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At least 91 records · Page 5Linked to original sources
USGS1964

Vapor pressure and vapor fractionation of silicate melts of tektite composition

The total vapor pressure of Philippine tektite melts of approximately 70 per cent silica has been determined at temperatures ranging from 1500 to 2100°C. This pressure is 190 ± 40 mm Hg at 1500°C, 450 ± 50 mm at 1800°C and 850 ± 70 mm at 2100° C. Determinations were made by visually observing the temperature at which bubbles began to form at a constant low ambient pressure. By varying the ambient pressure, a boiling point curve was constructed. This curve differs from the equilibrium vapor pressure curve due to surface tension effects. This difference was evaluated by determining the equilibrium bubble size in the melt and calculating the pressure due to surface tension, assuming the latter to be 380 dyn/cm. The relative volatility from tektite melts of the oxides of Na, K, Fe, Al and Si has been determined as a function of temperature, total pressure arid roughly, of oxygen fugacity. The volatility of SiO 2 is decreased and that of Na 2 O and K 2 O is increased in an oxygen-poor environment. Preliminary results indicate that volatilization at 2100°C under atmospheric pressure caused little or no change in the percentage Na 2 O and K 2 O. The ratio Fe 3 Fe 2 "> Fe 3 Fe 2 of the tektite is increased in ambient air at a pressure of 9 × 10 −4 mm Hg (= 10 6.5 atm O 2 , partial pressure) at 2000°C. This suggests that tektites were formed either at lower oxygen pressures or that they are a product of incomplete oxidation of parent material with a still lower ferricferrous ratio.

Geochimica et Cosmochimica Acta↗
USGS1964

Submicroscopic spherules and color of tektites

Magnetic susceptibility measurements of 18 tektites from various strewn fields have been made as a function of temperature from 77°K to room temperature. A relatively large temperature-independent component of the magnetic susceptibility was observed in all cases, and an analysis of the data shows that this component is the result of submicroscopic iron spherules in the tektites. An analysis of the color of tektites in terms of the magnetic measurements and also of the optical absorption spectra suggests that the basic color of all tektites is green or greenish-blue, and that the brown to black coloration in some tektites is due to finely dispersed Fe 2 O 3 and/or many metallic spherules, both probably of colloidal size.

Geochimica et Cosmochimica Acta↗
USGS1964

The electrum-tarnish method for the determination of the fugacity of sulfur in laboratory sulfide systems

A new method for the determination of the fugacity of sulfur in laboratory systems consists of visual observation of the development and decomposition of a sulfide tarnish phase on silver-gold alloy (electrum) of precisely known composition. The alloy system is calibrated against pure sulfur. The method has the following advantages: simple apparatus; ability to cover a large range of fugacity of S 2 ; ability to cover a large temperature range by permitting runs of long duration; ability to tolerate other components in the gas phase; and ease of recovery of the quenched charges for determinations of phases and compositions. Results obtained by the electrum-tarnish method are in satisfactory agreement with those obtained by other workers for the f s2 vs. T curves for the assemblage Ni (1– x ) S + NiS 2 . The electrum-tarnish method shows promise for investigating many other reactions. Univariant reactions studied by this method can be represented as lines forming a genetic grid in terms of the environmental parameters f s2 and T , The slopes of such lines can yield valuable thermodynamic data for the phases involved, but activity coefficients must be known for phases of variable composition.

Geochimica et Cosmochimica Acta↗
USGS1964

A thermodynamic study of pyrite and pyrrhotite

Through the use of the electrum-tarnish method the following equation has been found to interrelate the composition of pyrrhotite, fugacity of sulfur, and temperature: In this equation f s2 is the fugacity of sulfur relative to the ideal diatomic gas at 1 atm, N is the mol fraction of FeS in pyrrhotite (in the system FeS-S 2 ), and T the absolute temperature. The experimental uncertainty in the equation is 0–003 in N . The activity of FeS ( a FeS ) in pyrrhotite relative to the pure substance at the temperature of consideration follows from the above equation by virtue of the Gibbs-Duhem relation; it is given by: The electrum-tarnish method has permitted us to determine the f s2 vs. T curve for the univariant assemblage pyrrhotite-pyrite-vapor from 743 to 325°C. Our determinations of the composition of pyrrhotite are in excellent agreement with the results of Arnold. The activity of FeS in pyrite-saturated pyrrhotite is very different from unity, a fact that greatly influences the interpretation of some other phase equilibrium studies involving pyrrhotite and their application to sulfide mineral assemblages, but has little effect on the more general calculations of composition of hydrothermal or magmatic fluids. Pressure effects calculated from available volumetric data on the phases are small.

