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At least 253 records · Page 14
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NASA NTRS1989

Moessbauer spectroscopy and scanning electron microscopy of the Murchison meteorite

Meteorites provide a wealth of information about the solar system's formation, since they have similar building blocks as the Earth's crust but have been virtually unaltered since their formation. Some stony meteorites contain minerals and silicate inclusions, called chondrules, in the matrix. Utilizing Moessbauer spectroscopy, we identified minerals in the Murchison meteorite, a carbonaceous chondritic meteorite, by the gamma ray resonance lines observed. Absorption patterns of the spectra were found due to the minerals olivine and phyllosilicate. We used a scanning electron microscope to describe the structure of the chondrules in the Murchison meteorite. The chondrules were found to be deformed due to weathering of the meteorite. Diameters varied in size from 0.2 to 0.5 mm. Further enhancement of the microscopic imagery using a digital image processor was used to describe the physical characteristics of the inclusions.

Brown, Christopher L.↗
NASA NTRS1991

Solution and shock-induced exsolution of argon in vitreous carbon

To add to the knowledge of noble gas solution and exsolution in carbonaceus material, experiments were performed on vitreous carbon. Ar-rich vitreous carbon samples were prepared under vapor-saturated conditions using argon as the pressurizing medium. Solubility data were obtained for temperatures of 773 to 973 K and pressures of 250 to 1500 bars. Up to 7 wt pct Ar was dissolved in the carbon. The solubility data were compared to a thermodynamic model of argon atoms dissolving into a fixed population of 'holes' in the carbon. Two variations of the model yielded estimates of the enthalpy of solution of Ar in vitreous carbon equal to about -4700 cal/mole. Preliminary shock experiments showed that 28 percent of the total argon was released by driving 4 GPa shocks into the argon-rich carbon. It was demonstrated that shock-induced argon loss is not simply caused by the impact-induced diminution of grain size. The present value of shock pressure required for partial impact devolatilization of Ar from carbon is below the range (5-30 GPa) at which H2O is released from phyllosilicates.

Gazis, Carey↗
NASA NTRS1991

Clay minerals in primitive meteorites and interplanetary dust 2. Smectites and micas

The classification is briefly summarized of stony meteorites and cosmic dust, and the mineralogy and chemistry is described of serpentine group minerals. The occurrence of smectites and micas in extraterrestrial materials is examined. The characterization of fine grained minerals in meteorites and IDPs relies heavily on electron beam instruments, especially the transmission electron microscope (TEM). Typically, phyllosilicates are identified by a combination of high resolution imaging of basal spacings, electron diffraction, and chemical analysis. Smectites can be difficult to differentiate from micas because the smectites lose their interlayer water and the interlayer partly collapse in the high vacuum of the TEM.

Keller, L. P.↗
NASA NTRS1991

Pre-terrestrial oxidation products in carbonaceous meteorites identified by Mossbauer spectroscopy

The occurrence of ferric bearing assemblages, comprising phyllosilicates, oxide hydroxides and magnetite, in carbonaceous chondrites (CC) indicates that these meteorites underwent pre-terrestrial, sub-aqueous oxidation reactions. Reported here are results of a Mossbauer spectral study of a suite of CC demonstrating that a variety of ferrous and ferric bearing phases may be distinguished in different classes of this meteorite type.

Burns, Roger G.↗
NASA NTRS1991

Compositional studies of primitive asteroids

The aqueous alteration history in the solar system are studied through acquiring additional CCD reflectance spectra in the blue-UV through near-IR spectral region and analyzing these spectra for information about iron oxides in phyllosilicates identified in the CM and CI carbonaceous chondrites. Emphasis is on the main-belt and Cybele primitive asteroids, as these asteroids show spectral diversity and are also spectral analogues for known meteorite samples. The porphyrin bands found in organics near 0.4 micron is also sought.

Vilas, Faith↗
NASA NTRS1991

Aqueous alteration on the parent bodies of carbonaceous chondrites: Computer simulations of late-stage oxidation

CI carbonaceous chondrites may be products of hydrous alteration of CV- or anhydrous CM-type materials. The CIs typically contain veins filled with carbonates and sulfates, probably indicating a period of late stage aqueous alteration under oxidizing conditions. To test this idea, computer simulations of aqueous alteration of CV- and CM-type carbonaceous were performed. Simulations were restricted to the oxidation of hydrous mineral assemblages produced in previous simulations in order to determine whether further reaction and oxidation results in the phyllosilicate, carbonate, sulfate and oxide vein assemblages typical of CI carbonaceous chondrites. Our simulations were performed at 1, 25, 100, and 150 C (the appropriate temperature range) for the CV and CM mineral assemblages and using the computer code EQ3/6.

