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At least 487 records · Page 27

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NASA NTRS1985

Cyanobacterial mats: Microanalysis of community metabolism

The microbial communities in two sites were studied using several approaches: (1) light microscopy; (2) the measurement of microprofiles of oxygen and sulfide at the surface of the microbial mat; (3) the study of diurnal variation of oxygen and sulfides; (4) in situ measurement of photosynthesis and sulfate reduction and study of the coupling of these two processes; (5) measurement of glutathione in the upper layers of the microbial mat as a possible oxygen quencher; (6) measurement of reduced iron as a possible intermediate electron donor along the established redoxcline in the mats; (7) measurement of dissolved phosphate as an indicator of processes of break down of organic matter in these systems; and (8) measurement of carbon dioxide in the interstitial water and its delta C-13 in an attempt to understand the flow of CO2 through the systems. Microbial processes of primary production and initial degradation at the most active zone of the microbial mat were analyzed.

Cohen, Y.↗

NASA NTRS1984

Electrical properties of metal-doped pyridine-derived polyimide films

A series of polyimide films containing pyridine coordinating groups doped with cobalt or nickel ions has been examined. Using 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride, films were prepared in combination with 4,4'-oxydianiline, 4,4'-dianiline sulfide, 3,3'-bis(aminopyridine) sulfide and 2,2'-dioxyphenelenebis (5,5'-aminopyridine). Room temperature volume resistivities have been found to decrease slightly with sulfur and pyridine containing diamines relative to diamines which do not contain these functionalities. Doping with cobalt and nickel additives results in further lowering of room temperature volume resistivity. Preliminary temperature profiling experiments have shown that volume resistivity decreases as T(g) is approached.

Khor, E.↗

NASA NTRS1986

Theoretical dissociation energies for ionic molecules

Ab initio calculations at the self-consistent-field and singles plus doubles configuration-interaction level are used to determine accurate spectroscopic parameters for most of the alkali and alkaline-earth fluorides, chlorides, oxides, sulfides, hydroxides, and isocyanides. Numerical Hartree-Fock (NHF) calculations are performed on selected systems to ensure that the extended Slater basis sets employed for the diatomic systems are near the Hartree-Fock limit. Extended Gaussian basis sets of at least triple-zeta plus double polarization equality are employed for the triatomic system. With this model, correlation effects are relatively small, but invariably increase the theoretical dissociation energies. The importance of correlating the electrons on both the anion and the metal is discussed. The theoretical dissociation energies are critically compared with the literature to rule out disparate experimental values. Theoretical (sup 2)Pi - (sup 2)Sigma (sup +) energy separations are presented for the alkali oxides and sulfides.

Langhoff, S. R.↗

NASA NTRS1985

Oxygen extraction from lunar materials - An experimental test of an ilmenite reduction process

The reaction of ilmenite with hydrogen to produce water has been studied experimentally in order to evaluate the effectiveness of using a cold trap to improve yields in a continuous flow process. Yields were enhanced, but not to the degree observed in batch processing systems. The terrestrial simulant used in these studies contained traces of iron sulfide, which released H2S during processing with a deleterious effect on several components of the test system. More sophisticated testing should be undertaken to obtain kinetic data and attention given to the removal of sulfides in the pre-process beneficiation.

Williams, R. J.↗

NASA NTRS1986

Roedderite in the Qingzhen (EH3) chondrite

The rare mineral roedderite, (Na1.09 K0.89 Ca0.02)2.00 (Mg4.71 Fe0.27)4.98 (Si11.80 Al0.09)11.89 O30 has been found in accessory amounts in the highly unequilibrated enstatite chondrite, Qingzhen. It occurs in association with minor amounts of albite and SiO2 as inclusions within the metal or sulfide phases of metal-sulfide assemblages. The roedderite crystals are connected through oxide and silicate veins to the surrounding matrix. The presence of glass coated vesicles on the surface of the assemblages strongly suggests that roedderite originated in the presence of a fluid phase, presumably during post-accretional planetary processes.

