poor, but goethites synthesized by oxidation of Fe2+ solutions at room temperature had higher chromas than goethites synthesized hydrothermally from an Fe3+ system. Visually determined colors agreed well with calculated ones. Second-derivative spectra and color designations calculated from visible spectra appear to be potentially useful for quickly estimating other properties of goethite and hematite, such as Al substitution and particle size.
Effect of Adsorbed Iron on Thermoluminescence and Electron Spin Resonance Spectra of Ca-Fe-Exchanged Montmorillonite
Authors: Coyne Lelia M. , Banin Amos
Published: September 2006

The electron spin resonance (ESR) spectra and the natural and gamma-induced thermoluminescence (TL) glow curves of a series of variably cation-exchanged Fe-Ca-clays prepared from SWy-1 montmorillonite were examined. The ESR signal (g = 2) intensity associated with the surface Fe was found to increase linearly with surface Fe content up to a nominal concentration of 50% exchangeable Fe. At > 50% exchangeable Fe, no appreciable increase in the signal was noted. The TL intensity decreased linearly with increasing surface Fe up to 50% nominal exchangeable Fe. At > 50%, the signal was not appreciably further diminished. The natural TL showed only a high-temperature peak, but irradiation produced an additional low-temperature peak. One month after gamma-irradiation, the integrated TL signal was still 10–100 times higher than that from the non-irradiated material. Thus, (1) surface iron clusters may form above a certain critical Fe concentration; (2) the Fe clusters are probably less effective in quenching TL than are single Fe atoms, implying interaction between surface Fe and the stored energy content of the material; and (3) the electronic energy stored in the material as the result of gamma-irradiation is only slowly dissipated.
Compositional Variation in Component Layers in Natural Illite/Smectite
Authors: Velde B. , Brusewitz A. M.
Published: September 2006

Published chemical data for suites of mixed-layer minerals from diagenetic sedimentary rocks, hydrothermally altered tufts, and a metasomatic bentonite bed indicate that the layer charge and composition of the different components of illite/smectite (I/S) differ from one geological environment to another. It appears that the composition of the elemental smectite and illite layers in the I/S is more or less constant for samples within each geologic setting. In the examples considered, the smectite layers are predominantly montmorillonitic in character (i.e., the charge is in the octahedral site), whereas the illite layers show different types of charge sites, depending upon the suite studied. Illite layers appear to have about the same charge in all three suites studied, slightly more than 0.7 per O10(OH)2 unit, whereas the smectite layers in the different suites range in charge from about 0.3 to 0.7 per O10(OH)2 unit. Cation-exchange capacities reflect these differences in charge, although not ideally. The differences in the composition of the component layers in each geologic suite of mixed-layer clays are probably due either to differences in the bulk chemistry of the rocks in the different suites or to differences in intensive variables, such as temperature and pressure, of the regime under which they have formed.
Synthesis and Properties of Titanium Oxide Cross-Linked Montmorillonite
Authors: Sterte Johan
Published: September 2006

Titanium was introduced into the montmorillonite structure by cation exchange with polymeric Ti cations, formed by partial hydrolysis of TiCl4 in HCl. On further hydrolysis and heating, TiO2 pillars in the form of anastase were formed between the montmorillonite layers. The resulting TiO2-cross-linked montmorillonites possessed surface areas in the range 200–350 m2/g and pore volumes of about 0.2 cm3/g and were thermally and hydrothermally stable to 700°C. The basal spacing of products heated at temperatures > 200°C was about 28Å, as determined by X-ray powder diffraction and by N2-desorption pore-size analysis. The surface area increased and the pore volume decreased with increasing HCl-concentration in the Ti-solution. The uptake of TiO2 by the montmorillonite, the surface area, and the pore volume increased with increasing amount of Ti added in the preparation, to about 10 mmoles of Ti/g of montmorillonite. A further increase in the amount of Ti added resulted in a decrease in surface area, but the pore volume and the uptake of TiO2 remained almost constant. The high porosity and the interlayer spacing of the product are consistent with a structure similar to that previously proposed for smectites, cross-linked with hydroxy-Al oligocations.
Curved Smectite in Soils from Volcanic ash in Kenya and Tanzania: A Low-Angle X-Ray Powder Diffraction Study
Authors: van der Gaast S. J. , Mizota Chitoshi , Jansen J. H. F.
Published: September 2006