Bourcier, W. L.↗
NASA NTRS1992

Spectral identification of chemisorbed CO2 and application to Mars analog materials

The goal of this work is to identify the spectral signature of chemisorbed CO2, to test the efficacy of carbonate formation on Mars-analog materials via CO2 chemisorption, and to identify the surface-chemical characteristics of good chemisorbents, with the intent of assessing the possible geochemical importance of CO2 chemisorption as a quasipermanent CO2 sink in the Martian environment. Our approach is to search for infrared spectral bands that result from chemisorption of CO2 molecules onto chemical reagents and Mars-analog materials, and to identify the salient differences in adsorbents that favor strong, permanent CO2 chemisorption. The total amount of CO2 in the early Martian atmosphere, and consequent surface temperatures, are unknown. A CO2 greenhouse may not have been an adequate mechanism under any circumstances; however, it if were, then most of that CO2 must still be in the near-surface environment; no escape mechanism that could remove it after the decline of channeling has been identified. The only plausible reservoir is carbonate, and there are various remote sensing techniques that can be used to search for it. We are investigating CO2 chemisorption as a permanent CO2 sink, and to aid in interpretation of remotely sensed IR spectra of Mars. A common effect reported in CO2 adsorption studies is the formation of a layer of carbonate or bicarbonate anions on adsorbents that have OH- groups available on their surfaces. Inorganic hydroxyls occur on phyllosilicates, amorphous silicates, metal oxides and hydroxides; it is the most abundant and reactive surface functional group on the surfaces of terrestrial silicates. The process responsible for the reaction is chemisorption. Chemisorption is distinguished from physical adsorption in that there is a transfer of electrons between species, and the formation of a chemical bond. The heat of chemisorption is typically of the same order as heats of chemical reaction (i.e., a few hundred to a few thousand kJ/mole), as opposed to heats of physical adsorption (a few kJ per mole). Chemisorption is an activated process that is promoted by an increase in temperature - quite the opposite of physical adsorption. Chemisorption is not reversible in the sense that physical adsorption is.

Zent, A. P.↗
NASA NTRS1992

Preliminary results of an experimental study of the interactions of basalt glass and a water vapor atmosphere: Implications for weathering on Mars

Models of weathering processes on the surface of Mars invoke hydrothermal alteration as the primary mechanism responsible for clay formation. Previous experimental studies of basalt glass interactions with water under hydrothermal conditions demonstrate that phyllosilicates and zeolites are the primary alteration minerals. Gas-solid weathering is thought to be less thermodynamically favorable and relatively unimportant, however, the experimental alteration of basalt glass under vapor ion conditions (large SA/V) can result in the formation of clay minerals, zeolites, and hydrated calcium silicates. We have undertaken a study of the reacted layers formed on basalt glasses experimentally altered under vapor hydration conditions to resolve this issue. High SA/V ratios promote reaction product buildup in solution and promote alteration mineral formation. It was previously shown that these reaction conditions promote weathering processes similar to those found in nature for tektite glasses, rhyolitic glasses, and basalt glasses. The hydration of basalt glass is described for 100 percent relative humidity experiments at temperatures of 150, 175, and 200 C for up to 400 days. Preliminary characterization of the alteration layers with analytical electron and scanning microscopy suggest that the reaction mechanism includes precipitation of a smectite clay on the outermost surface of the glass. Between the clay and the unreacted glass is an amorphous gel-like phase (palagonite). Our results provide an experimental basis for proposing that basalt glass interactions with vapor atmospheres can be an important source of clay minerals.

Mazer, J. J.↗
NASA NTRS1992

Alteration of hydrovolcanic basaltic ash - Observations with visible and near-infrared spectrometry

Altered basaltic tephras from tuff rings and tuff cones across the Basin and Range were examined using visible and NIR (Vis/IR) reflectance spectrometry and a variety of other techniques. It was found that Vis/IR spectrometry is sensitive to subtle changes in the hydration and oxidation state of these tephras. In the tuffs examined, ferrous and ferric iron minerals produce one composite absorption feature rather than two resolvable bands. Different styles of alteration were noted between thinly bedded tuff ring deposits emplaced by high energy, largely dry, pyroclastic surge and massively bedded tuff cone deposits emplaced by wetter, lower energy pyroclastic flow. The former class of tephras become hydrated and only moderately oxidized with some limited development of dioctehedral smectite clay minerals and minimal palagomitization. The latter class of tephras are highly palagonitized which implies a high degree of hydration and oxidation. In the most of highly altered tephras, all the Fe(2+) that is initially in the volcanic glass is converted to Fe(3+) within nanophase and bulk ferric oxide phases in the palagonite. There is also more extensive development in tuff cone beds of zeolite and phyllosilicate minerals. The differences in reflectance spectra of altered basaltic tephras can be traced to initial differences in the water/magma ratio extant at the volcanic vent.