Rambaldi, E. R.↗

NASA NTRS1986

Primary magmatic carbon in ureilites - Evidence from cohenite-bearing metallic spherules

The compositions and mineralogy of metal-sulfide spherules trapped in ureilites are studied. Electron microprobe analyses of metallic phases and sulfides in spherules were conducted and backscattered electron images of metallic spherules in olivine and in ureilites are presented. It is observed that the spherules are composed of cohenite, Fe-Ni metal, troilite, and rare phosphorus-bearing minerals. The intergranular metal and graphite in the ureilites are analyzed. The phase relations of cohenite-bearing spherules are considered. Spherules represent Fe-Ni-C-S alloys that exist as immiscible metallic liquid droplets in the silicate magma from which ureilite olivine and pigeonite crystallized. The data reveal that graphite crystals occur as inclusions in olivine and ureilites and therefore graphite in ureilites is a primary magnetic component. It is hypothesized that graphite is a residue from the solid source materials that melted to produce the ureilite parent magma or crystallized from the magma.

Goodrich, C. A.↗

NASA NTRS1987

Gossans on Mars: Spectral features attributed to jarosite

Jarosites, which are present in terrestrial gossans capping oxidized sulfides associated with mafic igneous rocks, may also be present in Martian regolith. Spectral characteristics of jarosites are displayed in remote sensed reflectance spectra of bright regions of Mars surface. The occurrence of jarosite in the regolith would imply that acidic permafrost and sulfide ores exist beneath the surface of Mars.

Burns, Roger G.↗

NASA NTRS1986

A comparative petrologic study of iron meteorites with radiogenic Ag-107 anomalies

The characteristics of the sulfide and metal phases of Derrick Peak, Cape York, Grant, Gibeon, and Santa Clara iron meteorite samples are examined. The textures and mineralogies of the samples are described. The effects of thermal and shock events on the distribution of Ag, and the relationship between Ag-107 and Pd are investigated. The correlation between the thermal histories and metal and sulfide nodules of the Gibeon and Santa Clara specimens is analyzed. It is noted that the Pd-107-Ag107 systematics involve the isotopic chemistry and cooling history of iron meteorites, the intensity of late extraterrestrial shock occurrences, and thermal processing.

Teshima, J.↗

NASA NTRS1987

High-temperature oxidation/corrosion of iron-based superalloys

The oxidation and sulfidation of several novel iron-base superalloys were evaluated in high-temperature cyclic tests. The experimental austenitic alloys examined were modifications of NASAUT-4GA which were developed for Stirling-engine application. The weight gains and resulting surface scales were measured and analyzed. Mixed oxide scales were found to form on all specimens exposed above 871 C. The build-up of these scales led to a depletion of Mn and Cr in a zone adjacent to the oxides. In addition, the initial oxidation of the Fe-rich alloy was inhibited by a thin but tenacious Si layer which formed at the interface between oxides and the parent layer. Sulfidation tests using Na2SO4 coatings resulted in the formation of a protective spinel and alpha-Fe2O3 phases. Preferential attack of the carbide phase by hydrogen was not observed after 350 h at 871 C.

Lemkey, F. D.↗

NASA NTRS1987

Sodium sulfate-induced corrosion of pure nickel and superalloy Udimet 700 in a high velocity burner rig at 900 C

Sodium sulfate-induced corrosion of pure nickel and a commercial nickel-base superalloy, Udimet 700 (U-700), were studied at 900 C in a Mach 0.3 burner rig with different Na levels in the combustor. The corrosion rate of Ni was independent of the Na level in the combustor and considerably lower than that measured in laboratory salt spray tests. The lower rates are associated with the deposition of only a small amount of Na2SO4 on the surface of the NiO scale. Corrosion of U-700 was observed to occur in two stages. During the first stage, the corrosion proceeds by reaction of Cr2O3 scale with the Na2SO4 and evaporation of the Na2CrO4 reaction product from the surface of the corroding sample. Cr depletion in the alloy occurs and small sulfide particles are formed in the Cr depletion zone. Extensive sulfidation occurs during the second state of corrosion, and a thick scale forms. The relationship between the corrosion rate of U-700 and the Na level in the combustor gives a good correlation in the range of 0.3 to 1.5 ppm by weight Na. Very low levels of Na in the combustor cause accelerated oxidation of U-700 without producing the typical hot corrosion morphology.