Low-angle X-ray powder diffraction (XRD) measurements of soil samples, made at controlled relative humidities, showed the presence of major reflections at 20–33, 10–27, and 7–8 Å. The first reflection, which increased in intensity but did not shift in spacing with decreasing relative humidity, represents curved smectite layers. This spacing was also observed by high-resolution transmission electron microscopy. The value of 10–27 Å for the second reflection, the 001 reflection of smetite, is unusually high, probably due to poorly stacked, irregularly curved layers. The 7–8-Å reflection originates from kaolinite or dehydrated halloysite, which also contain curved layers. The more curved the layer structure of the smectite, the more difficult it is to detect this phase; therefore the XRD relative peak heights are not directly proportional to the percentages of the smectite.
Sorption, Desorption, and Isotope Exchange of Cesium (10−9-10−3 M) On Chlorite
Authors: Grütter A. , von Gunten H. R. , Rössler E.
Published: September 2006

Batch cation-exchange experiments were performed on chlorite, using synthetic ground water containing cesium in the concentration range 10−9–10−3 M. The sorption behavior was complex; with increasing Cs concentration, the distribution ratio of Cs decreased at first, but passed through a pronounced maximum at high Cs loadings. The desorption data differed from the sorption data only by a shift towards larger distribution ratios in the region of the maximum. The distribution ratios for isotope exchange were even larger, but varied less with Cs loadings. Similar results were obtained for KCl-treated chlorite; however, grinding or acid-treatment of the chlorite considerably altered its behavior, the absence of the maximum being the most striking change. Most of the effects at higher Cs loadings can be explained by postulating structural changes induced by the sorption of Cs.
The CS/K Exchange in Muscovite Interlayers: An Ab Initio Treatment
Authors: Rosso Kevin M. , Rustad James R. , Bylaska Eric J.
Published: September 2006

Plane-wave pseudopotential total energy calculations have been applied to investigate the structure and energetics of the Cs/K exchange into interlayer sites in muscovite mica. Novel muscovite structures were designed to isolate the effects of 2:1 layer charge, cation size/interlayer site shape, and tetrahedral Al/Si substitutions on the exchange. All atom and cell-parameter optimizations were performed with the intention to mimic the constant pressure, non-isovolumetric exchange conditions thought to be found at frayed-edge sites. Under conditions where the cell parameters are allowed to relax, the overall Cs/K exchange reaction is surprisingly close to isoenergetic. The forward reaction is more strongly favored with increasing layer charge. For the condition of zero layer charge and no interlayer site distortion, the difference in the optimal interlayer spacing for Cs relative to K is very small, indicating a baseline indifference of the muscovite structure to cation size. The presence of 2:1 layer charge or tetrahedral rotations arising from Al/Si substitutions clearly change this outcome. Analysis of the dependence of the interlayer spacing on layer charge shows that while the spacing collapses with increasing layer charge for K as the interlayer cation, the reverse is true for Cs. We attribute the contrasting behavior to inherent differences in the ability of these cations to screen 2:1 layer-layer repulsions. Such effects might be involved during exchange at frayed-edge sites where interlayer spacings are increased. This is known, from experiment, to be very selective for Cs. Overall, the exchange energetics are so low that the Cs/K exchange rate and degree of irreversibility are likely to be dominated by diffusion kinetics.
A Deep-Water Glauconitization Process on the Ivory Coast—Ghana Marginal Ridge (ODP Site 959): Determination of Fe3+-Rich Montmorillonite in Green Grains
Authors: Wiewióra A. , Giresse P. , Petit S. , Wilamowski A.
Published: September 2006