Farrand, William H.↗
NASA NTRS1993

The CR (Renazzo-type) carbonaceous chondrite group and its implications

A petrologic, geochemical, and oxygen isotropic study of the CR chondrites including Renazzo, Al Rais, El Djouf 001 and the paired Acfer meteorites, EET87770 and the paired samples, MAC87320, Y790112, Y793495, and Y791498 is presented. It is concluded that the CR group is characterized by abundant large multilayered, Fe, Ni metal-rich, type I chondrules; abundant matrix and dark inclusions; unique assemblages of serpentine and chlorite-rich phyllosilicates and Ca-carbonates; Ca-carbonate rims on chondrules; abundant Fe, Ni metal with a positive Ni vs. Co trend and a solar Ni:Co ratio; and amoeboid olivine aggregates with Mn-rich and Mn-poor forsterite.

Weisberg, Michael K.↗
NASA NTRS1993

Iron oxide bands in the visible and near-infrared reflectance spectra of primitive asteroids

High resolution reflectance spectra of primitive asteroids (C, P, and D class and associated subclasses) have commonly revealed an absorption feature centered at 0.7 microns attributed to an Fe(2+)-Fe(3+) charge transfer transition in iron oxides and/or oxidized iron in phyllosilicates. A smaller feature identified at 0.43 microns has been attributed to an Fe(3+) spin-forbidden transition in iron oxides. In the spectra of the two main-belt primitive asteroids 368 Haidea (D) and 877 Walkure (F), weak absorption features which were centered near the location of 0.60-0.65 microns and 0.80-0.90 microns prompted a search for features at these wavelengths and an attempt to identify their origin(s). The CCD reflectance spectra obtained between 1982-1992 were reviewed for similar absorption features located near these wavelengths. The spectra of asteroids in which these absorption features have been identified are shown. These spectra are plotted in order of increasing heliocentric distance. No division of the asteroids by class has been attempted here (although the absence of these features in the anhydrous S-class asteroids, many of which have presumably undergone full heating and differentiation should be noted). For this study, each spectrum was treated as a continuum with discrete absorption features superimposed on it. For each object, a linear least squares fit to the data points defined a simple linear continuum. The linear continuum was then divided into each spectrum, thus removing the sloped continuum and permitting the intercomparison of residual spectral features.

Jarvis, Kandy S.↗
NASA NTRS1993

Bright soil units on Mars determined from ISM imaging spectrometer data

The lithology of bright Martian soil provides evidence for chemical and physical processes that have modified the planet's surface. Data from the ISM imaging spectrometer, which observed much of the equatorial region at a spatial resolution of approximately 22 km, cover the NIR wavelength range critical to ascertaining the presence and abundance of Fe-containing phases, hydroxylated silicates, and H2O in the bright soil. ISM data previously have revealed spatial variations in depth of the 3.0-microns H2O absorption suggesting differences in water content, a weak absorption at 2.2 microns indicative of metal-OH in phyllosilicate, and variations in the 1-micron Fe absorption indicative of differences in Fe mineralogy. This paper summarizes first results of a systematic investigation of spectral heterogeneity in bright soils observed by ISM. At least seven 'units' with distinctive properties were discriminated. Comparison of their spatial distributions with Viking data shows that they generally correspond with previously recognized morphologic, color, and thermal features. These correspondences and the units' spectral attributes provide evidence for lithologic differences between the soils in different geologic settings.

