Basic Melts Research Articles

Elucidation of Rate Variations for a Diels-Alder Reaction in Ionic Liquids from QM/MM Simulations.

The impact of acidic and basic ionic liquid 1-ethyl-3-methylimidazolium chloride (EMIC) melts upon cyclopentadiene and methyl acrylate Diels-Alder reaction rates has been investigated using QM/MM calculations. The ability of the ionic liquid to act as a hydrogen bond donor (cation effect), moderated by its hydrogen bond accepting ability (anion effect), has been proposed previously to explain observed endo/exo ratios. However, the molecular factors that endow ionic liquids with their rate enhancing potential remain unknown. New OPLS-AA force field parameters in conjunction with potentials of mean force (PMF) derived from free energy perturbation calculations in Monte Carlo simulations (MC/FEP) are used to compute activation energies. QM/MM simulations using a periodic box of ions reproduce relative rate enhancements for the EMIC melts compared to water and 1-chlorobutane that reproduce kinetic experiments. Solute-solvent interactions in acidic and basic ionic liquid melts have been analyzed at key stationary points along the reaction coordinate. The reaction rate was found to be greater in the acidic rather than the basic melt due to less-dominant ion-pairing in the acidic melt, enabling the EMI cation to better coordinate to the dienophile at the transition state. The simulations suggest that the hydrogen on C2 of the EMI cation does not contribute to stabilization of the transition state, as previously believed, and the interactions with the more sterically exposed hydrogens on C4 and C5 play a larger role. In addition, the relative stabilization of the transition state through electrostatic interactions with the EMI cation in the acidic melt is also greater than that afforded by the weaker Lewis-acid effect provided by hydrogen bonding with water molecules in aqueous solution.

Electrodeposition of aluminum from molten AlCl3–NaCl–KCl mixture

Electrochemical deposition of aluminum from basic and acidic molten NaCl-KCl-AlCl3 mixture on a graphite electrode at 140 °C was studied by voltammetry, chronopotentiometry and constant current deposition. The deposition of aluminum was found to proceed via a nucleation/growth mechanism in basic melt, while it was found to be diffusion controlled in acidic melt. The diffusion coefficient calculated for Al2Cl−7 ions in acidic melt by voltammetry was in agreement with the deductions of voltammetry. The morphology of the aluminum deposits was examined using a metallographic microscope and by SEM. It was shown that, depending on the current density (c.d.) and AlCl3 concentration (acidic or basic melt) different aluminum morphologies were evident but a silver-bright, compact, and very stable metallic form of aluminum deposit was obtained in acidic melt with KI addition as surfactant.

Evidence of diverse depletion and metasomatic events in harzburgite–lherzolite mantle xenoliths from the Iberian plate (Olot, NE Spain): Implications for lithosphere accretionary processes

