HABIT PLANES OF PLATELIKE SCHREIBERSITE IN HEXAHEDRITES*

We have measured the habit planes of plate rhabdites [(Fe, Ni)₃P] with respect to the parent kamacite phase (α, Fe-Ni) in eight hexahedrites. This was accomplished using x-ray diffraction plus two-surface trace analysis. Results show that plate rhabdites form on either {001} or {122} planes in the parent kamacite. The meteorite Uwet exhibits a {112} habit plane which is possibly an anomaly due to the occurrence of a large shock event during rhabdite nucleation (～ 550–650 °C).     

Shock‐thermal history of the Agoudal (IIAB) iron meteorite from microstructural studies

The Agoudal IIAB iron meteorite exhibits only kamacite grains (~6 mm across) without any taenite. The kamacite is homogeneously enriched with numerous rhabdite inclusions of different size, shape, and composition. In some kamacite domains, this appears frosty due to micron‐scale rhabdite inclusions (~5 to 100 μm) of moderate to high Ni content (~26 to 40 wt%). In addition, all the kamacite grains in matrix are marked with a prominent linear crack formed during an atmospheric break‐up event and subsequently oxidized. This feature, also defined by trails of lowest Ni‐bearing (mean Ni: 23 wt%) mm‐scale rhabdite plates (fractured and oxidized) could be a trace of a pre‐existing γ–α interface. Agoudal experienced a very slow rate of primary cooling ~4 °C Ma^?1 estimated from the binary plots of true rhabdite width against corresponding Ni wt% and the computed cooling rate curves after Randich and Goldstein (1978). Chemically, Agoudal iron (Ga: 54 ppm; Ge: 140 ppm; Ir: 0.03 ppm) resembles the Ainsworth iron, the coarsest octahedrite of the IIAB group. Agoudal contains multiple sets of Neumann bands that are formed in space and time at different scales and densities due to multiple impacts with shock magnitude up to 130 kb. Signatures of recrystallization due to postshock low temperature mild reheating at about 400 °C are also locally present.     

The cooling rates of iron meteorites—A new approach

Measurements of Ni concentration profiles of a large number of neighboring kamacite and taenite lamellae in the iron meteorite Cape York (IIIA) have revealed that the kamacite plates have nucleated in a taenite of varying Ni concentration, equal to or above the bulk Ni concentration of the meteorite. This variation indicates that the kamacite plates nucleated stepwise (i.e., independently) during cooling through a certain temperature interval, rather than simultaneously after more or less undercooling of the meteorite. The latter is assumed in most previous cooling rate determinations (e.g., Moren and Goldstein, 1978). In this paper the measured local bulk Ni concentrations are used in the computer simulation of the evolution of the Widmannstaetten pattern in order to calculate the cooling rate of the meteorite. The cooling rate obtained for Cape York is 1.3°K/my. In most previous work, a correlation is seen between the resulting taenite width and the cooling rate in one and the same meteorite. No such correlation is seen using the present method.     

MINERALOGICAL RELATIONSHIPS IN SEYMOUR,A COARSE OCTAHEDRITE

Seymour is a coarse octahedrite weighing 24.5 kg. It contains two types of troilite-graphite nodules (massive graphite nodules with irregular troilite rims, and troilite nodules with irregular, partial or total, rims of graphite); four morphologically distinct types of phosphide (lamellar, swathing, grain boundary, and rhabdite); and three types of cohenite (lamellar, swathing, and inclusions in kamacite). It is believed that lamellar and swathing schreibersite crystallized from taenite at approximately 900°C. This was followed by the crystallization of kamacite, then the development of cohenite, and finally a second period of phosphide nucleation giving rise to grain boundary schreibersite and rhabdite. Compositional data indicates that grain boundary schreibersite and rhabdite grew simultaneously.     

