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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Temperature-dependent Mg, Fe distribution in a lunar olivine.

The nuclear hyperfine doublets of Fe-57 at the two nonequivalent octahedrally coordinated positions M1 and M2 in an olivine from lunar basalt 12018 have been analyzed in the temperature range between 250 and 450 C. The Mg(2+), Fe(2+) is somewhat ordered, the Fe(2+) occupancy being greater at one of the two M positions by approximately 20%. After heating at 1155 C for seven days, the distribution was almost completely disordered. Studies of natural and heated olivines from terrestrial rocks show that the Mg(2+), Fe(2+) exchange reaction in olivines is considerably more sluggish than in chain silicates. Generally no change in site occupancy can be observed after heating at temperatures of approximately 900 C over several days. The observed degree of ordering in the lunar olivine is indicative of slow cooling during crystallization.

Virgo, D.↗
NASA NTRS1974

A scanning electron microscope study of olivine crystal surfaces

SEM photographs were taken of euhedral olivine grains from the Murchison C2 chondrite and several terrestrial and lunar occurrences. In general, the crystal faces of the meteorite grains are rough and uneven, with irregular growth patterns. They are very similar to crystal faces on terrestrial olivine grains that formed by sublimation from a vapor phase. They are very different from the relatively smooth and featureless surfaces of magmatic olivine crystals that precipitated from igneous melts. Qualitatively, the surface morphology of the crystal supports the contention that many euhedral crystals of olivine in C2 meteorites condensed from a gas phase.

Olsen, E. J.↗
NASA NTRS1975

Calculated diffusion coefficients and the growth rate of olivine in a basalt magma

Concentration gradients in glass adjacent to skeletal olivines in a basalt have been examined by electron probe. The glass is depleted in Mg, Fe, and Cr and enriched in Si, Al, Na, and Ca relative to that far from olivine. Ionic diffusion coefficients for the glass compositions are calculated from temperature, ionic radius and melt viscosity, using the Stokes-Einstein relation. At 1170 C, the diffusion coefficient of Mg(2+) ions in the basalt is 4.5 billionths sq cm per sec. Comparison with measured diffusion coefficients in a mugearite suggests this value may be 16 times too small. The concentration gradient data and the diffusion coefficients are used to calculate instantaneous olivine growth rates. Growth necessarily preceded emplacement such that the composition of the crystals plus the enclosing glass need not be that of a melt. The computed olivine growth rates are compatible with the rate of crystallization deduced for the Skaegaard intrusion.

Donaldson, C. H.↗
NASA NTRS1975

Pyroxenes and olivines: Structural implications of shock-wave data for high pressure phases

The nature of the shock-induced, high-pressure phases of olivine and pyroxene rocks is examined in the light of data for the densities of a new class of perovskite-related silicate structures. Also examined are some new Hugoniot and release adiabat data for bronzite. Reexamining available shock data for magnesian pyroxenes and olivines leads to the conclusion that they define a mixed phase (or disequilibrium) region to about the 100 GPa range, related to the kinetics of phase transformation in these silicates. By recognizing this point, certain discrepancies in previous interpretations of shock data can be explained. A set of theoretical Hugonoits for pyroxene and olivine stoichiometry, perovskite-bearing assemblages was constructed based on their properties deduced from high-pressure work, showing that the shock data is compatible with transformations to perovskites in the 45-7GPa region. Finally, the shock data indicate very similar properties for olivine and pyroxene at high pressures making them both equally likely candidates for the lower mantle.

Jeanloz, R.↗
NASA NTRS1978

Textural evidence bearing on the origin of isolated olivine crystals in C2 carbonaceous chondrites

In some cases the mechanical competence of chondrules in carbonaceous chondrites has been reduced by alteration of their mesostasis glass to friable phyllosilicate, providing a mechanism by which euhedral olivines can be separated from chondrules. Morphological features of isolate olivine grains found in carbonaceous chondrites are similar to those of olivine phenocrysts in chondrules. These observations suggest that the isolated olivine grains formed in chondrules, by crystallization from a liquid, rather than by condensation from a vapor.