Mantle xenoliths from the Olot volcanic district (NE Spain) comprise a bi-modal suite consisting of protogranular spinel lherzolites (cpx 12–14%) sometimes with pargasitic amphibole, and highly refractory spinel harzburgites (cpx ≤ 1%) with coarse-grained granular textures. The lherzolites range from slightly depleted to moderately LREE-enriched with flat HREE patterns between 1.5 and 2.7 × chondrite (Ch). In contrast, the harzburgites are extremely depleted in HREE (down to 0.2 × Ch) and strongly LREE-enriched (La N /Yb N = 12.3–17.2). LA-ICP-MS analyses of clinopyroxene and amphibole of the lherzolites highlight variable degrees of LREE depletion (HREE up to 13 × Ch, La N /Yb N down to 0.01), with the exception of a single sample in which both clinopyroxene and amphibole are LREE-enriched (La N /Yb N up to 19). In the harzburgites, clinopyroxenes display totally different REE distributions, characterized by extreme HREE depletion (down to 0.4 × Ch) and upward convex positively fractionated middle-light REE patterns (Nd N /Yb N up to 20.7 × Ch; La N /Yb N up to 12 × Ch). Sr–Nd–Hf isotopic data for both whole-rocks and cpx separates, coherently indicate depleted mantle (DM) compositions for the lherzolites ( εSr = − 15 to − 26, εNd = + 9 to + 17, εHf = + 18 to + 68) and enriched mantle (EM) compositions for the harzburgites ( εSr = − 10 to + 36, εNd = − 1 to − 6, εHf = + 3 to + 8). Modelling of the clinopyroxene REE data and isotopic systematics suggest that some lherzolites were affected by pre-Paleozoic (0.6–1 By) low-degree partial melting processes, while others probably reflect some extent of refertilization of the mantle protolith by metasomatizing melts similar to the Triassic rift-related tholeiites reported from several Pyrenean localities. The harzburgites represent extreme refractory residua, resulting from a complex depletion history due to multistage melt extraction as often observed in the cratonic mantle. The distinctive REE patterns and isotopic systematics of their clinopyroxenes suggest that the harzburgites were formed by the interaction of an ultra-depleted peridotite matrix with highly alkaline basic melts similar in composition to the Permo-Triassic alkaline lamprophyres which are widespread within the Iberian plate. Lherzolites possibly represent younger lithosphere (accreted asthenosphere?) up-lifted and juxtaposed to the older subcontinental lithospheric mantle (harzburgites) during the post-Variscan rifting of the Iberian margin. These two genetically different, but adjoining, mantle domains intimately mingled along the northern Iberian margin during the subsequent plate convergence processes, leading to the close association of harzburgites and lherzolites observed in the Olot mantle xenoliths and in some Pyrenean peridotite massifs.

Interaction between basic and acid magmas during the latest stages of the post-collisional Variscan evolution: Clues from the gabbro–granite association of Ota (Corsica–Sardinia batholith)

The intrusive association of Ota (post-Variscan Corsica–Sardinia batholith) consists of mafic rocks and granites displaying mingling relations. The mafic rocks are olivine gabbros to quartz-diorites, and the granites are subsolvus and characterised by biotite as the only mafic phase. Minor amounts of amphibole-bearing granodiorites to granites develop along the contacts between mafic rocks and granites. The rather high initial ε Nd (+ 4.5) of the most primitive mafic sample indicates that parental basic melts had an extensive contribution from a depleted mantle source. In the mafic rocks, the decrease of initial ε Nd (to + 2.1) is associated with an increase of Th and U concentrations in clinopyroxene, thus indicating that the Ota basic melts underwent a process of crustal contamination. Associated biotite granites have low initial ε Nd (+ 0.4) and high Th and U concentrations. The chemical evolution of Ota basic melts most likely occurred through fractional crystallisation and concomitant contamination with the acid magma that gave rise to associated biotite granites. The incompatible element signature of late-stage anhedral amphibole from variably evolved mafic rocks is similar, thus suggesting that the whole Ota mafic sequence recorded the contamination process, possibly by percolation within the gabbroic crystal mush of a melt contaminated by the acid magma. Hybridisation between basic and acid magmas is also documented by contact amphibole-bearing granitoids. In particular, major and trace element zoning of amphibole from granodiorites reveals that such a hybridisation was associated with reactions between crystals from the basic magma (amphibole and clinopyroxene) and the acid melt. The occurrence of chemically homogeneous amphiboles in associated granites suggests that “contact” hybridisation comprised also a process of gradient-induced chemical diffusion.

Density and surface tension of the systems CaO-FeO-Fe2O3-MgO, CaO-FeO-Fe2O3-ZnO and CaO-Fe2O3-Cu2O

AbstractThe density and surface tension of melts of the systems CaO-FeO-Fe2O3 MgO at the temperature 1623 K, CaO-FeO-Fe2O3-ZnO at 1573 K, and CaO-Fe2O3-Cu2O at 1573 K were determined using the maximum bubble pressure method. The molar volume, the excess molar volume, and the excess surface tension were calculated on the basis of the obtained data. From these properties information on the interactions of components and possible chemical reactions between them was obtained. Due to the absence of silica and the low concentration of other network-forming oxides, only isolated FeO45− tetrahedra and the CaO·FeO ionic pairs are formed in these basic melts, the donor of the oxygen atoms being either CaO, FeO, or both CaO+FeO oxides. Even the observed ternary interactions may be attributed to the formation of the anions FeO45− only.

Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica)

Peridotite xenoliths found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite to harzburgite. They often contain glass-bearing pockets formed after primary pyroxenes and spinel. Few samples are composite and consist of depleted spinel lherzolite crosscut by amphibole veins and/or lherzolite in contact with poikilitic wehrlite. Peridotite xenoliths are characterized by negative Al2O3–Mg# and TiO2–Mg# covariations of clino- and orthopyroxenes, low to intermediate HREE concentrations in clinopyroxene, negative Cr–Al trend in spinel, suggesting variable degrees of partial melting. Metasomatic overprint is evidenced by trace element enrichment in clinopyroxene and sporadic increase of Ti–Fetot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fetot contents constrain the metasomatic agent to be an alkaline basic melt. In composite xenoliths, clinopyroxene REE contents increase next to the veins suggesting metasomatic diffusion of incompatible element. Oxygen isotope data indicate disequilibrium conditions among clinopyroxene, olivine and orthopyroxene. The highest δ18O values are observed in minerals of the amphibole-bearing xenolith. The δ18Ocpx correlations with clinopyroxene modal abundance and geochemical parameters (e.g. Mg# and Cr#) suggest a possible influence of partial melting on oxygen isotope composition. Thermobarometric estimates define a geotherm of 80°C/GPa for the refractory lithosphere of NVL, in a pressure range between 1 and 2.5 GPa. Clinopyroxene microlites of melt pockets provide P–T data close to the anhydrous peridotite solidus and confirm that they originated from heating and decompression during transport in the host magma. All these geothermometric data constrain the mantle potential temperature to values of 1250–1350°C, consistent with the occurrence of mantle decompressional melting in a transtensive tectonic regime for the Ross Sea region.

Granitic Rocks of the Rio dos Bugres Mine: Host Rocks of Fluorite Deposits in Southernmost Brazil

Most fluorite deposits in southernmost Brazil are hosted by Neoproterozoic granitic associations. Fluorite deposits are, nevertheless, much younger than granitic magmatism, and the only probable role of granitic rocks in fluorite genesis was as a source of F. This paper characterizes granitic-dioritic associations in which these fluorite lodes and veins are localized. The granitic suite includes quartz diorites, tonalites, biotite-bearing syenogranites, monzogranites, and rhyolite dikes. It is interpreted as formed by partial mixing of basic and granitic melts, generating hybrid products, such as the tonalitic rocks. The granitic magmatism is correlated with the late-transcurrent magmatism that characterizes the post-collisonal stages of the Brasiliano-Pan-African orogenic cycle in southernmost Brazil. Basic rocks have mildly alkaline or continental tholeiitic affinities. The associated granitic magmatism was probably produced by crustal melting or, alternatively, as a reaction product of mantle-derived basic melts with quartz-feldspathic gneissic crustal sequences. Trace-element and REE patterns of the basic rocks are consistent with melts produced from a mantle previously modified by subduction-related metasomatism. The granitic rocks show trace-element compositions compatible with comagmatic products derived from the basic magmatism, which can also be interpreted as an effect of mixing between two non cogenetic magmas. Hydrothermally altered granites exhibit HREE-enriched patterns that can be used as a prospective tool for fluorite deposits.