MOSSBAUER SPECTRA FOR IRON BEARING PHASES IN THE METEORITE TOLUCA

Room temperature Mossbauer spectra have been obtained for several iron bearing phases in the octahedrite Toluca. The spectrum for kamacite contains six lines, as expected for a ferromagnetic material. That for taenite contains a strong six-line pattern, closely similar to that for kamacite, plus a weak singlet. The former is due to the ferromagnetic form of taenite, which predominates, and the latter to the non-equilibrium paramagnetic form. The spectra for troilite, schreibersite and cohenite are similar to those for terrestrial troilite, synthetic schreibersite and cementite, respectively. With some troilite samples, a weak doublet due to some non-magnetic phase was obtained. This was found to match the doublet reported for terrestrial pyrite, but the results are not such as to make a positive identification possible. This exploratory study of the Mossbauer spectra of some of the principal phases found in iron-nickel meteorites suggests that the main value of Mossbauer spectroscopy in the study of meteorites lies in its ability to detect relatively small amounts of paramagnetic phases such as the paramagnetic form of taenite.     

Modeling of Carbon Migration in Meteorite Olivines Influenced by Ionizing Radiation

The thermal diffusion of carbon in model crystals of forsterite was studied experimentally and compared with radiation-enhanced diffusion in olivines. A hypothesis is proposed concerning the radiation nature of carbon migration in natural crystals of meteorite olivines at low temperatures. For the first time, the effect of the radiation of natural radionuclides on carbon migration was examined in detail by means of numerical modeling. A vacancy mechanism was assumed for the radiation-enhanced diffusion of carbon. The maximum carbon content (6 × 10^–4 wt %) in crystals of meteorite olivines, which is related to the concentration of vacancies from the radiation of natural radionuclides, was estimated for the first time.     

The New Peruvian Meteorite Carancas: Mössbauer Spectroscopy and X-Ray Diffraction Studies

The Carancas meteorite fell on 15 September 2007 approximately 10 km south of Desaguadero, near Lake Titicaca, Peru, producing bright lights, clouds of dust in the sky and intense detonations. The Carancas meteorite is classified as a H4–5 ordinary chondrite with shock stage S3 and a degree of weathering W0. The Carancas meteorite is characterized by well defined chondrules composed either of olivine or pyroxene. The Mössbauer spectra show an overlapping of paramagnetic and magnetic phases. The spectra show two quadrupole doublets associated to olivine and pyroxene; and two magnetic sextets, associated with the primary phases kamacite/taenite and Troilite (Fe²⁺). Metal particles were extracted from the bulk powdered samples exhibit only kamacite and small amounts of the intergrowth tetrataenite/antitaenite. X-Ray diffractogram shows the primary phases olivine, pyroxene, troilite, kamacite, diopside and albite. Iron oxides has not been detected by Mössbauer spectroscopy or XRD as can be expected for a meteorite immediately recovered after its fall.     

Mass‐dependent fractionation of nickel isotopes in meteoritic metal

Abstract— We measured nickel isotopes via multicollector inductively coupled plasma mass spectrometry (MC‐ICPMS) in the bulk metal from 36 meteorites, including chondrites, pallasites, and irons (magmatic and non‐magmatic). The Ni isotopes in these meteorites are mass fractionated; the fractionation spans an overall range of ～0.4‰ amu^?1. The ranges of Ni isotopic compositions (relative to the SRM 986 Ni isotopic standard) in metal from iron meteorites (～0.0 to ～0.3‰ amu^?1) and chondrites (～0.0 to ～0.2‰ amu^?1) are similar, whereas the range in pallasite metal (～–0.1 to 0.0‰ amu^?1) appears distinct. The fractionation of Ni isotopes within a suite of fourteen IIIAB irons (～0.0 to ～0.3‰ amu^?1) spans the entire range measured in all magmatic irons. However, the degree of Ni isotopic fractionation in these samples does not correlate with their Ni content, suggesting that core crystallization did not fractionate Ni isotopes in a systematic way. We also measured the Ni and Fe isotopes in adjacent kamacite and taenite from the Toluca IAB iron meteorite. Nickel isotopes show clearly resolvable fractionation between these two phases; kamacite is heavier relative to taenite by ～0.4‰ amu^?1. In contrast, the Fe isotopes do not show a resolvable fractionation between kamacite and taenite. The observed isotopic compositions of kamacite and taenite can be understood in terms of kinetic fractionation due to diffusion of Ni during cooling of the Fe‐Ni alloy and the development of the Widmanstätten pattern.     