Geochimica et Cosmochimica Acta↗
USGS1965

Some aspects of the geochemistry of yttrium and the lanthanides

Recent data on the relative abundances of the lanthanides and yttrium in meteorites, basaltic rocks, granitic rocks and sedimentary rocks are reviewed. It is shown that the data are inadequate to substantiate or to disprove Taylor's derivation from these data of a 1:1 abundance ratio of basaltic to granitic rocks in the continental crust. Graphs are given to illustrate the variation of lanthanides in minerals with paragenesis. Both the paragenesis and the crystal chemistry of minerals affect the composition of the lanthanides.

Geochimica et Cosmochimica Acta↗
USGS1965

Alkali content of alpine ultramafic rocks

The lower limit of abundance of sodium and potassium in ultramafic rocks is less than the threshold amount detectable by conventional analytical methods. By a dilutionaddition modification of the flame-spectrophotometric method, sodium and potassium have been determined in 40 specimens of alpine ultramafic rocks. Samples represent six regions in the United States and one in Australia, and include dunite, peridotite, pyroxenite, and their variably serpentinized and metamorphosed derivatives. The median value found for Na 2 O is 0.004 per cent, and the range of Na 2 O is 0.001–0.19. The median value for K 2 O is 0.0034 per cent and the range is 0.001–0.031 per cent. Alkali concentrations are below 0.01 per cent Na 2 O in 28 samples and below 0.01 per cent K 2 O in 35. Derivation of basalt magma from upper-mantle material similar to such ultramafic rocks, as has been postulated, is precluded by the relative amounts of sodium and potassium, which are from 200 to 600 times more abundant in basalt than in the ultramafic rocks. Similar factors apply to a number of other elements. No reasonable process could produce such concentrations in, for example, tens of thousands of cubic miles of uniform tholeiitic basalt. The ultramafic rocks might have originated either as magmatic crystal precipitates or as mantle residues left after fusion and removal of basaltic magma. Injection of ultramafic rocks to exposed positions is tectonic rather than magmatic.

Geochimica et Cosmochimica Acta↗
USGS1965

Age and metamorphism of some massive sulfide deposits in Virginia, North Carolina and Tennessee

Isotopic ages of vein and wall-rock samples have been determined on five massive sulflde deposits of the southern Appalachians. Vein mineral ages of about 1100 m.y. indicate that some ore bodies formed at least as early as the Grenville metamorphism, and probably soon after the formation of the enclosing gneiss and schist. Present textures of the ore were formed during subsequent metamorphic periods at about 450 m.y. and 300 to 330 m.y. ago.

North Carolina, Tennessee, Virginia↗
USGS1965

Glass electrode determination of sodium in closed basin waters

Because of its direct relation to total dissolved solids content, sodium concentration is the most useful single hydrochemical parameter of closed basin waters in the western United States. Therefore, it is advantageous to have a rapid method for sodium determination which could be readily adapted to field use. Accordingly, thirty waters of highly variable concentration and anionic composition from the western Great Basin have been analyzed with a sodium-sensitive glass electrode. Na + values ranged from 10 to 130,000 ppm in waters containing from 102 to 401,000 ppm total dissolved solids. The electrode values were compared with conventional sodium determinations; results differed by as much as 11 per cent, although over two-thirds were within 5 per cent. No significant relation was found between the glass electrode and conventional method differences and the total dissolved solids or sodium concentration. The comparative accuracy of electrode and conventional procedures was checked, employing solutions of known sodium concentration. It was concluded that for the analysis of sodium in natural waters the glass electrode method offers results comparable in accuracy to those obtained by conventional methods, but with increased simplicity, and therefore the possibility of rapid, accurate field measurements.