Richardson, S. M.↗
NASA NTRS1977

Cooling rate estimations based on kinetic modelling of Fe-Mg diffusion in olivine

A finite one-dimensional kinetic model was developed to estimate the cooling rates of lunar rocks. The model takes into consideration the compositional zonation of olivine and applies Buening and Buseck (1973) data on ion diffusion in olivine. Since the 'as-solidified' profile of a given olivine is not known, a step-function, with infinite gradient, is assumed; the position of this step is based on mass balance considerations of the measured compositional profile. A minimum cooling rate would be associated with the preservation of a given gradient. The linear cooling rates of lunar rocks 12002 and 15555 were estimated by use of the olivine cooling-rate indicator to be 10 C/day and 5 C/day, respectively. These values are lower than those obtained by dynamic crystallization studies (10-20 C/day).

Taylor, L. A.↗
NASA NTRS1977

Applications of dynamic crystallization studies - Lunar olivine-normative basalts

Dynamic crystallization studies were performed on two synthetic glasses similar in composition to the mare olivine-normative basalt samples 12009 and 15555. The effects of viscosity (primarily a function of FeO) and the initial temperature of cooling on mineral chemistry, texture, and temperature of appearance of phases were investigated. Olivine compositions seem to indicate that, for the two types of melt cooled at the same rate, there are no significant differences in the degree of undercooling at which olivine nucleates, but it is found that olivine-nucleated densities differ. The cooling rate of 15555 is estimated. Since the temperature at which cooling is initiated affects texture, mineral chemistry, and temperature of appearance of phases so greatly, caution is recommended in the application of experimental data to natural rock systems.

Bianco, A. S.↗
NASA NTRS1979

Optical absorption and radiative heat transport in olivine at high temperature

Results are presented of measurements of the optical absorption spectra (300-8000 nm) of olivine as a function of temperature (300-1700 K) under conditions of controlled and known oxygen fugacity within the stability field of the samples. The absorption spectra are used to calculate the temperature-dependent radiative transfer coefficient of olivine and to numerically study the accuracy of the method. The present absorption measurements in olivine under oxidizing conditions known to be within the olivine stability field indicate that the effective radiative conductivity K(R) is lower than that obtained in previous studies under different experimental conditions. The lower value of K(R) makes it more likely that some of the earth's internal heat is removed by convection and less likely that thermal models involving conduction and radiation alone will satisfactorily explain thermal conditions in the earth's mantle.

Shankland, T. J.↗
NASA NTRS1978

Genesis of Apollo 15 olivine normative mare basalts - Trace element correlations

Chemical and petrographic studies of 21 Apollo 15 rake basalts are reported; one pyroxene-phyric basalt, four olivine-phyric basalts, 15 olivine microgabbros, and one brecciated basalt are distinguished. The olivine-normative basalts (i.e., all samples but the pyroxene-phyric basalt) show correlation trends of La/Sm vs. La and Sm/Eu vs. La that are within the observed dispersion ranges of La/Sm and Sm/Eu ratios for a single lava flow. Trace element fractionation trends in the sample could be due to mobilization (perhaps by filter pressing) of fractionated residual liquid during the crystallization of a lava flow. However, it is also possible that the suite of olivine-normative basalts represents several basalt flows produced by partial melting of cumulate layers having interstitial liquids with variable trace-element compositions.

Ma, M.-S.↗
NASA NTRS1979

Trace elements in the Allende meteorite. IV - Amoeboid olivine aggregates

INAA data for Ca, Sc, Hf, La, Ce, Sm, Eu, Tb, Yb, Lu, Os, Ir, Ru, Na, Cl, Br, Fe, Mn, Cr, Co, Au, As, and Sb are presented for ten amoeboid aggregates from the Allende meteorite. Only one lacks olivine. Seven of the remainder, as a group, have cosmic proportions of refractory lithophile and siderophile elements and appear to have formed when coarse-grained Allende inclusion material underwent partial reaction with a low-temperature nebular gas and mixture with FeO-rich olivine. The other two have highly fractionated abundances of refractory elements relative to one another compared to Cl chondrites, including Group II REE patterns, and probably formed by the mixing of fine-grained Allende inclusion material with FeO-rich olivine. Non-refractory siderophile components are also different in composition in each type of amoeboid olivine aggregate.