Magma-mixing in the genesis of Hercynian calc-alkaline granitoids: an integrated petrographic and geochemical study of the Sázava intrusion, Central Bohemian Pluton, Czech Republic

The Devonian–early Carboniferous (354.1±3.5 Ma: conventional zircon U–Pb age) Sázava intrusion (biotite-amphibole quartz diorite, tonalite and granodiorite) of the Central Bohemian Pluton (CBP) associated with bodies of (olivine, pyroxene-) amphibole gabbro, gabbrodiorite, (quartz) diorite and rare hornblendite, gives an opportunity for a comparative study of a rather shallow, calc-alkaline magma-mixing zone at two levels, separated by a vertical difference of approximately 1 km. The deeper section (Příbram) displays the direct evidence for the existence of a long-lived, periodically tapped and replenished, floored magma chamber (MASLI). The contacts between the subhorizontal sheet-like basic bodies and the surrounding, commonly cumulus-rich, Sázava granitoid, are arcuate, and cut by a series of veins and ascending pipes. Shallow-dipping swarms of strongly elongated and flow-aligned mafic microgranular enclaves (MME), concordant with the contacts of the basic bodies, are commonplace. The higher level (Teletín) section shows relatively independent basic intrusions, some of them distinctly hybrid in character and mainly of quartz dioritic composition, surrounded by relatively homogeneous, nearly cumulus-free Sázava tonalite rich in texturally variable MME. Larger quartz microdiorite bodies and the MME, both interpreted as hybrids, contain varying proportions of highly heterogeneous plagioclase megacrysts with complex zoning, particularly well shown by cathodoluminescence (CL). Most often the megacrysts have cores of labradorite-anorthite, partly resorbed and overgrown by andesine rims but some are strongly brecciated and fragments have been annealed by rim growth. Also characteristic are long prisms of apatite, oikocrysts of quartz and K-feldspar and zoned amphibole. The latter has brown pargasite and magnesiohastingsite cores, resorbed and overgrown by magnesiohornblende, compositionally similar to the amphibole in the Sázava tonalite. The brown cores are probably relics of a higher P–T stage, with temperature exceeding 900 °C. Plots of major-element analyses of the Teletín quartz microdiorite and the MME fall close to the line joining the compositions of the evolved Sázava granodiorite-tonalite and the associated gabbros. The estimated minimal proportions of the acidic end-member range from ca. 15% to 20% for the quartz microdiorite and ca. 40% to 60% for the enclaves. Field relationships, macroscopic and mineralogical features and whole-rock geochemistry are consistent with a hybrid origin of the quartz microdiorite masses and enclaves. Unzoned calcic plagioclase cores are interpreted as having crystallized from a basic magma prior to its injection into a high-level acidic magma chamber. The sudden decrease in Ca activity and introduction of water, due to magma-mixing with tonalitic melt, led to their resorption and brecciation. Quartz ocelli that are abundant locally in some basic bodies are most likely xenocrysts introduced from the felsic magma. In the basic melt, they were resorbed and became a substrate for heterogeneous nucleation of the amphibole or pyroxene coronas. Strong undercooling of the basic magma resulted in the growth of long prismatic apatite together with dendritic, boxy cellular plagioclase. Both types of plagioclase cores were subsequently overgrown by more sodic mantles. At the same time, small unzoned plagioclase crystals nucleated in the groundmass. Following thermal re-adjustment, crystallization of the hybrid magma ended with considerably slower growth of quartz and K-feldspar oikocrysts.

Density of peridotite melts at high pressure

Densities of ultramafic melts were determined up to 22 GPa by relative buoyancy experiments. Olivine and diamond were used as buoyancy markers. We confirmed that the density crossover of PHN 1611 melt and its equilibrium olivine (Fo94) occurs at around 13.5 GPa and 2030 °C and that olivine floats from deeper regions in the magma ocean of the primordial terrestrial mantle. The comparison of the compression curves of basic and ultrabasic melts implies that the basic melt is more compressible. This can be explained by the difference in the amount of compressible linkage of SiO n and AlO n polyhedra. The interstitial melt trapped by the density crossover can be the cause of the impedance anomaly of the seismic wave in the deep upper mantle.

