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NASA NTRS1975

Mineralogy of lunar norite 78235 - Second lunar occurrence of P2 sub 1 ca pyroxene from Apollo 17 soils

A thin section of lunar rock 78235 is examined optically as well as with an electron microprobe and is shown to be a highly-shocked coarse norite. The rock is found to consist of approximately 30% low-Ca pyroxene, 55% Ca-rich plagioclase, and 15% glass veins. The composition of the glass veins is analyzed, and it is suggested that they resulted from shock melting of the norite. Comparisons with other noritic rocks indicate that the pyroxene in rock 78235 has P2 sub 1 ca symmetry. Based on this symmetry plus the coarse grain size, it is concluded that rock 78235 was formed under plutonic conditions. The origin of the minor phases in this rock is briefly considered.

Steele, I. M.↗

NASA NTRS1976

Garnet and pyroxene compositions in some diamondiferous eclogites

Analyses are reported for garnet and pyroxene from 17 eclogites that contain diamond. The garnets contain small but significant contents of Na, Ti and P and the pyroxenes contain traces of K. The diamond-bearing eclogites do not constitute a unique compositional group but show a range of mineral compositions consistent with a very high P-T environment.

Reid, A. M.↗

NASA NTRS1975

Relative cooling rates of mare basalts at the Apollo 12 and 15 sites as estimated from pyroxene exsolution data

X-ray single-crystal diffraction studies, supplemented by electron microprobe analyses of core pigeonites and rim augites from rocks 12031, 15085, 15475, and 15495, have been used to suggest cooling rates for these and other Apollo 12 and 15 rocks studied by similar methods. The extent of subsolidus phase separation of pyroxenes is used as a measure of the cooling rate. The results were interpreted in terms of model cooling histories of a lava flow whose thickness was estimated from the absolute cooling rates obtained by cooling rate experiments and by temperature-time variation through an extrusive flow computed by employing simplified theory of Jaeger. All available data on the exsolution and cation distributions of pyroxenes from Apollo 12 and 15 samples at present are consistent with the hypothesis that these rocks were derived from the top and interior of lava flows with thicknesses of 4-10 m. A process for exsolution of augite from a host pigeonite on (100) has been related to its low-calcium content.

Takeda, H.↗

NASA NTRS1981

A thermochemical calculation of the pyroxene saturation surface in the system diopside-albite-anorthite

The pyroxene saturation surface in the system diopside-albite-anorthite may be calculated to + or - 10 C from thermochemical data over most of its composition range. The thermochemical data used are the experimentally determined enthalpies of mixing of the ternary liquids and the enthalpy of fusion of diopside. These are combined with a mixing model for the configurational entropy in the melt and the activity of CaMgSi2O6 in the clinopyroxene, which is less than unity due to departures from CaMgSi2O6 stoichiometry. The two-lattice melt model appears to work satisfactorily throughout the pyroxene primary phase field but probably needs modification at more anorthite-rich compositions.

Hon, R.↗

NASA NTRS1981

Near-infrared spectral reflectance of mineral mixtures - Systematic combinations of pyroxenes, olivine, and iron oxides

Near-infrared spectral reflectance data are presented for systematic variations in weight percent of two component mixtures of ferromagnesium and iron oxide minerals used to study the dark materials on Mars. Olivine spectral features are greatly reduced in contrast by admixture of other phases but remain distinctive even for low olivine contents. Clinopyroxene and orthopyroxene mixtures show resolved pyroxene absorptions near 2 microns. Limonite greatly modifies pyroxene and olivine reflectance, but does not fully eliminate distinctive spectral characteristics. Using only spectral data in the 1 micron region, it is difficult to differentiate orthopyroxene and limonite in a mixture. All composite mineral absorptions were either weaker than or intermediate in strength to the end-member absorptions and have bandwidths greater than or equal to those for the end members. In general, spectral properties in an intimate mixture combine in a complex, nonadditive manner, with features demonstrating a regular but usually nonlinear variation as a function of end-member phase proportions.