MÖssbauer spectroscopy,x-ray diffraction,and electron microprobe analysis of the New Haifa meteorite

Abstract— MÖssbauer spectroscopy, x-ray diffraction (XRD) measurements, and electron microprobe analysis (EMPA) have been carried out for the investigation of a newly fallen Sudanese meteorite named New Haifa. The room temperature MÖssbauer spectrum is fitted with three sextets and two doublets. The sextets are assigned to Fe in troilite, kamacite, and taenite, and the two doublets are assigned to Fe²⁺in olivine and pyroxene (no Fe³⁺was found). The microprobe trace of Ni concentration across a kamacite-taenite-kamacite area shows a high-Ni concentration at the interfaces between kamacite and taenite. From the microprobe analysis, olivine appears to have a constant composition, whereas pyroxene has a varying composition. The mole fractions of the Fe end members of olivine (fayalite) and pyroxene (ferrosilite) are found to be 23.5% and 23.2%, respectively. Accordingly, the New Haifa meteorite is classified as an ordinary L-type chondrite.     

Diffusion‐driven kinetic isotope effect of Fe and Ni during formation of the Widmanstätten pattern

Abstract— Iron meteorites show resolvable Fe and Ni isotopic fractionation between taenite and kamacite. For Toluca (IAB), the isotopic fractionations between the two phases are around +0.1‰/amu for Fe and ?0.4‰/amu for Ni. These variations may be due to i) equilibrium fractionation, ii) differences in the diffusivities of the different isotopes, or iii) a combination of both processes. A computer algorithm was developed in order to follow the growth of kamacite out of taenite during the formation of the Widmanstätten pattern as well as calculate the fractionation of Fe and Ni isotopes for a set of cooling rates ranging from 25 to 500 °C/Myr. Using a relative difference in diffusion coefficients of adjacent isotopes of 4‰/amu for Fe and Ni (β = 0.25), the observations made in Toluca can be reproduced for a cooling rate of 50 °C/Myr. This value agrees with earlier cooling rate estimates based on Ni concentration profiles. This supports the idea that the fractionation measured for Fe and Ni in iron meteorites is driven by differences in diffusivities of isotopes. It also supports the validity of the value of 0.25 adopted for β for diffusion of Fe and Ni in Fe‐Ni alloy in the temperature range of 400–700 °C.     

The Santa Filomena meteorite shower: Trajectory,classification, and opaque phases as indicators of metamorphic conditions

On August 19, 2020, at 13:18—UTC, a meteor event ended as a meteorite shower in Santa Filomena, a city in the Pernambuco State, northeast Brazil. The heliocentric orbital parameters resulting from images by cameras of the weather broadcasting system were semimajor axis a = 2.1 ± 0.1 au, eccentricity e = 0.55 ± 0.03, and inclination i = 0.15^o ± 0.05. The data identified the body as an Apollo object, an Earth-crossing object with a pericenter interior to the Earth's orbit. The chemical, mineralogical, and petrological evaluations, as well as the physical analysis, followed several traditional techniques. The meteorite was identified as a H5-6 S4 W0 ordinary chondrite genomict breccia. The large amount of metal in the meteorite made a metallographic evaluation based on the opaque phases possible. The monocrystalline kamacite crystals suggest a higher petrological type and the distorted Neumann lines imply at least two different shock events. The absence of the plessite phase shows that the meteorite did not reach the highest shock levels S5 and S6. The well-defined polycrystalline taenite is indicative of petrologic types 4 and 5 due to the conserved internal tetrataenite rim at the boundaries. The presence of polycrystalline taenites and the characteristics of the Agrell Effect suggest that the Santa Filomena meteorite did not reheat above 700°C. The absence of martensite confirms reheating temperatures <800°C and a slow cooling rate. The Ni contents and sizes of the zoned taenite particles indicate a slow cooling rate ranging from 1 to 10 K Myr⁻¹.     