The mineral and chemical composition of green glauconitic grains from ODP Site 959 (2100 m water depth) located on the northern flank of the Ivory Coast—Ghana Marginal Ridge was studied. Recurrent winnowing of a 20 m thick Pleistocene succession resulted in a low accumulation rate and stratigraphic hiatuses. The green clay material typically occurs as fillings in the chambers of pelagic foraminifers. The amount of green clay present in sediments older than 1 Ma is small, and greater in younger material. Mud composed of smectite, kaolinite, traces of mica, calcite and quartz was the precursor material that filled the chambers of the foraminifers. Processes at the water-sediment interface slowly modified this composition. Kaolinite was dissolved; smectite lost Al but gained Fe, K and layer charge. In that matrix, the nanocrystals of neoformed smectite are observed. The infrared (IR) spectra showed OH-stretching and bending vibrations due to groups incorporating Fe3+. The spectra are in agreement with the crystallochemical formulae of Fe3+-rich montmorillonite as determined by point-by-point analyses on the neoformed crystallites and on the surrounding matrix. The layer charge in this Fe3+-rich montmorillonite is almost wholly octahedral as shown in crystallochemical formulae and documented independently by a new IR method. The tetrahedral charge appeared when the Fe content increased by > 1.2 Fe per formula unit. With the maturation process, the increased role of the closed layers is observed, with the color of grains becoming greener. We have documented for the first time glauconitization proceeding at a depth of 2100 m at a temperature near 3°C. The most important factors of the process are: accumulation of terrigenous clayey material in the foraminiferal chambers, Fe supply from a nearby continent, and a lengthy residence at the water-sediment interface in the zone of the winnowing and low sediment accumulation rate.
Authigenic Phyllosilicates in the Middle Triassic Kremikovtsi Sedimentary Exhalative Siderite Iron Formation, Western Balkan, Bulgaria
Authors: Damyanov Zhelyazko , Vassileva Margarita
Published: September 2006

Two distinct assemblages of authigenic phyllosilicates were distinguished in the Kremikovtsi sedimentary exhalative (SEDEX) siderite iron formation (SIF) and noted as important tracers of two styles of mineralization characteristic of this type of ore deposit. Hydrothermal-sedimentary layer silicates are represented by rare occurrences of relict microcrystalline Mg-rich berthierine with a relatively low degree of structural ordering, associated closely with framboidal pyrite as an intergranular matrix cementing sparry siderite grains; the larger silicates are also represented by the diagenetic transformation product of berthierine, chamosite. Berthierine precipitated under anoxic conditions during advanced early diagenesis after chert deposition. Hydrothermal-epigenetic phyllosilicates (berthierine, chamosite, illite-smectite (I-S), and kaolinite) from the barite-sulfide assemblage are characterized by: crystalline and undeformed habits; relatively larger particle size, low-temperature polytypes, low to no mixed layering, and a high degree of crystallinity; absence of impurities and dominant monomineralic aggregates; affiliation to initial open spaces and deposition mainly as vug fillings and linings. They formed under pronounced control by the vuggy porosity of the siderite host caused by the invasion of acid (pH = 3–5), hot (200–230°C) hydrothermal fluids probably at the stage of burial diagenesis of the SIF under relatively stable reducing conditions fluctuating near the sulfide/sulfate stability boundary <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:c="http://cambridge.org/core/content" xmlns:core="http://cambridge.org/core" xmlns:m="http://cambridge.org/core/metadata" xmlns:cup="http://contentservices.cambridge.org" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:mnf="http://cambridge.org/core/manifest">(logPO2≅−30). The greater Al concentration in hydrothermal solution than in seawater determines the affiliation of phyllosilicates in the Phanerozoic SEDEX SIFs to aluminous species (berthierine, chamosite) unlike low to non aluminous ones (greenalite, stilpnomelane) in the Precambrian IFs. The berthierine compositions, expressed by the Mg/Fe vs. Al/Si ratios, are a sensitive indicator of the geological conditions under which they formed (marine, non-marine, hydrothermal ore and pre-ore), thus allowing the genetic discrimination of this mineral from various localities.
Kaolinite to Halloysite-7 Å Transformation in the Kaolin Deposit of São Vicente De Pereira, Portugal
Authors: Bobos Iuliu , Duplay Joelle , Rocha João , Gomes Celso
Published: September 2006