Murchie, Scott↗
NASA NTRS1993

Comparison of visible and near-infrared reflectance spectra of CM2 carbonaceous chondrites and primitive asteroids

Spectra of primitive asteroids (defined as C, P, and D classes and associated subclasses) were compared to the limited number of spectra of CM2 carbonaceous chondrites. An absorption feature located at 0.7 microns attributed to an Fe(+2) - Fe(+3) charge transfer absorption in iron oxides in phyllosilicates is apparent in some of the CM2 carbonaceous chondrite spectra and many of the asteroid spectra. Sawyer found a correlation between the area of the 0.7 micron feature and the mean semimajor axis of the asteroids. Spectra of a larger sample of carbonaceous chondrites, including 7 CM2 chondrites, covering a spectral interval of 0.30-2.5 microns were recently obtained using the Relab instrument at Brown University. These spectra were compared with spectrophotometric asteroid observations in a separate abstract. Those spectra of CM2 chondrites were isolated into the UV, visible and near-infrared spectral regions in order to compare them with high-quality narrowband reflectance spectra.

Vilas, F.↗
NASA NTRS1994

Alteration and formation of rims on the CM parent body

All types of coarse-grained components in CM chondrites are surrounded by fine-grained dust coatings, but the origin of these rims is not yet clear. Although a strictly nebular origin seems likely for rims in the relatively unaltered type 3 chondrites, the rims in CM chondrites are dominated by secondary alteration phases. It has been argued that either the coarse-grained cores accreted altered rim materials while still in the nebula or that alteration of primary rim phases occurred on the CM parent body. To constrain the origin of alteration phases in rim material, we have analyzed the textures and mineral associations from 10 CM chondritic falls by optical and scanning electron microscopy. Our results indicate that the secondary phases in CM chondritic rims were produced by parent body fluid-rock interactions which redefined some primary rim textures and may have produced, in some cases, both coarse-grained components and the rims that surround them. Textural features demonstrate the interactive exchange of alteration fluids between rims, matrix, and chondrules on the CM parent body. For example, most matrix-rim contacts are gradational, suggesting the synchronous alteration of both components. Several observations suggest the possibility of in situ rim production. For example, tochilinite and phyllosilicates commonly form rims around matrix carbonates, which are generally believed to have precipitated from alteration fluids on the CM parent body. This suggests that the rims surrounding matrix carbonates may also have been produced by alteration processes. Partially replaced chondrule olivines bear a striking resemblance to many rimmed olivines in the matrix which suggests, by analogy, that site-specific precipitation of S-bearing phases may also be responsible for the occurrence of many tochilinite-rich rims around isolated matrix olivines. Non-silicate rims precipitate around olivines of any composition, but the process is most effective for fayalitic olivines. Most of the remaining olivines in CM chondrites are relatively Mg-rich, which suggests that the precipitation of S-bearing rims on olivines may not have been an important process in the aqueous alteration of CM chondrites. We conclude that: (1) precursor rim materials in CM chondrites were subjected to pervasive aqueous alteration on the CM parent body; and (2) textures and mineral associations observed in CM chondrites also suggest the possibility of in situ rim production.

Browning, Lauren B.↗
NASA NTRS1994

A proposition for the classification of carbonaceous chondritic micrometeorites

Classification of interplanetary dust particles (IDP's) should be unambiguous and, if possible, provide an opportunity to interrelate these ultrafine IDP's with the matrices of undifferentiated meteorites. I prefer a scheme of chemical groupings and petrologic classes that is based on primary IDP properties that can be determined without prejudice by individual investigators. For IDP's of 2-50 microns these properties are bulk elemental chemistry, morphology, shape, and optical properties. The two major chemical groups are readily determined by energy dispersive spectroscopic analysis using the scanning or analytical electron microscope. Refinement of chondritic IDP classification is possible using the dominant mineral species, e.g. olivine, pyroxene, and layer silicates, and is readily inferred from FTIR, and automated chemical analysis. Petrographic analysis of phyllosilicate-rich IDP's will identify smectite-rich and serpentine-rich particles. Chondritic IDP's are also classified according to morphology, viz., CP and CF IDP's are aggregate particles that differ significantly in porosity, while the dense CS IDP's have a smooth surface. The CP IDP's are characterized by an anhydrous silicate mineralogy, but small amounts of layer silicates may be present. Distinction between the CP and CF IDP's is somewhat ambiguous, but the unique CP IDP's are fluffy, or porous, ultrafine-grained aggregates. The CP IDP's, which may contain silicate whiskers, are the most carbon-rich extraterrestrial material presently known. The CF IDP's are much less porous that CP IDP's. Using particle type definitions, CP IDP's in the NASA JSC Cosmic Dust Catalogs are approx. 15 percent of all IDP's that include nonchondritic spheres. Most aggregate particles are of the CF type.