Interpreting the I‐Xe system in individual silicate grains from Toluca IAB

Abstract— Detailed isotopic and mineralogical studies of silicate inclusions separated from a troilite nodule of the Toluca IAB iron meteorite reveal the presence of radiogenic ¹²⁹Xe in chlorapatite, plagioclase, perryite, and pyroxene grains. Subsequent I‐Xe studies of 32 neutron‐irradiated pyroxene grains indicate that high‐Mg and low‐Mg pyroxenes have distinctive I‐Xe signatures. The I‐Xe system in high‐Mg pyroxenes closed at 4560.5 ± 2.4 Ma, probably reflecting exsolution of silicates from the melt, while the low‐Mg pyroxenes closed at 4552.0 ± 3.7 Ma, 8.5 Ma later, providing a means for determining the cooling rate at the time of exsolution. If the host Toluca graphite‐troilite‐rich inclusion formed after the breakup and reassembly of the IAB parent body as has been suggested, the I‐Xe ages of the high‐Mg pyroxenes separated from this inclusions indicate that this catastrophic impact occurred not later than 4560.5 Ma, 6.7 Ma after formation of CAIs. The cooling rate at the time of silicates exsolution in Toluca is 14.5 ± 10.0 °C/Ma.     

THE IJOPEGA CHONDRITE: A NEW H6 FALL

The Ijopega (Papua New Guinea) meteorite is a new H6 group chondrite fall which contains olivine (Fa 19.9 mole %), bronzite (Fs 17.8 mole %), plagioclase (An 12.1 Or 6.3 Ab 81.6 mole %), diopside, kamacite, taenite, troilite, chromite and whitlockite. The meteorite is extensively recrystallized and brecciated, and shows evidence of moderate shock deformation. Examination of Fe²⁺ and Mg partitioning between ortho- and clinopyroxene indicates a high equilibration temperature (940° or 880 °C). Chemical analysis shows the meteorite to be rich in S, containing about twice the average H-group abundance. Trace elements, including REE, are in accord with established H-group chondrite abundances.     

Some characteristic magnetic properties of lunar materials

One of the typical magnetic characteristics of lunar materials is the composition of their ferromagnetic constituent. Lunar breccias often contain kamacite (less than 7 weight per cent of Ni content) as well as almost pure metallic iron. Metallic ferromagnetics in most igneous rocks are almost pure iron, but the kamacite phase also has been found in some Apollo 15 igneous rocks. It seems likely therefore the metallic ferromagnetics in the lunar crust are more or less similar to those in chondrites.Another typical magnetic characteristic of lunar materials is the presence of a considerable amount of superparamagnetically fine particles of metallic iron. A higher relative content of such fine iron particles results in a higher value of the ratio of magnetic susceptibility (o) to saturation magnetization (I _s), a smaller ratio of the coercive force (H _c) to remanence coercive force (H _RC), and an extremely higher ratio of the viscous component (I _v) to the stable one (I _s) of the remanent magnetization.Communication presented at the Lunar Science Institute Conference on Geophysical and Geochemical Exploration of the Moon and Planets, January 10–12, 1973.     

On the Origin of the Last Meteorite Parent Bodies

The existence of gaps in the perihelion distribution of the orbits of multikilometer-sized asteroids that approach the orbits of terrestrial-group planets is confirmed. This property of the orbits of large asteroids suggests the existence among them of a family of last meteorite parent bodies. Astrophysical data were considered for S-asteroids of the Main Belt and those that approach terrestrial planets. The u–x color index, which is related to the position of the absorption band at 950 nm in the asteroid spectra, was chosen for a qualitative comparison of the surface composition of these asteroids (identical or differing composition). The u – x color-index distributions were analyzed statistically according to the perihelion (q) and mean heliocentric distances (a) of the S-asteroids. It is shown that these distributions are -shaped, peaking at q 1.8 AU and a 2.2 AU. The wings of the distributions can be approximated by linear regressions. A comparison of the u – x color-index mean values for S-asteroids in the regions of the Earth and Mars and of the Main Belt prompts the conclusion that the last meteorite parent bodies in the vicinity of the orbits of the Earth and Mars come primarily from various regions of the asteroid Main Belt.     