The transformation of kaolinite to halloysite-7 Å was identified in the kaolin deposit of São Vicente de Pereira (SVP), using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). Both the 02, 1̄ and 13̄, 13 reflections show changes in the XRD patterns along the kaolinite to halloysite-7 Å transition, and the FTIR spectra show changes corresponding to both OH− and Si-O-stretching bands and Al-O-Si-bending vibrations. The interlayer water content in the kaolinite structure increases during transition. The two-layer periodicity of well-ordered kaolinite and rolling up of kaolinite plates are observed using high-resolution transmission electron microscopy (HRTEM). Long and short tubes exhibit halloysite-7 Å. No structural Fe was found in the kaolinite samples. Analytical electron microscopy (AEM) indicates no substitution of Al3+ for Si4+. The Si/Al ratio shows values of ∼1 for the kaolinite and rolled kaolinite plates. The 27Al magic angle spinning neutron magnetic resonance (MAS-NMR) spectra display a resonance centered at ∼1 ppm, assigned to six-coordinated aluminum. The transformation of kaolinite to halloysite-7 Å is controlled by surface reaction.
α-Band Electroencephalographic Activity over Occipital Cortex Indexes Visuospatial Attention Bias and Predicts Visual Target Detection
Authors: Thut Gregor , Nietzel Annika , Brandt Stephan A. , Pascual-Leone Alvaro
Published: September 2006

Covertly directing visual attention toward a spatial location in the absence of visual stimulation enhances future visual processing at the attended position. The neuronal correlates of these attention shifts involve modulation of neuronal “baseline” activity in early visual areas, presumably through top-down control from higher-order attentional systems. We used electroencephalography to study the largely unknown relationship between these neuronal modulations and behavioral outcome in an attention orienting paradigm. Covert visuospatial attention shifts to either a left or right peripheral position in the absence of visual stimulation resulted in differential modulations of oscillatory α-band (8–14 Hz) activity over left versus right posterior sites. These changes were driven by varying degrees of α-decreases being maximal contralateral to the attended position. When expressed as a lateralization index, these α-changes differed significantly between attention conditions, with negative values (α_right < α_left) indexing leftward and more positive values (α_left ≤ α_right) indexing rightward attention. Moreover, this index appeared deterministic for processing of forthcoming visual targets. Collapsed over trials, there was an advantage for left target processing in accordance with an overall negative bias in α-index values. Across trials, left targets were detected most rapidly when preceded by negative index values. Detection of right targets was fastest in trials with most positive values. Our data indicate that collateral modulations of posterior α-activity, the momentary bias of visuospatial attention, and imminent visual processing are linked. They suggest that the momentary direction of attention, predicting spatial biases in imminent visual processing, can be estimated from a lateralization index of posterior α-activity.
TRPV1 Receptors in the CNS Play a Key Role in Broad-Spectrum Analgesia of TRPV1 Antagonists
Authors: Cui M. , Honore P. , Zhong C. , Gauvin D. , Mikusa J. , Hernandez G. , Chandran P. , Gomtsyan A. , Brown B. , Bayburt E. K. , Marsh K. , Bianchi B. , McDonald H. , Niforatos W. , Neelands T. R. , Moreland R. B. , Decker M. W. , Lee C.-H. , Sullivan J. P. , Faltynek C. R.
Published: September 2006