Exploration of molten hydroxide electrochemistry for thermal battery applications

The electrochemistry of molten LiOH–NaOH, LiOH–KOH, and NaOH–KOH was investigated using platinum, palladium, nickel, silver, aluminum and other electrodes. The fast kinetics of the Ag+/Ag electrode reaction suggests its use as a reference electrode in molten hydroxides. The key equilibrium reaction in each of these melts is 2 OH− = H2O + O2− where H2O is the Lux-Flood acid (oxide ion acceptor) and O2− is the Lux–Flood base. This reaction dictates the minimum H2O content attainable in the melt. Extensive heating at 500 °C simply converts more of the alkali metal hydroxide into the corresponding oxide, that is, Li2O, Na2O or K2O. Thermodynamic calculations suggest that Li2O acts as a Lux–Flood acid in molten NaOH–KOH via the dissolution reaction Li2O(s) + 2 OH− = 2 LiO− + H2O whereas Na2O acts as a Lux–Flood base, Na2O(s) = 2 Na+ + O2−. The dominant limiting anodic reaction on platinum in all three melts is the oxidation of OH− to yield oxygen, that is 2 OH− → 1/2 O2 + H2O + 2 e−. The limiting cathodic reaction in these melts is the reduction of water in acidic melts ([H2O] ≫ [O2−]) and the reduction of Na+ or K+ in basic melts. The direct reduction of OH− to hydrogen and O2− is thermodynamically impossible in molten hydroxides. The electrostability window for thermal battery applications in molten hydroxides at 250–300 °C is 1.5 V in acidic melts and 2.5 V in basic melts. The use of aluminum substrates could possibly extend this window to 3 V or higher. Preliminary tests of the Li–Fe (LAN) anode in molten LiOH–KOH and NaOH–KOH show that this anode is not stable in these melts at acidic conditions. The presence of superoxide ions in these acidic melts likely contributes to this instability of lithium anodes. Thermal battery development using molten hydroxides will likely require less active anode materials such as Li–Al alloys or the use of more basic melts. It is well established that sodium metal is both soluble and stable in basic NaOH–KOH melts and has been used as a reference electrode for this system.

Hydromagmatic amphibole in komatiitic, tholeiitic and ferropicritic units, Abitibi greenstone belt, Ontario and Qu�bec: evidence for Archaean wet basic and ultrabasic melts

¶Detailed petrographic, electron microprobe and ion probe studies of Archaean hydromagmatic amphiboles from the Abitibi greenstone belt, Canada, yield new insights into the origin of Al-undepleted komatiitic and Al-depleted tholeiitic and ferropicritic melts. The amphiboles are present in peridotite layers and basal chill zones of thick differentiated basic and ultrabasic sills and flows, and are titanian pargasite–hastingsite in composition. They can be grouped into two petrographic types: (1) amphibole in the groundmass; and (2) amphibole-bearing melt inclusions. The groundmass amphiboles are oikocrysts, rims and interstitial grains, present in minor to major amounts. The oikocrysts host cumulus olivines (Fo83–84) that are rounded in shape, embayed, and smaller in size. The amphibole-bearing melt inclusions are hosted in cumulus olivines (Fo83–84 in komatiitic rocks and Fo79 in tholeiitic rocks), spherical to ovoid in shape, 50–500 µm in size, and dominated modally by amphibole. The melt inclusions also contain euhedral chromite and aluminous spinel and micrometric clinopyroxene and glass, and sub-micrometric iron–nickel sulphide, chloro-apatite and ilmenite. In-situ ion probe analyses indicate the amphibole is: (1) enriched in Nb, LREE and Zr and depleted in Sr and HREE relative to primitive mantle; (2) contains up to 1–3 wt% H2O; and (3) overall displays δD values from 50‰ to −140‰, including many values in the accepted magmatic range of −60‰ to −90‰.