The Lueders,Texas, IAB iron meteorite with silicate inclusions

Abstract The Lueders iron meteorite with silicate inclusions was recovered as a single specimen of ～35.4 kg in Shackelford County, Texas, in 1973 and recognized as a meteorite in 1993. Siderophile element concentrations indicate chemical classification as a low-Ni IAB iron meteorite closely related to Landes; like Landes, it has a Cu content ～4σ above the main IAB-IIICD trend and therefore we also designate Lueders as an anomalous member of IAB. The metallic host is composed of equigranular kamacite but with a suggestion of octahedral structure and with a bandwidth of 1.4 mm, suggesting structural classification as a coarse octahedrite (Og). The meteorite contains ～23 wt% of roughly millimeter to centimeter-sized angular silicate inclusions. Classification as a IAB is confirmed by O isotopic analysis of silicate inclusions. These inclusions contain an assemblage rich in silicates, troilite and graphite; lack certain minor phases (e.g., daubreelite); and have angular shapes. A variety of processes (e.g., fragmentation, partial melting, reduction) appear to have played a significant role in the formation of Lueders and all IAB iron meteorites. Petrologic and chemical differences confirm that Lueders is not paired with the widely distributed Odessa meteorite.     

Meteoritic iron from the Libyan glass area,southwestern Egypt

Abstract— A fragment of a weathered iron meteorite was collected from the Libyan Desert glass area of southwestern Egypt in 1991 May. The specimen is of irregular shape, measuring 5 × 3.5 × 2 cm and weighing 110 g. It is covered by a shiny black layer of magnetite ～1 mm thick. The interior is brownish-black in color owing to terrestrial oxidation. An unetched polished surface shows some areas still having remnants of kamacite and taenite. Remnants of the lamellar octahedral structure have been detected. The kamacite bandwidth ranges from 0.2 mm to 0.8 mm. Chemical analysis shows that the meteorite contains 3.4% Ni, 0.24% Co, 85 ppm Cu, 4 ppm As, 132 ppm Au and 2530 ppm Ir. The meteorite was discovered after the discovery of two different chondritic meteorites in the same area; so, it will be named Great Sand Sea 003.     

Video observations, atmospheric path, orbit and fragmentation record of the fall of the Peekskill meteorite

Large Near-Earth-Asteroids have played a role in modifying the character of the surface geology of the Earth over long time scales through impacts. Recent modeling of the disruption of large meteoroids during atmospheric flight has emphasized the dramatic effects that smaller objects may also have on the Earth's surface. However, comparison of these models with observations has not been possible until now. Peekskill is only the fourth meteorite to have been recovered for which detailed and precise data exist on the meteoroid atmospheric trajectory and orbit. Consequently, there are few constraints on the position of meteorites in the solar system before impact on Earth. In this paper, the preliminary analysis based on 4 from all 15 video recordings of the fireball of October 9, 1992 which resulted in the fall of a 12.4 kg ordinary chondrite (H6 monomict breccia) in Peekskill, New York, will be given. Preliminary computations revealed that the Peekskill fireball was an Earth-grazing event, the third such case with precise data available. The body with an initial mass of the order of 10⁴ kg was in a pre-collision orbit with a = 1.5 AU, an aphelion of slightly over 2 AU and an inclination of 5. The no-atmosphere geocentric trajectory would have lead to a perigee of 22 km above the Earth's surface, but the body never reached this point due to tremendous fragmentation and other forms of ablation. The dark flight of the recovered meteorite started from a height of 30 km, when the velocity dropped below 3 km/s, and the body continued 50 km more without ablation, until it hit a parked car in Peekskill, New York with a velocity of about 80 m/s. Our observations are the first video records of a bright fireball and the first motion pictures of a fireball with an associated meteorite fall.     