Vanilloid receptor type 1 (TRPV1) is a ligand-gated nonselective cation channel that is considered to be an important integrator of various pain stimuli such as endogenous lipids, capsaicin, heat, and low pH. In addition to expression in primary afferents, TRPV1 is also expressed in the CNS. To test the hypothesis that the CNS plays a differential role in the effect of TRPV1 antagonists in various types of pain, the analgesic effects of two TRPV1 antagonists with similarin vitropotency but different CNS penetration were comparedin vivo. Oral administration of either A-784168 (1-[3-(trifluoromethyl)pyridin-2-yl]-N-[4-(trifluoromethylsulfonyl)phenyl]-1,2,3,6-tetrahydropyridine-4-carboxamide) (good CNS penetration) or A-795614 (N-1H-indazol-4-yl-N′-[(1R)-5-piperidin-1-yl-2,3-dihydro-1H-inden-1-yl]urea) (poor CNS penetration) blocked capsaicin-induced acute pain with the same potency. In complete Freund's adjuvant (CFA)-induced chronic inflammatory pain, oral administration of either compound blocked thermal hyperalgesia with similar potency. Furthermore, intraplantar or intrathecal administration of A-784168 blocked CFA-induced thermal hyperalgesia, suggesting that both peripheral and CNS TRPV1 receptors may play a role in inflammatory thermal hyperalgesia. The effects of the two TRPV1 antagonists were further assessed in models presumably mediated by central sensitization, including CFA- and capsaicin-induced mechanical allodynia and osteoarthritic pain. In these models, the potency of the two compounds was similar after intrathecal administration. However, when administered orally, A-784168, with good CNS penetration, was much more potent than A-795614. Together, these results demonstrate that TRPV1 receptors in the CNS play an important role in pain mediated by central sensitization. In addition, these results demonstrate that significant CNS penetration is necessary for a TRPV1 antagonist to produce broad-spectrum analgesia.
Estrogen Receptor Protein Interaction with Phosphatidylinositol 3-Kinase Leads to Activation of Phosphorylated Akt and Extracellular Signal-Regulated Kinase 1/2 in the Same Population of Cortical Neurons: A Unified Mechanism of Estrogen Action
Authors: Mannella Paolo , Brinton Roberta Diaz
Published: September 2006

17β-Estradiol (E2)-induced neuroprotection is dependent on mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-kinase (PI3K) signaling cascades. We sought to determine whether E2neuroprotective mechanisms are mediated by a unified signaling cascade activated by estrogen receptor (ER)–PI3K interaction within the same population of neurons or whether E2activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt are independent signaling events in different neuronal populations. Immunoprecipitation of E2-treated cortical neurons was conducted to determine a protein–protein interaction between ER and the PI3K regulatory subunit p85. Subsequently, cortical neurons were treated with E2alone or in presence of MAPK inhibitors or PI3K inhibitors. Results of these analyses indicated a protein–protein interaction between ER and p85 that was time-dependent and consistent with the temporal profile for generation of Akt (pAkt) and ERK1/2 phosphorylation (pERK1/2). E2-induced phosphorylation of Akt, was first apparent at 10 min and maximal at 30 min. Simultaneously, E2-induced pERK1/2 was first apparent at 5–10 min and maximal at 30 min. Inhibition of PI3K completely blocked E2activation of pAkt at 10 and 30 min and blocked E2activation of ERK1/2 at 10 min, which revealed a PI3K-independent activation of ERK at 30 min. Double immunocytochemical labeling for pERK1/2 and pAkt demonstrated that E2induced both signaling pathways in the same neurons. These results indicate a unified signaling mechanism for rapid E2action that leads to the coordinated activation of both pERK1/2 and pAkt in the same population of neurons. Implications of these results for understanding estrogen mechanism of action in neurons and therapeutic development are considered.
Neural Dynamics in Monkey Parietal Reach Region Reflect Context-Specific Sensorimotor Transformations
Authors: Gail Alexander , Andersen Richard A.
Published: September 2006

We investigated the neural dynamics of sensorimotor transformations in the parietal reach region (PRR) of monkeys. To dissociate sensory from motor goal representations, we used a memory-guided anti-reach task. The monkeys had to reach either to a visually instructed, memorized peripheral target position (pro-reach) or to a diametrically opposed position (anti) while keeping central ocular fixation. Pro- and anti-reaches were randomly interleaved and indicated by a color instruction from the beginning of each trial. We analyzed spatiotemporal single-cell tuning and performed time-resolved population decoding to quantify the dynamic representation of the spatial visual cue, the reach goal, and the currently valid task rule (pro/anti mapping). Sensory information regarding the visual cue position was represented weakly during a short period of cue visibility. PRR predominantly encoded the reach goal from the end of the cue period on. The representation of the reach goal in the memory task evolves later for the anti- compared with pro-reaches, consistent with a 40–50 ms difference in reaction time between the two task rules. The task rule could be decoded before the appearance of the spatial cue, which indicates that abstract rule information is present in PRR that is independent of spatial cue or motor goal representations. Our findings support the hypothesis that PRR immediately translates current sensory information into reach movement plans, rather than storing the memorized cue location in the instructed-delay task. This finding indicates that PRR represents integrated knowledge on spatial sensory information combined with abstract behavioral rules to encode the desired movement goal.
Expression of Multiple P2X Receptors by Glossopharyngeal Neurons Projecting to Rat Carotid Body O2-Chemoreceptors: Role in Nitric Oxide-Mediated Efferent Inhibition
Authors: Campanucci Verónica A. , Zhang Min , Vollmer Cathy , Nurse Colin A.
Published: September 2006

