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At least 145 records · Page 8
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NASA NTRS1979

Post-shock temperatures in minerals

An experimental technique was developed for measuring post-shock temperatures in a wide variety of materials, including those of geophysical interest such as silicates. The technique uses an infrared radiation detector to determine the brightness temperature of samples shocked to pressures in the range 5 to approximately 30 GPa; in these experiments measurements were made in two wavelength ranges (4.5 to 5.75 microns and 7 to 14 microns). Reproducible results, with the temperatures in the two wavelength bands generally in excellent agreement, were obtained for aluminum-2024 (10.5 to 33 GPa, 125 to 260 C), stainless steel-304 (11.5 to 50 GPa, 80 to 350 C), crystalline quartz (5.0 to 21.5 GPa, 80 to 250 C), forsterite (7.5 to 28.0 GPa, approximately 30 to 160 C) and Bamble bronzite (6.0 to 26.0 GPa, approximately 30 to 225 C). It is concluded that release adiabat data should be used, wherever available, for calculations of residual temperature, and that adequate descriptions of the shock and release processes in minerals are more complex than generally assumed.

Raikes, S. A.↗
NASA NTRS1978

Clast assemblages of possible deep-seated /77517/ and immiscible-melt /77538/ origins in Apollo 17 breccias

Breccia samples 77517 and 77538 are composed of abundant mineral and lithic clasts set in porous, poorly sintered matrices. Clast assemblages in the two rocks are of contrasting composition and origin. Breccia 77517 has Mg-rich olivine and pyroxene and calcic plagioclase clasts, indicating limited, almost exclusively ANT-suite parentage. A significant feature is the presence of an assemblage (aluminous enstatite, forsterite, anorthite, aluminous spinel) corresponding to spinel cataclasite, a rock type of deep-seated (about 60 km) crustal origin. Breccia 77538 contains Fe-rich pyroxene and rather sodic plagioclase clasts, indicative of predominantly KREEP and/or mare derivation. An important feature is the occurrence of high-K and high-Fe lithic clasts whose compositions resemble those of immiscible-melts produced during late-stage magmatic crystallization, and which probably originated via silicate liquid immiscibility in a KREEP or mare basalt magma. Both rocks contain numerous fine-grained breccia clasts which represent material that has been modified by impact processes at or very near the moon's surface.

Warner, R. D.↗
NASA NTRS1979

Post-shock temperatures in minerals

An experimental technique for the measurement of post-shock temperatures in minerals is presented, and silicate post-shock temperature measurements are compared with theoretical calculations. The technique involves the use of infrared detectors to determine the brightness temperatures at 4.5 to 5.75 microns and 7 to 14 microns of samples shocked to between 5 and 30 GPa. Results obtained for aluminum 2024 and stainless steel 304, as well as for Bamble bronzite and synthetic crystal forsterite are found at low pressures to be considerably in excess of the temperatures predicted assuming a hydrodynamic rheology and isentropic release parallel to the Hugoniot. The results are shown to be in better agreement, however, with values calculated assuming elastoplastic behavior, and the post-shock temperatures of crystalline quartz are found to be in good agreement with those calculated by Mashimo et al. (1979) from release adiabat data.

Raikes, S. A.↗
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 NTRS1981

Mechanical aggregation of enstatite chondrites from an inhomogeneous debris cloud

Enstatite chondrites have oxygen isotope ratios closer to those of the earth and moon than other meteorites. Their minerals are chemically reduced; metal contains Si, and some Ca, Ti, Mg and Mn are incorporated in sulphides rather than silicates. Clinoenstatite and olivine are virtually Fe-free. Two types of clinoenstatite in the Indarch enstatite chondrite have been reported, one luminescing blue and one red. Similar clinoenstatites in the Kota-Kota enstatite chondrite are associated with two distinct types of forsteritic olivine, one luminescing orange and the other blue. The textural relations and differences in chemical composition cannot be explained by progressive condensation from the solar nebula and require the mechanical mixing of material from at least two sources. It is suggested here that enstatite chondrites result from mechanical and chemical processes during aerodynamic sorting and gravitational settling of debris from a hot cloud of dust, liquid and gas produced during collision of planetesimals.