The Mariánské-Lázně Complex, NW Bohemian Massif: development and destruction of an early Palaeozoic seaway

Abstract The Mariánské-Lázně Complex is a Cambro-Ordovician terrane of oceanic affinity tectonically emplaced between the Saxothuringian Zone and Teplá-Barrandian Unit, NW Czech Republic. It forms a SE-dipping allochthonous body that comprises the largest contiguous exposure of metamorphosed basic and ultrabasic lithologies in the Bohemian Massif. Petrographic evidence indicates that a significant proportion of protoliths underwent eclogite facies metamorphism (570 to 720°C, 1.44 to 2.10 GPa), followed by an increase in temperature (up to around 800°C) and a subsequent widespread retrograde amphibolite facies event (550 to 680 °C, 0.75 to 1.20 GPa). New major and trace element geochemical analyses of metamorphosed basic and ultrabasic lithologies indicate that they exhibit geochemical characteristics attributable to a sea floor origin. The metabasites were generated at a spreading centre that interacted with deep-seated upwelling mantle asthenosphere. Separate, independently fractionating basic melt batches existed: these were derived from depleted and enriched asthenosphere and depleted sub-continental lithosphere sources. Geochemical correlation of the Mariánské-Lázně Complex with other early Palaeozoic metabasic provinces facilitates comparison of metabasic lithologies occurring in tectonically dislocated nappe pile thrust sheets, and allows delineation of important suture zones in the European Variscides.

The Influence of Lewis Acid/Base Chemistry on the Removal of Gallium by Volatility from Weapons-Grade Plutonium Dissolved in Molten Chlorides

It has been proposed that GaCl3 can be removed by direct volatilization from a Pu-Ga alloy that is dissolved in a molten chloride salt. Although pure GaCl3 is quite volatile (boiling point: 201°C), the behavior of GaCl3 dissolved in chloride salts is quite different because of solution effects and is critically dependent upon the composition of the solvent salt (i.e., its Lewis acid/base character). In this technical note, the behavior of gallium in prototypical Lewis acid and Lewis base salts is contrasted. It is found that gallium volatility is suppressed in basic melts and promoted in acidic melts. These results have an important influence on the potential for simple gallium removal in molten salt systems.

Multistage evolution of the European lithospheric mantle: new evidence from Sardinian peridotite xenoliths

Peridotite xenoliths entrained in Plio-Pleistocene alkali basalts from Sardinia represent fragments of the uppermost lithospheric mantle, and are characterised by an anhydrous four-phase mineral assemblage. They range in bulk rock composition from fertile spinel–lherzolites to residual spinel–harzburgites. The Sr–Nd isotope and trace element composition of clinopyroxene mineral separates varies between LREE-depleted samples with 87Sr/86Sr as low as 0.70262 and 143Nd/144Nd up to 0.51323 and LREE-enriched samples with 87Sr/86Sr up to 0.70461 and 143Nd/144Nd down to 0.51252. The available data suggest that all the studied peridotite samples suffered variable degrees of partial melting during Pre-Mesozoic times (based on Nd model ages relative to CHUR and DMM). The overprinted enrichment is related to a subsequent metasomatism, induced by fluids rising through the lithosphere that preferentially percolated the originally most depleted domains. Despite the occurrence of orogenic volcanism in the area, preferential enrichment in elements typically associated with slab derived fluids/melts (K, Rb, Sr, Th) relative to LREE has not been detected, and metasomatism seems to be more likely related to the infiltration of highly alkaline basic melts characterised by an EM-like Sr–Nd isotopic composition. Similar 87Sr/86Sr–143Nd/144Nd compositions, characterised by an EM signature, are observed in anorogenic mafic lavas and peridotite xenoliths from widespread localities within the "European" plate, whereas they have not previously been recorded in peridotite xenoliths and associated alkaline mafic lavas from the stable "African" lithospheric domain.

Dispersions of Anatase in Ambient Temperature Ionic Liquids

1-Ethyl-3-methylimidazolium (EMI) tetrachloroaluminate (“basic melt,” AlCl3:EMICl ratio 0.94) and EMIBF4 were studied as two examples of ambient temperature ionic liquids (salts that are liquid at room temperature). Dispersions of graphite and anatase (300 nm in diameter particles) in these media are surprisingly stable considering the high (about 4 mol dm−3) ionic strength. Electrostatic interactions play a rather insignificant role in the stability of anatase in EMIBF4, with the absolute value of electrophoretic mobility of the particles not exceeding 3×10−10 m2V−1 s−1. The sign of electrophoretic mobility depends on the presence of traces of water in EMIBF4. The particles carry a positive electrokinetic charge in dehydrated EMIBF4, and in the presence of 1% water (added as liquid water or absorbed from the atmosphere) the sign is reversed to negative.