In mammals, ventilation is peripherally controlled by the carotid body (CB), which receives afferent innervation from the petrosal ganglion and efferent innervation from neurons located along the glossopharyngeal nerve (GPN). GPN neurons give rise to the “efferent inhibitory” pathway via a plexus of neuronal nitric oxide (NO) synthase-positive fibers, believed to be responsible for CB chemoreceptor inhibition via NO release. Although NO is elevated during natural CB stimulation by hypoxia, the underlying mechanisms are unclear. We hypothesized that ATP, released by rat CB chemoreceptors (type 1 cells) and/or red blood cells during hypoxia, may directly activate GPN neurons and contribute to NO-mediated inhibition. Using combined electrophysiological, molecular, and confocal immunofluorescence techniques, we detected the expression of multiple P2X receptors in GPN neurons. These receptors involve at least four different purinergic subunits: P2X2[and the splice variant P2X2(b)], P2X3, P2X4, and P2X7. Using a novel coculture preparation of CB type I cell clusters and GPN neurons, we tested the role of P2X signaling on CB function. In cocultures, fast application of ATP, or its synthetic analog 2′,3′-O-(4 benzoylbenzoyl)-ATP, caused type I cell hyperpolarization that was prevented in the presence of the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide potassium. These data suggest that ATP released during hypoxic stress from CB chemoreceptors (and/or red blood cells) will cause GPN neuron depolarization mediated by multiple P2X receptors. Activation of this pathway will lead to calcium influx and efferent inhibition of CB chemoreceptors via NO synthesis and consequent release.
Odorant Deprivation Reversibly Modulates Transsynaptic Changes in the NR2B-Mediated CREB Pathway in Mouse Piriform Cortex
Authors: Kim Hyun H. , Puche Adam C. , Margolis Frank L.
Published: September 2006

The olfactory system is an outstanding model for understanding activity-dependent neuronal plasticity in mammals. Olfactory sensory neurons (OSNs) in the periphery project onto mitral/tufted cells in the olfactory bulb (OB) and these mitral/tufted cells in turn project to piriform cortex (PC). Numerous studies have examined changes in OB after a permanent OSN ablation, but little is known about “trans-transsynaptic” changes taking place in the PC. Permanent zinc sulfate lesion of the olfactory epithelium resulted in a selective loss of the NMDA receptor NR2B protein and mRNA expression in pyramidal cells in layer IIb of PC after 2–7 d. Regulatory elements affected by NR2B signaling, namely the phosphorylation of CREB, were also downregulated only in layer IIb neurons. These changes could be caused by OSN axon loss in the zinc sulfate lesion, or to a reduced activity. To test this hypothesis, we performed both permanent and reversible naris occlusion, which blocks odorant access to the nasal cavities and OSN activity. The expression of NR2B and phospho-CREB were downregulated 5 d after occlusion and this reduction was fully restored 10 d after reopening of the naris. Subsequently, we identified the subset of pyramidal cells in layer IIb that are especially sensitive to the loss of odor-evoked activity using double retrograde tracers. In summary, the present study provides an initial characterization of the molecular mechanisms associated with odor stimulation on second order neuronal plasticity and phenotype in the olfactory system.
Context-Dependent Human Extinction Memory Is Mediated by a Ventromedial Prefrontal and Hippocampal Network
Authors: Kalisch Raffael , Korenfeld Elian , Stephan Klaas E. , Weiskopf Nikolaus , Seymour Ben , Dolan Raymond J.
Published: September 2006