Singer, R. B.↗

NASA NTRS1981

Cristobalite-pyroxene in an L6 chondrite - Implications for metamorphism

CRISPY is a cristobalite-pyroxene assemblage in the L6 chondrite ALHA 76003. It was formed by reaction of a very siliceous inclusion with the surrounding olivine-rich rock. Oxygen isotopes show that the inclusion was derived from a source with non-chondritic isotopic composition. The isotopes also show that the oxygen of the pyroxene reaction product was derived by simple mixing of oxygen from the inclusion and its immediately adjacent surroundings, with exchange with the bulk meteorite limited to a distance of about a millimeter. The persistence of cristobalite in close proximity to olivine, and the lack of isotopic equilibration, show that the metamorphic processes that form petrographic grade 6 chondrites involve transport of major elements over distances only on the order of millimeters.

Olsen, E. J.↗

NASA NTRS1982

Conditions of formation of pyroxene excentroradial chondrules

Understanding the exact mechanism of origin of chondrules and the attendant physical conditions remains a key factor in understanding the early evolution of the solar system. The present investigation is concerned with an interpretation of the range of cooling histories needed to make chondrules. Chondrule-like spherules were formed in dynamic crystallization experiments, using gas-mixing facilities. It is found that textures in pyroxene-rich chondrules are well reproduced in the laboratory by cooling melts of chondrule composition from just above the liquidus. The range of pyroxene dendrite widths is slightly greater for natural chondrules than for droplets cooled from 50 C/hr to greater than 3000 C/hr. A blanketing medium of variable thickness explains the cooling rates lower than expected for radiative cooling. Experimental results, textures indicating incomplete melting, and isotopic disequilibrium are compatible with reheating of primitive material in the early solar nebula.

Hewins, R. H.↗

NASA NTRS1984

Phosphoran pyroxene and olivine in silicate inclusions in natural iron-carbon alloy, Disko Island, Greenland

A survey of literature data on the P contents of olivine, pyroxene, and garnet in mantle xenoliths and the bulk P contents of mantle xenoliths shows that available data is ambiguous as to whether mantle P is primarily contained in silicates or in phosphates, even though phosphates are rarely observed in mantle xenoliths. Garnet probably contains more of the mantle's P than olivine or pyroxene. Bulk P contents of mantle xenoliths vary significantly. Estimation of the primitive mantle P content from xenoliths requires interpretation of the partial melting and contamination histories of the xenoliths.

Goodrich, C. A.↗

NASA NTRS1986

Dynamic crystallization of chondrule melts of porphyritic and radial pyroxene composition

Dynamic crystallization experiments in which heterogeneous nucleation is an important variable have been completed on four melts of chondrule composition. Compositions were chosen to best represent chondrules with porphyritic pyroxene and radial pyroxene textures. Experimental results show that heterogeneous nucleation is essential for the formation of porphyritic textures. Without preexisting nuclei, too much supercooling is established before crystallization is initiated and the textures are more likely to be dendritic or radial. In the near total absence of nuclei, radial textures can form at cooling rates as slow as 5 C/hr in this study. By varying the heterogeneous nucleation conditions and having a melt in which the appropriate phases are stable or metastable, analogs to most of the recognized chondrule textures can be produced in a single melt composition.

Lofgren, G.↗

NASA NTRS1987

Spectral properties of plagioclase and pyroxene mixtures and the interpretation of lunar soil spectra

Pyroxene and plagioclase mixtures' spectral characteristics are investigated with a view to particle size and mineral proportion effects in the cases of variously proportioned labradorite/enstatite compositional mixtures. The amount of plagioclase that will allow its band to be observed in discrete absorption is particle size-dependent; plagioclase is detectable in the flattening of the pyroxene reflectance peak that lies between the 0.9 and 1.9 micron absorption bands, if sufficient plagioclase is present. Attention is given to the combined effects of mineral proportions and particle size for plagioclase/pyroxene mixtures in the case of analogs of mature and immature lunar highland soils.