Investigation of the oxygen electrode reaction in basic molten carbonates using electrochemical impedance spectroscopy

The oxygen electrode reaction at a gold electrode in a molten 53 mol% Li 2CO 3+47 mol% Na 2CO 3 eutectic has been studied at 923 and 1000 K by electrochemical impedance spectroscopy. The studies were generally performed in a basic melt, i.e. at the level of the CO 2 partial pressure corresponding to the thermal decomposition pressure of the carbonate ion. It was found that a semi-infinite diffusion controlled the rate of the electrode process and a modified Randles–Ershler model described the data obtained well. The dependence of the Warburg coefficient at the rest potential on the partial pressure of oxygen over the melt was analysed to verify an involvement of peroxide/oxide ions in the reduction/oxidation process at both the temperatures. Further analysis of the data allowed us to determine γ= D(O 2 2−) 1/2 C*(O 2 2−)/ D(O 2−) 1/2 C*(O 2−), where C*(O 2 2−) and C*(O 2−) are the bulk concentrations of O 2 2− and O 2− ions and D(O 2 2−) and D(O 2−) are their respective diffusion coefficients. The C*(O 2 2−) and C*(O 2−) were also estimated. The conclusion about the reaction mechanism was corroborated by the effect of electrode potential on the Warburg coefficient.

Interpretation of Vibrational Spectra Using Molecular Orbital Theory Calculations

A great deal of mineralogy has focused on determining crystal structures of minerals. Amorphous materials are often compared to crystalline materials of the same composition in order to predict atomic structure in the disordered phase. However, there are many instances when such analogies will break down or no crystalline counterpart exists. Even mineral surface structures may be significantly different from the bulk mineral structure, particularly when the mineral surface has undergone reaction with aqueous solutions (e.g., Hellman et al. 1997). Although unambiguous determination of the atomic structure of an amorphous phase is generally more problematic than for crystalline phases, modern spectroscopic techniques (see Hawthorne 1988 for a review) can shed a great deal of light on the atomic structure of phases with long-range disorder. Vibrational (infrared and Raman) spectroscopy is a particularly useful tool for studying many types of phases (McMillan and Hofmeister 1988 and references therein). Unfortunately, infrared and Raman spectra may be subject to various interpretations. Molecular modeling of vibrational spectra can be useful for sorting out these debates. For example, the mechanism of CO2 dissolution in albitic melts is difficult to understand. Fine and Stolper (1985) measured infrared spectra and Mysen and Virgo (1980) measured Raman spectra of CO2-bearing albite (NaAlSi3O8) glasses. Both studies indicated the presence of a carbonate species. Although carbonate readily forms in more basic melts through the combination of CO2 with a “free” oxygen atom (where “free” means not bonded to a tetrahedral cation (Mysen and Virgo 1980), there were thought to be no “free” oxygen atoms in fully-polymerized Na-aluminosilicates. Mysen and Virgo (1980) interpreted the spectra in terms of a depolymerization of the melt with formation of octahedral Al from the previously tetrahedral Al atoms. Fine and Stolper (1985) proposed a different mechanism whereby a …

Anodic behaviour of carbon materials in NaCl saturated NaAlCl4 fused electrolyte at low temperatures : A cyclic voltammetric study