Leitch, C. A.↗
NASA NTRS1982

The evolution of an impact-generated atmosphere

The minimum impact velocities and pressures required to form a primary H2O atmosphere during planetary accretion from chondritelike planetessimals are determined by means of shock wave and thermodynamic data for rock-forming and volatile-bearing minerals. Attenuation of impact-induced shock pressure is modelled to the extent that the amount of released water can be estimated as a function of projectile radius, impact velocity, weight fraction of target water, target porosity, and dehydration efficiency. The two primary processes considered are the impact release of water bound in such hydrous minerals as serpentine, and the subsequent reincorporation of free water by hydration of forsterite and enstatite. These processes are described in terms of model calculations for the accretion of the earth. It is concluded that the concept of dehydration efficiency is of dominant importance in determining the degree to which an accreting planet acquires an atmosphere during its formation.

Lange, M. A.↗
NASA NTRS1982

Retention of ion-implanted-xenon in olivine: Dependence on implantation dose

The diffusion of Xe in olivine, a major mineral in both meteorites and lunar samples, was studied. Xe ions were implanted at 200 keV into single-crystal synthetic-forsterite targets and the depth profiles were measured by alpha particle backscattering before and after annealing for 1 hour at temperatures up to 1500 C. The fraction of implanted Xe retained following annealing was strongly dependent on the implantation dose. Maximum retention of 100% occurred for an implantion dose of 3 x 10 to the 15th power Xe ions/sq cm. Retention was less at lower doses, with (approximately more than or = 50% loss at one hundred trillion Xe ions/sq cm. Taking the diffusion coefficient at this dose as a lower limit, the minimum activation energy necessary for Xe retention in a 10 micrometer layer for ten million years was calculated as a function of metamorphic temperature.

Melcher, C. L.↗
NASA NTRS1982

The Tucson iron and its relationship to enstatite meteorites

The Tucson meteorite was studied in order to search for the origin of its silicates and to explore a possible relationship between Tucson and the enstatite meteorites. All samples were analyzed with an electron microprobe. The silicate inclusions are described in terms of size and texture, and the silicate mineralogy, which is that of a highly reduced assemblage, is described. The bulk composition of the silicate inclusions is discussed. The results provide evidence of fast cooling. The textural and mineralogical data all point to the presence of a high temperature melt followed by rapid cooling and quenching. Several hypotheses for the origin of the Tucson are discussed, and it is concluded that the preferred hypothesis is that the meteorite formed by the high temperature impact mixing of a forsterite-enstatite silicate assemblage with Si-bearing metal, both having formed in a highly reduced environment.

Nehru, C. E.↗
NASA NTRS1982

Nucleation and condensation in the primitive solar nebula

It is pointed out that the primitive solar nebula may be modeled using the frictionally induced transport theory of Lynden-Bell and Pringle (1974) if the principal frictional mechanism within the nebula is turbulent viscosity. The present investigation is concerned with the construction of a model of a section of the primitive solar nebula as a basis for the study of nucleation and condensation processes within this section. The construction involves a relatively simple application of the Lynden-Bell and Pringle theory subject to steady mass flow conditions. The calculations which are conducted in connection with the investigation indicate that by the time the gas in the primitive solar nebula has become sufficiently supercooled to nucleate condensation centers, several different compounds, including the magnesium silicates forsterite and enstatite (MgSiO3), will probably be able to condense on the growing condensation center.