Variations in impact effects among IIIE iron meteorites

Group‐IIIE iron meteorites can be ordered into four categories reflecting increasing degrees of shock alteration. Weakly shocked samples (Armanty, Colonia Obrera, Coopertown, Porto Alegre, Rhine Villa, Staunton, and Tanokami Mountain) have haxonite within plessite, unrecrystallized kamacite grains containing Neumann lines or possessing the ? structure, and sulfide inclusions typically consisting of polycrystalline troilite with daubréelite exsolution lamellae. The only moderately shocked sample is NWA 4704, in which haxonite has been partially decomposed to graphite; the majority of the kamacite in NWA 4704 is recrystallized, and its sulfide inclusions were partly melted. Strongly shocked samples (Cachiyuyal, Kokstad, and Paloduro) contain graphite and no haxonite, suggesting that pre‐existing haxonite fully decomposed. Also present in these rocks are recrystallized kamacite and melted troilite. Residual heat from the impact caused annealing and recrystallization of kamacite as well as the decomposition of haxonite into graphite. Severely shocked samples (Aliskerovo and Willow Creek) have sulfide‐rich assemblages consisting of fragmental and subhedral daubréelite crystals, 1–4 vol% spidery troilite filaments, and 30–50 vol% low‐Ni kamacite grains, some of which contain up to 6.0 wt% Co; haxonite in these inclusions has fully decomposed to graphite. The wide range of impact effects in IIIE irons is attributed to one or more major collision(s) on the parent asteroid that affected different group members to different extents depending on their proximity to the impact point.     

Birth of the presolar nebula: The sequence of condensation revealed in the Allende meteorite

This work applies the well-known supernova-trigger hypothesis for solar system formation to explain in detail many properties of the Allende meteorite. The Allende carbonaceous chondrite meteorite is an assemblage of millimetre- to centimetre-sized Ca-Al-rich inclusions (CAI's), fine-grained alkali-rich spinel aggregates, amoeboid olivine aggregates, olivine chondrules and sulfide chondrules set in an extremely fine-grained black matrix. Detailed isotopic, chemical and textural properties show that these components formed in the above order as independent cosmic grains. Some CAI's containmicron-sized metal nuggets in which the normally incompatible refractory (Mo, Re, W) and platinum group (Pt, Os, Ir, Ru) metals are alloyed together in approximately cosmic proportions, suggesting that these nuggets also condensed as cosmic grains.From the consistent pattern of enclosure of earlier components on the above list within later ones, it appears that in the environment where these materials formed, condensation moved inexorably in the direction of increasing olivine and decreasing refractory element and¹⁶O content (from 4% excess¹⁶O to normal terrestrial oxygen isotopic composition). Condensation sequences are all short and incomplete, from which it is concluded that condensing materials were soon separated from the condensing environment and isolated until all were brought together in a final snowstorm of fine-grained, olivine crystals constituting the meteorite matrix.These major properties can be accounted for in a model in which a supernova remnant (SNR) in the snowplow phase, whose oxygen was initially pure¹⁶O, pushes into a dark interstellar cloud. In the model, condensation of CAI's begins in the SNR shell when it has been diluted with 2500 times its mass of matter from the cloud, which also in part explains the rarity of observed isotopic anomalies in CAI's. The retardation of the SNR by the cloud propels condensed grains ahead toward the cloud under their own momentum. Continuing dilution by the cloud and continuing removal of the most refractory elements in grains can explain the evolving patterns of fractionation and depletion of refractory elements, including REE's, in successive condensates. Features such as rims on CAI's and concentric zonation of fine-grained aggregates can also be satisfied in the model. A presolar origin and a short ( 10 000 years) formation time for inclusions in carbonaceous chondrites are major implications of the model.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.