Geochimica et Cosmochimica Acta↗
USGS1965

Geochemistry of Birch Creek, Inyo County, California a travertine depositing creek in an arid climate

A small stream in eastern California was studied in detail to determine the causes of travertine deposition from the stream. Although the ground water feeding the stream is slightly supersaturated to just saturated with calcite, the ground water is supersaturated with CO 2 with respect to the overlying air. The water becomes increasingly supersaturated with calcite in a downstream direction as CO 2 loss to the air and by photosynthesis exceeds the rate of precipitation of calcite. Water loss by evaporation was below detection and does not play an important role in calcite deposition in this case. The water loss is by seepage through the stream bed. Observations showed variations in HCO − 3 , pH, discharge, Ca +2 , relative humidity, water and air temperatures, and dissolved CO 2 . Comparisons of observed and equilibrium states are given for five sampling stations along the stream.

California↗
USGS1966

Hot brines and recent iron deposits in deeps of the Red Sea

Sedimentary iron and heavy-metal deposits of undetermined size have been found in the middle of the Red Sea some 2000 meters below the surface of the sea (Fig. 1). This discovery has been made from the Research Vessel Atlantis II , which is still at sea engaged in a series of oceanographic investigations which ultimately will end in November 1965, after the ship has circumnavigated the globe. The discovery is significant because the environment and the processes controlling deposition of heavy metals are observable and appear to be still active.

Geochimica et Cosmochimica Acta↗
USGS1966

Some alkali and titania analyses of tektites before and after G-1 precision monitoring

A comparison of 55 older analyses of Australasian tektites with 110 modern precisely monitored analyses suggests that more than half of the older alkali and titania determinations are decidedly inaccurate and misleading. Deviations of the older analyses from the restricted values of the modern analyses are comparable to the imprecisions shown by early analyses of G-1 granite and W-1 diabase. This suggests that a high percentage of older alkali and titania analyses, such as those of Washington's tables, are of questionable quality.

Geochimica et Cosmochimica Acta↗
USGS1966

Chemical composition of phosphorites of the Phosphoria Formation

The chemical composition, both major and minor constituents, of 60 samples of phosphorite from the Phosphoria Formation was determined. Major constituents of the average phosphorite are, by weight per cent: SiO 2 , 11·9; Al 2 O 3 , 1·7; Fe 2 O 3 ,1·1; MgO, 0·3; CaO, 44·0; Na 2 O, 0·6; K 2 O, 0·5; total H 2 O, 2·2; H 2 O − , 0·6; TiO 2 , 0·1; P 2 O 5 , 30·5; CO 2 , 2·2; SO 3 , 1·8; F, 3·1; organic matter, 2·1; and oil, 0·2. Uranium averages 0·009 per cent. The phosphate mineral is basically apatite, Ca 5 (PO 4 ) 3 F, with small but significant and variable substitutions—Na, Sr, U and Th for Ca, and CO 3 and SO 4 for PO 4 . Rare metals not associated with apatite are associated principally with the organic-matter component of the rocks. This group includes As, Ag, Cd, Cr, Cu, Mo, Ni, Sb, Se, V and Zn. Chromium is the most abundant, having a modal abundance of 0·1 per cent and a maximum concentration of 0·3 per cent. The average phosphorite is composed of approximately 80 per cent apatite, 10 per cent quartz, 5 per cent muscovite-illite, 2 per cent organic matter, 1 per cent dolomite—calcite, 1 per cent iron oxide, and 1 per cent other components. It is texturally a medium-grained pellet phosphorite.

Geochimica et Cosmochimica Acta↗
USGS1967

U.S. Geological Survey silicate rock standards

The U.S. Geological Survey has processed six silicate rocks to provide new reference samples to supplement G-1 and W-1. Complete conventional, rapid rock, and spectrochemical analyses by the U.S. Geological Survey are reported for a granite (replacement for G-1), a granodiorite, an andesite, a peridotite, a dunite, and a basalt. Analyses of variance for nickel, chromium, copper, and zirconium in each rock sample showed that for these elements, the rocks can be considered homogeneous. Spectrochemical estimates are given for the nickel, chromium, copper, and zirconium contents of the samples. The petrography of five of the six rocks is described and CIPW norms are presented.