Grossman, L.↗
NASA NTRS1979

Petrography and olivine mineral chemistry of chondrules and inclusions in the Allende meteorite

The examination of 19 polished-thin sections from a sample of the Allende meteorite reveals that anhedral olivine chondrules (OCs) are the most abundant chondrule type (19.7% of total chondrules + inclusions), followed by recrystallized OCs (7.5%), euhedral OCs (2.9%), glassy/skeletal OCs (2.7%), and barred OCs (2.3%). The most abundant inclusions are those comprised of many small olivine crystals with varying amounts of opaque minerals and glass (31.5%), followed by fine grained inclusions (16.1%) and single olivine crystals (15.7%). Microprobe analyses of olivines show that recrystallized and barred chondrules are the most forsteritic and exhibit relatively narrow compositional ranges, whereas single crystals have the widest range, from Fa sub 0.3 to Fa sub 49.

Simon, S. B.↗
NASA NTRS1982

Olivine flotation and settling experiments on the join Mg2SiO4-Fe2SiO4

Results are presented of a study of some unusual density relations between olivine and coexisting liquid in the system fosterite-fayalite. At 1 atmosphere pressure it is found that olivine floats on its coexisting liquid for intermediate compositions on this binary because of extreme partitioning of Fe into the melt phase. At 20 kilobars, the usual behavior of olivine settling is found to occur because the partitioning of Fe in the melt is reduced, aided possibly by the dissolution of CO2 in the melt from the use of a graphite container. It is determined that olivine flotation and settling are rapid in a time period of only a few hours because viscosities are slightly greater than that of paraffin oil at room temperature. Some adcumulate textures with good triple junction grain boundaries are found to be developed. Observations of differentiated magmatic systems suggest that the mechanisms by which magmas can differentiate vary considerably in the ultramafic to tholeiitic compositional range.

Herzberg, C. T.↗
NASA NTRS1984

The meteorite-asteroid connection - Two olivine-rich asteroids

Two asteroids have clear indications of olivine-rich surface petrology without any indication of pyroxene or plagioclase, suggesting that the olivine may be quite pure. They provide probable examples of mantles of differentiated parent asteroids exposed by fragmentation and are good candidates for the parent bodies of the unusual olivine meteorite Brachina or the olivine-iron alloy meteorites called pallasites.

Cruikshank, D. P.↗
NASA NTRS1984

Petrography and classification of Ca, Al-rich and olivine-rich inclusions in the Allende CV3 chondrite

The results of a detailed, systematic petrographic survey of Ca, Al-rich and olivine-rich inclusions in the Allende CV3 chondrite are reported, and a new classification system based on clearly defined and readily applied petrographic criteria is presented. Most Allende inclusions are aggregates containing one or more of three distinct constituents: (1) rimmed concentric objects enriched in Al- and Ti-rich oxide minerals and various amounts of Ca-rich silicates; (2) porous, 'fine-grained' chaotic material enriched in Ca-rich silicates, especially clinopyroxenes and garnets; and (3) porous, 'fine-grained', mafic inclusion matrix, enriched in olivine, pyroxene, and feldspathoids. Two texturally distinct varieties of inclusions consist primarily of inclusion matrix: unrimmed olivine aggregates and rimmed olivine aggregates. Ca, Al-rich inclusions are classified on the basis of the size and abundance of their constituent concentric objects. Some fundamental relationships among Allende inclusions that previusly have not been emphasized are discussed.