Cameron, A. G. W.↗
NASA NTRS1983

Retention of ion-implemented-xenon in olivine - Dependence on implantation dose

The diffusion of Xe in olivine, a major mineral in both meteorites and linear samples, was studied. Xe ions were implanted at 200 keV into single-crystal synthetic-forsterite targets and the depth profiles were measured by alpha particle backscattering before and after annealing for 1 hour at temperatures up to 1500 C. The fraction of implanted XE retained following annealing was strongly dependent on the implantation dose. Maximum retention of 100 percent occurred for an implanting dose of 3 x 10 to the 15th power Xe ions/sq cm. Retention was less at lower doses, with approximately more than or = 50 percent loss at one hundred trillion Xe ions/sq cm. Taking the diffusion coefficient at this dose as a lower limit, the minimum activation energy necessary for Xe retention in a 10 micrometer layer for ten million years was calculated as a function of metamorphic temperature. Previously announced in STAR as N83-18528

Melcher, C. L.↗
NASA NTRS1984

Petrographic studies of refractory inclusions from the Murchison meteorite

Textural and mineral-chemical data on freeze-thaw disaggregated refractory inclusions from the Murchison meteorite are reported. The data were obtained with neutron activation analysis, SEM, and spectroscopy, the study revealed corundum-bearing inclusions, spinel-hibonite aggregates and spherules, and spinel-pyroxene and elivine-pyroxene inclusions. One of the three spinel-, pyroxene-, forsterite-rich inclusions had an amoeba-shaped spinel-pyroxene core, implying vapor-to-solid condensation and therefore crystallization from a melt. It is concluded that the meteorite formation encompassed diverse nebular materials, and that further studies of the meteorite will enhance the data base on the planetary nebular processes.

Macpherson, G. J.↗
NASA NTRS1986

Analytical electron microscopy of Mg-SiO smokes - A comparison with infrared and XRD studies

Analytical electron microscopy conducted for Mg-SiO smokes (experimentally obtained from samples previously characterized by IR spectroscopy) indicates that the microcrystallinity content of unannealed smokes increases with increased annealing for up to 30 hr. The growth of forsterite microcrystallites in the initially nonstoichiometric smokes may give rise to the contemporaneous growth of the SiO polymorph tridymite and MgO; after 4 hr of annealing, these react to form enstatite. It is suggested that XRD analysis and IR spectroscopy should be conducted in conjunction with detailed analytical electron microscopy for the detection of emerging crystallinity in vapor-phase condensates.

Rietmeijer, F. J. M.↗
NASA NTRS1989

Nonradiative relaxation and laser action in tunable solid state laser crystals

Room-temperature pulsed laser action was obtained in chromium-activated forsterite (Cr:Mg2SiO4) for both 532 and 1064 nm pumping. Free running laser emission in both cases is centered at 1235 nm and has a bandwidth of approximately 30 nm. Slope efficiency as high as 22 percent was measured. Using different sets of output mirrors and a single birefrigent plate as the intracavity wavelength selecting element tunability over the 1167 to 1268 nm spectral range was demonstrated. Continuous wave laser operation at room temperature was obtained for 1064 nm pumping from a CW Nd:YAG laser. The output power slope efficiency is 6.8 percent. The gain cross section is estimated to be 1.1 x 10 to the 19th sq cm. Spectroscopic studies suggest that the laser action is due to a center other than the trivalent chromium (Cr 3+), presumably the tetravalent chromium (Cr 4+) in a tetrahedrally coordinated site.

Petricevic, V.↗
NASA NTRS1988

A new tunable solid-state laser

A tunable solid state laser system, chromium-activated forsterite (Cr3+: Mg2SiO4), which has the potential for tunability from 850-1400 nm, is introduced. The characteristics of the host crystal are examined. Laser experiments and measurements are presented, including the observation of pulsed laser action at room temperature. The possibilities of the system are also considered.

Gayen, S. K.↗
NASA NTRS1989

Lea County 001, an H5 chondrite, and Lea County 002, an ungrouped type 3 chondrite

A search of active deflation basins near Jal, Lea County, New Mexico resulted in the discovery of two meteorites, Lea County 001 and 002. Lea County 001 has mean olivine and low-Ca pyroxene compositions of Fa(19) and Fs(17), respectively. These and all other mineralogical and petrological data collected indicate a classification of H5 for this stone. Lea County 002 has mean olivine and low-Ca pyroxene compositions of Fa(2) and Fs(4), and is unequilibrated. Although it is mineralogically most similar to Kakangari and chondritic clasts within Cumberland Falls, the high modal amount of forsterite makes Lea County a unique type 3 chondrite. Oxygen isotope data for Lea County 002 fall on an 0-16-mixing line through those of the enstatite meteorites and IAB irons, a feature shared by Kakangari.