In fear extinction, an animal learns that a conditioned stimulus (CS) no longer predicts a noxious stimulus [unconditioned stimulus (UCS)] to which it had previously been associated, leading to inhibition of the conditioned response (CR). Extinction creates a new CS–noUCS memory trace, competing with the initial fear (CS–UCS) memory. Recall of extinction memory and, hence, CR inhibition at later CS encounters is facilitated by contextual stimuli present during extinction training. In line with theoretical predictions derived from animal studies, we show that, after extinction, a CS-evoked engagement of human ventromedial prefrontal cortex (VMPFC) and hippocampus is context dependent, being expressed in an extinction, but not a conditioning, context. Likewise, a positive correlation between VMPFC and hippocampal activity is extinction context dependent. Thus, a VMPFC–hippocampal network provides for context-dependent recall of human extinction memory, consistent with a view that hippocampus confers context dependence on VMPFC.
Postreactivation Glucocorticoids Impair Recall of Established Fear Memory
Authors: Cai Wen-Hui , Blundell Jacqueline , Han Jie , Greene Robert W. , Powell Craig M.
Published: September 2006

Pavlovian fear conditioning provides one of the best rodent models of acquired anxiety disorders, including posttraumatic stress disorder. Injection of a variety of drugs after training in fear-conditioning paradigms can impair consolidation of fear memories. Indeed, early clinical trials suggest that immediate administration of such drugs after a traumatic event may decrease the risk of developing posttraumatic stress disorder in humans (Pitman et al., 2002; Vaiva et al., 2003). The use of such a treatment is limited by the difficulty of treating every patient at risk and by the difficulty in predicting which patients will experience chronic adverse consequences. Recent clinical trials suggest that administration of glucocorticoids may have a beneficial effect on established posttraumatic stress disorder (Aerni et al., 2004) and specific phobia (Soravia et al., 2006). Conversely, glucocorticoid administration after training is known to enhance memory consolidation (McGaugh and Roozendaal, 2002; Roozendaal, 2002). From a clinical perspective, enhancement of a fear memory or a reactivated fear memory would not be desirable. We report here that when glucocorticoids are administered immediately after reactivation of a contextual fear memory, subsequent recall is significantly diminished. Additional experiments support the interpretation that glucocorticoids not only decrease fear memory retrieval but, in addition, augment consolidation of fear memory extinction rather than decreasing reconsolidation. These findings provide a rodent model for a potential treatment of established acquired anxiety disorders in humans, as suggested by others (Aerni et al., 2004; Schelling et al., 2004), based on a mechanism of enhanced extinction.
Stromal-Derived Factor-1 (CXCL12) Regulates Laminar Position of Cajal-Retzius Cells in Normal and Dysplastic Brains
Authors: Paredes Mercedes F. , Li Guangnan , Berger Omri , Baraban Scott C. , Pleasure Samuel J.
Published: September 2006

Normal brain development requires a series of highly complex and interrelated steps. This process presents many opportunities for errors to occur, which could result in developmental defects in the brain, clinically referred to as malformations of cortical development. The marginal zone and Cajal-Retzius cells are key players in cortical development and are established early, yet there is little understanding of the factors resulting in the disruption of the marginal zone in many types of cortical malformation syndromes. We showed previously that treatment with methylazoxymethanol in rats causes marginal zone dysplasia with displacement of Cajal-Retzius cells to deeper cortical layers. Here we establish that loss of activity of the chemokine stromal-derived factor-1 (SDF1) (CXCL12), which is expressed by the leptomeninges, is necessary and sufficient to cause marginal zone disorganization in this widely used teratogenic animal model. We also found that mice with mutations in the main receptor for SDF1 (CXCR4) have Cajal-Retzius cells displaced to deeper cortical layers. Furthermore, by inhibiting SDF1 signalingin uteroby intraventricular injection of a receptor antagonist, we establish that SDF1 signaling is required for the maintenance of Cajal-Retzius cell position in the marginal zone during normal cortical development. Our data imply that cortical layering is not a static process, but rather requires input from locally produced molecular cues for maintenance, and that complex syndromes of cortical malformation as a result of environmental insults may still be amenable to explanation by interruption of specific molecular signaling pathways.