Rietmeijer, Frans J. M.↗
NASA NTRS1994

Carbonates in the Kaidun chondrite

Kaidun is a remarkable chondrite breccia fall containing lithic clasts that span a wide range of chondrite groups including C and E chondrites, as well as having clasts with characteristics not yet found in existing chondrite samples. The dominant lithology in Kaidun appears to be CR chondritic, consonant with recent O isotope data. The carbonates in Kaidun are presented as one mineralogical basis for comparing it to the other hydrated chondrites and to better understand its relative alteration history. The four polished thin sections of Kaidun studied contained a variety of lithologies that we classified into four groups -- CR, E, CM-like, and dark inclusions (DIs). DIs contain sulfide and magnetite morphologies that superficially resemble CI chondrites, and some of the previously reported CI lithologies in Kaidun may be what we term DIs. Carbonates were found in all lithologies studied. Carbonates in Kaidun are similar in composition to those in CR chondrites. Some of the DIs in Kaidun, previously characterized as CI, have carbonates similar to those in CR chondrites and are unlike those in CI or CM chondrites. Most carbonates in Kaidun and CR chondrites are calcites, some of which formed at temperatures above 250 C. Dolomite is less common and some may be metastable. Alteration temperatures in the Renazzo CR chondrite were estimated to be approximately 300 C, based on O isotope fractionation between phyllosilicates and magnetite. Temperatures of up to 450 C were proposed for the alteration of a CR-like dark inclusion in Kaidun, based on the presence of hydrothermal pentlandite veins. The alteration temperatures for Kaidun and the other CR chondrites are considerably higher than those suggested for CI or CM parent bodies.

Weisberg, M. K.↗
NASA NTRS1994

CM chondrites exhibit the complete petrologic range from type 2 to 1

Recognition and characterization of the different CM lithologies as components in all meteorites could reveal details of the nature and chronology of alteration and brecciation events on hydrous asteroids. The CM chondrites are of particular interest, as they are the most common carbonaceous chondrites and are found as clasts within other types of meteorites, which suggests that the CM parent asteroids are (or were) widespread in the sections of the asteroid belt providing samples to Earth. Some CM2s, including EET 90047, ALH 83100, and Y 82042, are more 'extensively' altered, and are distinguished by a high proportion of Mg-rich phyllosilicates and Ca-Mg carbonates, frequently in rounded aggregates, and near absence of olivine or pyroxene. 'Completely' altered CMs, called CM1s, essentially lack olivine or pyroxene; these include EET 83334, ALH 88045, and the CM1 clasts in Kaidun. Cold Bokkeveld and EET 84034, both highly brecciated CMs, consist of both extensively and completely altered lithologies. We describe how these lithologies further cosntrain physicochemical conditions on hydrous asteroids. We conclude that CM chondrites exhibit the petrologic range 2 through 1, and that progressive alteration on the parent hydrous asteroid(s) was accompanied by significant increases in temperature (to a peak of approximately 450 C), fO2, water-rock ratio, and (locally) degree of chemical leaching, all well beyond the conditions recorded by CM2s.

Zolensky, M. E.↗
NASA NTRS1993

Carbonaceous chondrite clasts in the howardites Bholghati and EET87513

Twenty-two carbonaceous chondrite clasts from the two howardites Bholghati and EET87513 were analyzed. Clast N from EET87513 is a fragment classified as CM2 material on the basis of texture, bulk composition, mineralogy, and bulk O isotopic composition. Carbonaceous chondrite clasts from Bholghati, for which less data are available because of their small size, can be divided into two petrologic types: C1 and C2. C1 clasts are composed of opaque matrix with rare coarse-grained silicates as individual mineral fragments; textures resemble CI meteorites and some dark inclusions from CR meteorites. Opaque matrix is predominantly composed of flaky saponite; unlike typical CI and CR meteorites, serpentine is absent in the samples we analyzed. C2 clasts contain chondrules, aggregates, and individual fragments of coarse-grained silicates in an opaque matrix principally composed of saponite and anhydrous ferromagnesian silicates with flaky textures similar to phyllosilicates. These anhydrous ferromagnesian silicates are interpreted as the product of heating of pre-existing serpentine. The carbonaceous chondrite clasts we have studied from these two howardites are, with one notable exception (clast N from EET87513), mineralogically distinct from typical carbonaceous chondrites. However, these clasts have very close affinities to carbonaceous chondrites and have also experienced thermal metamorphism and aqueous alteration, but to different degrees.

Buchanan, P. C.↗