Kormacki, A. S.↗
NASA NTRS1987

Barred olivine chondrules in ordinary chondrites

The characteristics of 125 barred olivine chondrules identified in polished thin sections of 26 H-L-LL3-6 chondrites were studied, using an electron microprobe for determining mineral abundances and a broad-beam electron microprobe for determining bulk compositions. Compared with all chondrule types in the H-L-LL3 ordinary chondrites, the barred olivine chondrules were found to be enriched in FeO, Al2O3, Na2O, and K2O and depleted in SiO2 and MgO; they were also found to be enriched in the feldspathic component. It is concluded that the precursor components of barred olivine chondrules consisted of a mixture of an olivine-rich component, a refractory-rich component containing Al2O3, TiO2, and CaO, and a volatile-rich alkalic component. This unusual combination implies high temperatures of melting for the refractory component and low temperatures for the volatile component, supporting the hypothesis of Grossman and Wasson (1983) that the alkalies condensed on the alumina already available.

Weisberg, Michael K.↗
NASA NTRS1987

Relation of the spectroscopic reflectance of olivine to mineral chemistry and some remote sensing implications

High-resolution visible and near-IR diffuse spectral reflectance are used to systematically investigate apparent wavelength shifts as a function of mineral chemistry in the Fe/Mg olivine series from Fo(11) to Fo(91). The study also shows that trace amounts of nickel can be spectrally detected in the olivine structure. Significant compositional information can only be extracted at relatively high resolution, because the overall spectral characteristics of the olivines change only subtly as a function of the Fe/Mg ratio. This laboratory study is expected to aid in the interpretation of remotely sensed data from both terrestrial and extraterrestrial bodies. Terrestrial applications may include the recognition of ultramafic, ultrabasic, and basaltic terrains which in themselves may have mineral potential. Among extraterrestrial applications, the asteroids are obvious candidates for further examination. Some permutations of Fe-Mg-Ni relations in olivines are discussed as they apply to the interpretation of asteroid surfaces and other extraterrestrial bodies.

King, Trude V. V.↗
NASA NTRS1990

Kamacite and olivine in ordinary chondrites - Intergroup and intragroup relationships

Results are presented from high-precision electron microprobe analyses of olivine and kamacite in a suite of 134 ordinary chondrites (OCs). The compositional ranges of these phases are defined for each OC group (high total Fe, low total Fe, and low total Fe/low metallic Fe). Anomalous OCs that have olivine and/or kamacite compositions that lie outside the established ranges are identified. The phases in the chondritic clasts of the Netschaevo iron meteorite are characterized to determine the relationship between Netschaevo and OCs. Intragroup variations of olivine and kamacite compositions with petrologic type are examined and OCs that contain olivine and/or kamacite grains with aberrant compositions are identified as fragmental breccias. Also, a search for new metallic Fe-Ni phases with extreme compositions is conducted. As a result of these analyses, several meteorites are reclassified.

Rubin, Alan E.↗
NASA NTRS1991

Moessbauer spectra of olivine-rich achondrites - Evidence for preterrestrial redox reactions

Moessbauer spectral measurements at 4.2 K were made on several ureilites and the two shergottites found in Antarctica, as well as two ureilite falls, three SNC meteorite falls, and two finds in order to distinguish products of preterrestrial redox reactions from phases formed during oxidative weathering on the earth. The spectra indicated that several ureilites contain major proportions of metallic iron, much of which resulted from preterrestrial carbon-induced reduction of ferrous iron in the outermost 10-100 microns of olivine grains in contact with carbonaceous material in the ureilites. The cryptocrystalline nature of these Fe inclusions in olivine renders the metal extremely vulnerable to aerial oxidation, even in ureilites collected as falls. It is inferred that the nanophase ferric oxides or oxyhydroxides identified in Brachina and Lafayette were produced by terrestrial weather of olivines before the meteorites were found. The absence of goethite in two olivine-bearing Antarctic shergottites suggests that the 2 percent ferric iron determined in their Moessbauer spectra also originated from oxidation on Mars.

Burns, R. G.↗