Crown, David A.↗

NASA NTRS1989

Unusual olivine and pyroxene composition in interplanetary dust and unequilibrated ordinary chondrites

The presence, in both a number of interplanetary dust particles (IDPs) and in meteorite matrices, of olivine and orthopyroxene grains, low in FeO but containing up to 5 wt pct MnO, is reported. The majority of olivines and pyroxenes in meteorites contain less than 0.5 wt pct MnO. The presence of these low-iron, manganese-enriched (LIME) olivines and pyroxenes in IDPs and meteorites may indicate a link between the origin and history of IDPs and the matrix material of primitive meteorites. The origin of the LIME silicates could be explained by condensation from a gas of solar composition. Forsterite is the first major silicate phase to condense from a solar nebula gas, and Mn, which is not stable as a metal under solar nebula conditions, would condense at about 1100 K as Mn2SiO4 in solid solution with forsterite.

Klock, W.↗

NASA NTRS1991

Spectral signature of oxidized pyroxenes - Implications to remote sensing of terrestrial planets

The effect of the structural Fe(3+) or disseminated ferric oxides in oxidized ferromagnesian silicates on the reflectance spectral profiles of the remnant Fe(2+) ions in oxidized silicates was investigated. Results of spectral measurements in the visible NIR region, performed on several well-characterized pyroxenes representative of different structural types and compositions, were correlated with data obtained from the Mossbauer spectra of the oxidation products. The measurements made it possible to determine the relative proportions of Fe(2+) and Fe(3+) ions present in the oxidized pyroxenes and provided information on the amorphous ferric oxide phases present in the oxidation products. The results also suggest a mechanism for producing the nanophase hematite popularly considered to be responsible for the features observed around 0.62 and 0.87 micron in the remotely sensed spectra of Mars.

Straub, D'arcy W.↗

NASA NTRS1993

Pyroxene equilibration temperatures in metamorphosed ordinary chondrites

Ordinary chondrites are divided into petrographic types based on observed mineralogical and textural properties consistent with progressive thermal metamorphism from low grade (type 3) to high (type 7). Regardless of the exact cause of the metamorphism, higher-type chondrites should retain information concerning peak temperatures reached and for what duration. Using the two-pyroxene geothermometer of Lindsley, we have calculated the equilibration temperatures for 26H, L and LL type 5 and 6 ordinary chondrites, to investigate the relative peak temperatures and equilibration-states reached by these various meteorite classes. The Lindsley thermometer relies on a detailed accounting of non-quadrilateral components in pyroxenes, whose recalculated compositions are then plotted onto an empirically-derived polythermal diagram from which temperatures can be interpolated. The reported uncertainty of this method is plus or minus 50 C; in addition, close spacing of isotherms on the graph (particularly for orthopyroxene compositions) increase this uncertainty. We have parameterized the Lindsley polythermal quadrilateral for 1 atm pressure (less than 2 bar), and interpreted recalculated coordinates directly in terms of 25 C temperature intervals. Meteorites selected for this study include both relatively shocked and unshoked specimens; heavily weathered or visibly brecciated specimens were avoided. Temperatures were calculated from orthopyroxene (opx) and clinopyroxene (cpx) analyses within one relative percent of ideal sums and stoichiometry. Histograms summarizing the calculated temperatures for type 5 and 6 ordinary chondrites are shown.

Harvey, R. P.↗

NASA NTRS1993

Zaoyang chondrite cooling history from pyroxene Fe(2+)-Mg intracrystalline ordering and exolutions

The Zaoyang ordinary chondrite fell as a single 14.15-kg mass in Hubey province (China) in October 1984 and was classified as a non-brecciated H5 chondrite, shock facies b. Cooling rate in pyroxenes can be calculated down to about 1000 C by using fine textures and at still lower temperatures (700 to 200 C) by intracrystalline ordering processes. The crystal chemistry of clinopyroxene and orthopyroxene from the matrix of the H5 Zaoyang chondrite has been investigated by X-ray structure refinement and detailed microprobe analysis. By comparison with terrestrial pyroxenes cell and polyhedral volumes in clino- and orthopyroxenes show a low crystallization pressure. Fe(2+) and Mg are rather disordered in M1 and M2 sites of clino- and orthopyroxenes; the closure temperatures of the exchange reaction are 600 and 512 C respectively, which is consistent with a quite fast cooling rate, estimated of the order of one degree per day. The closure temperature for the intercrystalline Ca-Mg exchange reaction for clino- and orthopyroxene showing clinopyroxene lamellae about 10 microns thick. Kinetic evaluations based on the thickness of exolved lamellae give a cooling rate of not more than a few degrees per 10(exp 4) years. The different cooling rates obtained from Fe(2+)-Mg intracrystalline partitioning and exolution lamellae suggest an initial episode of slow cooling at 900 C, followed by faster cooling at temperatures of 600-500 C at low pressure conditions. The most probable scenario of the meteorite history seems that the exolved orthopyroxene entered the parental chondrite body after exolution had taken place at high temperature. Subsequent fast cooling occurred at low temperature after the formation of the body.