Geochimica et Cosmochimica Acta↗
USGS1967

A low pressure investigation of the stability of phlogopite

Phlogopite reacts to form orthorhombic kalsilite, leucite, forsterite and fluid (H 2 O) at 1012 ± 7° C , 400 bars pressure, and 905 ± 5° C at 100 bars pressure. The heat of reaction is 39 ± 4 kcal compared to 44 ± 6kcal for the reaction of annite to form orthorhombic kalsilite, leueite, fayalite and fluid (H 2 O). The results are consistent with earlier experiments, but not with the interpretations of those experiments.

Geochimica et Cosmochimica Acta↗
USGS1967

K Ar and Rb Sr measurements on P-207, the U.S.G.S. interlaboratory standard muscovite

Potassium-argon analyses of P-207 in twenty-four laboratories and rubidium-strontium analyses in ten laboratories indicate that for material of this typo the average interlaboratory precision for KAr ages is 2.6% and for RbSr ages is 3.4%, and the average intralaboratory precision for KAr ages is 1.6% and for RbSr ages is 2.9%.

Geochimica et Cosmochimica Acta↗
USGS1967

Comparison of Macedon and Darwin glass

Chemical analyses are presented for major and minor elements in two specimens of natural glass reported from Macedon, Victoria, and are compared with new analyses of glass from Mt. Darwin, Tasmania. One specimen of Macedon glass is dark, the other light; both are spongy with relatively large cavities of size uncommon in Darwin glass. Some of the new analyses of Darwin glass extend considerably the compositional range previously reported for Mg, Ni and Co. The chemical composition of Macedon glass cannot be distinguished from that of Darwin glass for any of twenty-five elements investigated. It appears possible that the two specimens of glass reported from Macedon may represent either two mislabelled pieces of Darwin glass, or else a separate natural occurrence of Darwin glass 560 km north of Mt. Darwin.

Geochimica et Cosmochimica Acta↗
USGS1967

Petrology of eucrites, howardites and mesosiderites

The eucrite and howardite calcium-rich achondrites and many mesosiderites are considered as a coherent meteorite assemblage, their silicates consisting essentially of calciumpoor monoclinic and orthorhombic pyroxenes and calcium-rich plagioclase feldspar. The achondrites can be grouped according to their brecciated structure as follows: eucrites—unbrecciated and monomict brecciated achondrites; howardites—polymict brecciated achondrites. Many mesosiderites contain brecciated structures; they are distinguished from the achondrites by their large metallic fraction. The structure and composition of rock fragments in the breccias indicate a complicated sequence of events including magmatic differentiation, brecciation, recrystallization and refragmentation, and ejection from the parent body. Detailed mineralogical and chemical data suggest that the magmatic differentiation proceeded primarily by the separation of pyroxene from an ultrabasic parent material that had a much lower alkali content than ordinary chondrites. Magmatic crystallization took place in environments ranging from extrusive to deep-seated intrusive. Polymict breccias contain fragments with a wide variety of magmatic and recrystallization textures, which suggests that the breccias were formed either in very large or repeated fragmentation events. Monomict breccias contain fragments with a small range of similar magmatic textures, which suggests that these breccias were formed by small or single events. Petrographic evidence suggests that many of the breccias are impact breccias. Either in their original magmatic crystallization sites or in the sites of breccia accumulation, most of these meteorites apparently had a near-surface location prior to ejection from the parent body. Evidence obtained from eucrites, howardites and mesosiderites forms an important part of our understanding of the early evolution of the surface regions of their parent body. Chemical and oxidation conditions were different from those presently found in the Earth's crust and upper mantle, but the necessary conditions may have been present in the early history of the Earth. A lunar origin for eucrites, howardites and mesosiderites is proposed, but an asteroidal origin can not be presently excluded.

Geochimica et Cosmochimica Acta↗