The anodic behaviour of compacted graphite, graphite powder, glassy carbon and reticulated vitreous carbon electrodes in basic sodium chloroaluminate melt in the temperature range 428–573 K was studied using cyclic voltammetry. Chlorine evolution (> + 2.1 V vs Al) alone was the predominant reaction on the compact glassy carbon and fresh RVC electrodes. On compacted graphite, chlorine-assisted chloroaluminate intercalation was found to be a competitive process to the chlorine evolution. At high sweep rates, intercalation/deintercalation near the graphite lattice edges occur faster than chlorine evolution. Subsequent intercalation, however, is a slow process. Chlorine evolution predominates at higher temperatures and at higher anodic potentials. On graphite powders, a more reversible free radical chlorine adsorption/desorption process also occurs in the potential region below chlorine evolution. The process occurs at the grain boundaries, edges and defects of the graphite powder material. Intercalation/deintercalation processes are mainly responsible for the disintegration of graphitic materials in low-temperature chloroaluminate melts. Repeated intercalation/deintercalation cycles result in the irreversible transformation of the electrode surface and electrode characteristics. The surface area of the electrode is increased substantially on cycling. Electrode materials and operating conditions suitable for chlorine generation, intercalation/deintercalation and chlorine adsorption/desorption and power sources based on these processes are identified in this work.

Electrodeposition of Tungsten and Related Voltammetric Study in a Basic ZnCl2­NaCl (40-60 mol %) Melt

The electrodeposition of tungsten and voltammetric experiments were performed in basic (40-60 mol %) melts containing various tungsten compounds at 450°C. In constant potential electrolysis, metallic tungsten was obtained from and but not from and Similar to the case of acidic melt (60-40 mol %), the oxide ion or an oxochlorotungsten species may participate in the electrodeposition process. The presence of molecular oxygen does not interfere with the process. © 2000 The Electrochemical Society. All rights reserved.

Glasses in mantle xenoliths as geochemical indicators of metasomatic agents

An extensive comparison between world-wide occurrences of mantle glasses in both continental and oceanic settings is presented here. Several localities were chosen on the basis of the available major (and trace) element data, coupled with a clear identification of the nature of the metasomatizing agents. Xenoliths bearing amphibole and/or phlogopite, which can strongly affect the glass chemistry, were not considered. Despite their considerable geochemical variability world-wide, glasses are rather homogeneous within single xenoliths and their main geochemical features are broadly maintained in the same area. Orthopyroxene always plays a major role in glass genesis, as suggested by the relationship between its presence in the primary assemblage and the silica saturation degree, as well as by the FeO vs. SiO 2 decreasing trends. At the same time, the absence of any correlation between mg# and SiO 2 rules out the possibility of glass chemistry resulting from (or being affected by) secondary mineral fractionation. Thus, the geochemical features of mantle glasses are a powerful tool for investigating the nature of the metasomatic melts. According to available data, glasses in mantle xenoliths from Gran Comore, Samoa, Spitsbergen, Mongolia and Hoggar have been related to carbonatite metasomatism, whereas glasses in mantle xenoliths from Mt. Lessini, Hawaii, Cape Verde and Yitong have been related to metasomatism by alkali silicate basic melts. Moreover, the distinctly different alkali contents in Mt. Lessini and Hawaii glasses as compared with those of Cape Verde and Yitong allow a sodic signature to be attributed to the alkali silicate metasomatic melt of the former with respect to a potassic characteristic of the latter. Glasses related to carbonatite metasomatism are characterized by high CaO, Na 2O, and low SiO 2 and K 2O contents, with Na 2O/K 2O ratios usually greater than 2; they tend to have the highest Nb and LREE and the lowest Zr and Ti contents. On the other hand, glasses related to K-alkali silicate metasomatism are mainly characterized by high SiO 2 and K 2O and low CaO and NaO contents, with Na 2O/K 2O ratios less than 1; they generally present higher Rb and lower Nb and REE contents. Glasses related to Na-alkali silicate metasomatism are characterized by SiO 2 contents comparable to those related to K-alkali silicate metasomatism, but present higher Na 2O contents (in any case lower than those of carbonatite metasomatism), with Na 2O/K 2O ratios usually ranging between 1 and 2. On these bases, diagrams for discriminating between carbonatite- and Na-alkali or K-alkali silicate-related metasomatism are presented.