Misra, A. K.↗

NASA NTRS1988

Noble metals in mid-ocean ridge volcanism: A significant fractionation of gold with respect to platinum group metals

Hydrothermal precipitates, black smoker particulate, and massive sulphide dredge samples from the Explorer Ridge on the Juan de Fuca Plate and the TAG hydrothermal area on the Mid-Atlantic Ridge were analyzed for selected noble metals including Au, Ir and Pd by radiochemical neutron activation analysis. The preliminary results indicate that gold contents may reach the ppm range although values in the neighborhood of 100 to 200 ppb are more typical. The platinum group elements (PGE) represented by Ir and Pd are typically less than 0.02 ppb and less than 2 ppb respectively. These abundances represent a significant enrichment of gold relative to the PGE in comparison with average noble metal abundances in mid-ocean ridge basalts (MORB). A partial explanation of this distinctive fractionation can be found in the concepts of sulfur-saturation of basic magma in mid-ocean ridge (MOR) settings, and the origin of MOR hydrothermal fluids. Experimental and petrological data suggest that MORBs are sulfur-saturated at the time of magma generation and that an immiscible sulfide component remains in the mantle residue. Hence, MORBs are noble metal-poor, particularly with respect to PGE. Consequently, black smoker fluids can be expected to reflect the low Ir and Pd contents of the rock column. The average Au content of MORB is 1.3 ppb, and so the rock column is not significantly enriched in Au. The generation of fluids which precipitate solids with 200 ppb Au is apparently dependent on highly efficient fluid chemistry to mobilize Au from the rock column, high Au solubility in seawater hydrothermal fluids and efficient precipitation mechanisms to coprecipitate Au on Fe, Zn and Cu sulfides. Significant differences in these parameters appear to be the ultimate cause of the strong Au-PGE fractionation in the MOR setting. It does not appear from the current data base that MOR hydrothermal fluids are significant contributors to the Ir enrichment seen in Cretaceous-Tertiary boundary sediments.

Crocket, James H.↗

NASA NTRS1988

Geochemistry and origin of gold mineralization in the Kolar Schist Belt

Geological, mineralogical, mineral-textural and geochemical data of the sulfide lodes in the belt indicate that the gold mineralization could be related to low temperature, low Eh and high pH rock-dominated geothermal systems set up in the submarine volcanic pile prior to amphibolite metamorphism. A relatively long-lived geothermal system produced an economic deposit, whereas short-lived ones, because of rapid burial by younger basalts throttled the geothermal system and diffused the discharge yielding low grade ore bodies. The source for gold and iron could be iron enriched tholeiites derived from source regions enriched in komatiitic melt components and komatiitic rocks derived by very low extents of melting of metasomatised mantle sources. On the other hand, the geographical restriction of the quartz-calcite lodes, their mineralogical and geochemical data and their estimated temperature of formation all seem to suggest that a major part of the hydrothermal fluids, and a significant portion of gold could have been derived from mantle derived intrusive, sanukitoid type magma sources, similar to the Champion Gneiss occurring on the eastern part of the belt. However, the possibility of some input by remobilization of a premetamorphic sulfide protore to quartz lodes cannot be ruled out completely.