Molin, G. M.↗

NASA NTRS1993

The formation of FeO-rich pyroxene and enstatite in unequilibrated enstatite chondrites: A petrologic-trace element (SIMS) study

Enstatite (En) chondrites record the most reducing conditions known in the early solar system. Their oxidation state may be the result of condensation in a nebular region having an enhanced C/O ratio, reduction of more oxidized materials in a reducing nebula, reduction during metamorphic reheating in a parent body, or a combination of these events. The presence of more oxidized Fe-rich silicates, two types of En (distinguished by red and blue CL), and the juxtaposition of FeO-rich pyroxenes (Fe-pyx) surrounded by blue En (enstatite) in the UEC's (unequilibrated enstatite chondrites) is intriguing and led to the examination of the question of enstatite chondrite formation. Previously, data was presented on the petrologic-geochemical characteristics of the Fe-pyx and coexisting red and blue En. Here minor and trace element abundances (determined by ion probe-SIMS) on these three types of pyroxenes are reported on in the following meteorites: Kota Kota and LEW87223 (EH3), MAC88136 (EL3), St. Marks (EH4), and Hvittis (EL6). More data are currently being collected.

Weisberg, M. K.↗

NASA NTRS1994

Compositional variations of olivines and pyroxenes in chondritic interplanetary dust particles

We report here analyses of olivines and pyroxenes, and petrofabrics of 27 chondritic interplanetary dust particles (IDPs), comparing those from anhydrous and hydrous types. Approximately 40% of the hydrous particles contain diopside, a probable indicator of parent body thermal metamorphism, while this mineral is rarely present in the anhydrous particles. Based on this evidence, we find that hydrous and anhydrous IDPs are, in general, not directly related, and we conclude that olivine and pyroxene major-element compositions can be used to help discriminate between IDPs that are (1) predominantly nebular condensates, and lately resided in anhydrous or icy (no liquids) primitive parent bodies, and (2) those originating from more geochemically active parent bodies (probably hydrous and anhydrous asteroids).

Zolensky, Michael E.↗

NASA NTRS2001

REE and Strontium Partition Coefficients for Nakhla Pyroxenes

We present new partition coefficients for REE and Sr determined using a synthetic melt that crystallizes pyroxenes very similar in composition to Nakhla pyroxene cores. We believe these are the most appropriate partition coefficients to use in studying Nakhla Additional information is contained in the original extended abstract..

Oe, K.↗

NASA NTRS2001

Unambiguous Spectral Evidence for High- (and Low-) Calcium Pyroxene in Asteroids and Meteorites

Spectroscopy remains a powerful tool for inferring the modal mineralogy and mafic mineral composition of asteroid surfaces. Since similar measurements can be made on meteorite samples, spectroscopy can help link the two populations and add spatial and geologic context to detailed geochemical knowledge derived from meteorite samples. For example, analysis of the recent NEAR-Shoemaker mission to Eros include detailed study of NIS spectra to assess the affinity of Eros to ordinary chondrites. As discussed in these studies, pyroxene (PYX) and olivine (OLV) absorption are readily detectable in the spectra. Furthermore, subtleties in band parameters (position vs. area) suggest the presence of both low- and high-calcium pyroxene (LCP and HCP), as expected from the petrology of ordinary chondrites. However unambiguous identification and detailed compositional inferences for both LCP and HCP (and OLV) are difficult from band parameters analysis. In this study, we examine spectra of S-asteroids and meteorites with the Modified Gaussian Model (MGM), an absorption band model, to explore the role of HCP in these silicate-rich spectra.

Sunshine, J. M.↗