Zolensky, Michael E.↗
NASA NTRS1990

Petrology and mineralogy of Type II, FeO-rich chondrules in Semarkona (LL3.0) - Origin by closed-system fractional crystallization, with evidence for supercooling

The petrology of type II porphyritic olivine chondrules in Semarkona (LL3.0) has been studied in detail. Olivines in these chondrules are euhedral, Fe-rich, and are strongly zoned from cores to rims of grains in FeO (Fa10-30), Cr2O3 (0.2-0.6 wt pct), MnO (0.2-0.7 wt pct), and CaO (0.1-0.4 wt pct). Interstitial mesostasis is rich in Si, Al, and Ca and is glassy with abundant microcrystallites. Minor minerals include troilite, Fe,Ni metal, and chromite. Some olivine grains contain euhedral, fayalite-rich cores that are probably produced during initial supercooling of the chondrule melt. Rare relict grains of forsteritic olivine have compositions very similar to olivines in type IA chondrules in Semarkona and may result from disaggregation of such chondrules. Apart from these relics, all properties of type II chondrules can be described by closed-system fractional crystallization of droplets which were essentially entirely molten. Type IA chondrules may have formed from type II chondrules by loss of Fe and volatiles. Alternatively, the two chondrule types may have formed in regions of considerable diversity in the solar nebula from precursor materials with different Fe/Mg ratios.

Jones, Rhian H.↗
NASA NTRS1991

Enstatite chondrites and achondrites as asteroidal resources

The highly reduced nature of the enstatite meteorites (chondrites and achondrites) differs from that of all other types of stony meteorites. The silicates in the enstatite meteorites contain almost no FeO. In addition, many normally lithophile elements such as Na, Ca, Mg, Cr, and Ti display chalcophilic behavior. A study of the chemistry and mineralogy of the enstatite meteorites was undertaken to determine their resource potential. On average, about 99 percent of the volume of an enstatite achondrite, or aubrite, consists of only four silicate minerals: enstatite, Na-rich plagioclase, diopside, and forsterite, with enstatite being by far the most abundant mineral. The remaining one percent of the volume consists of troilite, kamacite, and trace amounts of oldhamite, daubreelite, ferromagnesian albandite, and schreibersite. Thus, the aubrites can be considered as a possible source for large quantities of Mg, Si, and O, but are of little interest as a source of anything else. The enstatite chondrites appear to be more promising candidates for resource utilization. The chondrites are generally divided into two groups: EH (high iron, fine-grained, with abundant chondrules); and EL (low iron, coarse-grained, with little or no evidence of chondrules). Metallic Ni-Fe makes up roughly 20-25 weight percent of each type of enstatite chondrite. These meteorites are also a good source of nitrogen. This is due in part to the presence of osbornite and sinoite. The latter mineral is restricted to EL chondrites, which typically have a higher bulk nitrogen content than the EH chondrites. Three valuable metals, Cr, Mn, and Ti, are concentrated in a few distinct sulfide phases in the enstatite chondrites. These sulfide phases are troilite and niningerite in EH chondrites and troilite, daubreelite, and ferroan alabandite in EL chondrites.

Hutson, M. L.↗
NASA NTRS1991

The composition of meteoroids impacting LDEF

So far we have completed an initial scanning electron microscopy (SEM) survey of craters on the exterior of the Long Duration Exposure Facility (LDEF) in the 100 micron to 1mm size range and done some quantitative analysis. In typical craters, the residue appears to be a mixture of glass and FeNi and sulfide beads with an overall chondritic elemental composition. In less than 10 percent of the craters, there is a substantial amount of meteoroid debris that also contains unmelted mineral grains. The relatively high abundance of forsterite and enststite among these irregular grains suggests that a high melting point probably plays a role in surviving impact without melting.

Brownlee, D. E.↗