Siddaiah, N. Siva↗

NASA NTRS1988

Gossans on Mars

The appearance of rusty iron-rich oxidized cappings over sulfide-bearing rocks on earth indicates that similar gossans may have formed on the surface of Mars. Electrochemical processes and thermodynamic relationships linking acidity to oxidation-reduction reactions between primary sulfide minerals and their oxidative weathering products present in the regolith of Mars are discussed. Remote-sensed visible spectra of the Martian surface are in keeping with the presence of poorly-crystalline FeOOH, jarosite, silica, and clay silicates found in gossans, while incompletely weathered pyrrhotite may represent the magnetic material observed in Martian regolith.

Burns, Roger G.↗

NASA NTRS1989

Chemokinetic motility responses of the cyanobacterium oscillatoria terebriformis

Oscillatoria terebriformis, a gliding, filamentous, thermophilic cyanobacterium, exhibited an inhibition of gliding motility upon exposure to fructose. The observed response was transient, and the duration of nonmotility was directly proportional to the concentration of fructose. Upon resumption of motility, the rate of motility was also inversely proportional to the concentration of fructose. Sulfide caused a similar response. The effect of sulfide was specific and not due to either anoxia or negative redox potential. Exposure to glucose, acetate, lactate, or mat interstitial water did not elicit any motility response.

Richardson, Laurie L.↗

NASA NTRS1989

Interfacial chemical reactions between MoS2 lubricants and bearing materials

XPS and conversion-electron Moessbauer spectroscopy (CEMS) were used to examine iron that was deposited on the basal plane of MoS2 single crystals and subjected to vacuum annealing, oxidizing, and reducing environments. Iron either intercalated into the MoS2 structure or formed oriented iron sulfides, depending on the level of excess S in the MoS2 structure. CEMS data demonstrated that iron sulfide crystal structures preferentially aligned with respect to the MoS2 basal plane, and that alignment (and potentially adhesion) could be varied by appropriate high-temperature annealing procedures.

Zabinski, J. S.↗

NASA NTRS1990

Wet chemical passivation of YBa2Cu3O(7-x)

Wet chemical techniques are described for treatment of YBa2Cu3O(7-x) surfaces, which result in the formation of native compounds known to have little or no reactivity to water. Suitable native compounds include CuI, BaSO4, CuS, Cu2S, YF3, and the oxalates. Formation of surface layers in which these nonreactive native compounds are major constituents is verified with X-ray photoelectron spectroscopy (XPS) measurements on YBa2Cu3O(7-x) films treated with dilute solutions of HI, H2SO4, Na2S, HF, or H2C2O4. No significant changes are observed in the XPS spectra when the sulfide, sulfate, or oxalate films are dipped in water, while the iodide and fluoride films show evidence of reaction with water. X-ray diffraction measurements show that the superconducting phase is absent in the sulfide film, but is unaffected by the oxalate and sulfate treatments.

Vasquez, R. P.↗

NASA NTRS1990

Interface properties of various passivations of HgCdTe

The effects of different surface treatments of the p-type Hg(x-1)Cd(x)Te (x about 0.2) alloy, including anodic-sulfide, anodic-oxide, and ZnS coatings, on the surface charge density and surface mobility were investigated. Measurements of surface recombination velocities in samples passivated by either a ZnS coating on a freshly etched surface or by anodic native sulfide combined with deposited ZnS showed essentially the same temperature dependence down to 50 K. For the same original material, both the surface state density and the activation energy were found to be similar. It is suggested that, for both passivations, the surface is controlled by traps that can be related to lattice defects, such as vacancies.

Schacham, S. E.↗

NASA NTRS1991

Passivation Of High-Temperature Superconductors

Surfaces of high-temperature superconductors passivated with native iodides, sulfides, or sulfates formed by chemical treatments after superconductors grown. Passivating compounds nearly insoluble in and unreactive with water and protect underlying superconductors from effects of moisture. Layers of cuprous iodide and of barium sulfate grown. Other candidate passivating surface films: iodides and sulfides of bismuth, strontium, and thallium. Other proposed techniques for formation of passivating layers include deposition and gas-phase reaction.

Vasquez, Richard P.↗