Fine Octahedrites Research Articles

Transjordanite, Ni2P, a new terrestrial and meteoritic phosphide, and natural solid solutions barringerite-transjordanite (hexagonal Fe2P–Ni2P)

Abstract This paper is a first detailed report of natural hexagonal solid solutions along the join Fe2P–Ni2P. Transjordanite, Ni2P, a Ni-dominant counterpart of barringerite (a low-pressure polymorph of Fe2P), is a new mineral. It was discovered in the pyrometamorphic phosphide assemblages of the Hatrurim Formation (the Dead Sea area, Southern Levant) and was named for the occurrence on the Transjordan Plateau, West Jordan. Later on, the mineral was confirmed in the Cambria meteorite (iron ungrouped, fine octahedrite), and it likely occurs in CM2 carbonaceous chondrites (Mighei group). Under reflected light, transjordanite is white with a beige tint. It is non-pleochroic and weakly anisotropic. Reflectance values for four COM recommended wavelengths are [Rmax/Rmin, % (λ, nm)]: 45.1/44.2 (470), 49.9/48.5 (546), 52.1/50.3 (589), 54.3/52.1 (650). Transjordanite is hexagonal, space group P62m; unit-cell parameters for the holotype specimen, (Ni1.72Fe0.27)1.99P1.02, are: a = 5.8897(3), c = 3.3547(2) Å, V = 100.78(1) Å3, Z = 3. Dcalc = 7.30 g/cm3. The crystal structure of holotype transjordanite was solved and refined to R1 = 0.013 based on 190 independent observed [I > 2σ(I)] reflections. The crystal structure represents a framework composed of two types of infinite rods propagated along the c-axis: (1) edge-sharing tetrahedra [M(1)P4] and (2) edge-sharing [M(2)P5] square pyramids. Determination of unit-cell parameters for 12 members of the Fe2P–Ni2P solid-solution series demonstrates that substitution of Ni for Fe in transjordanite and vice versa in barringerite does not obey Vegard’s law, indicative of preferential incorporation of minor substituent into M(1) position. Terrestrial transjordanite may contain up to 3 wt% Mo, whereas meteoritic mineral bears up to 0.2 wt% S.

Multi-Length Scale Characterization of the Gibeon Meteorite using Electron Backscatter Diffraction

Abstract The Gibeon meteorite is a differentiated iron meteorite that fell in Nambia, Africa in prehistoric times, with fragments spread over an area 70 miles wide and 230 miles long. The Gibeon fall was initially discovered in 1836, and hundreds of thousands of kilograms of fragments have been recovered. These fragments represent the iron core of a meteorite that cooled and crystallized over thousands of years (Norton 2002). The microstructure of the Gibeon meteorite, which is primarily an iron-nickel alloy, consists of two phases: kamacite, a body-centered cubic material and taenite, a face-centered cubic material that metallurgists would refer to as ferrite and austenite respectively. This material initially crystallizes as taenite, and as the temperature decreases, transforms into kamacite. This meteorite is classified as a Fine Octahedrite (Of) with an average Nickel content of approximately 7.9%

An Iron Meteorite from Akyumak

Abstract— The microstructure of an iron meteorite which fell near Akyumak, East Anatolia, Turkey on 2 August 1981 has been examined and its composition determined. The Ni content is 7.7% and kamacite bandwidth is 0.32 ± 0.06 mm. The kamacite contains Neumann bands and some daubreelite inclusions. Taenite and plessite account for about 45 to 50% of the metal; finger, cellular and net plessite are observed. Gallium (1.9 ppm), Ge (< 40 ppm) and Ir (1.81 ppm) were determined by neutron activation. The microstructural observations and chemical data show Akyumak to be a fine octahedrite and a member of group IVA.

A transmission electron microscopic study of the Bethany iron meteorite

The Bethany iron meteorite which is a part of the Gibeon shower is a fine octahedrite with zoned plessite fields of various sizes. The optically irresolvable microstructural details inside the plessitic fields have been studied by transmission electron microscopy, and the crystallographic relationships between the primary kamacite (α) and the parent taenite (γ), and between the α and γ particles in the coarse plessite, have been examined using electron diffraction. In the case of primary kamacite the orientation-relationship with γ was close to the Nishiyama-Wasserman relationship, whereas, for the plessitic α, the orientation-relationship with γ was close to Kurdjumov-Sachs. It was also found that the (111)γ and (110)α planes were not strictly parallel. Additionally, measurements of the composition profile through the zoned plessite have been made using STEM microanalysis technique, and related to microstructure.

RICA AVENTURA: A NEW IRON METEORITE FROM CHILE

Rica Aventura, a group IVA fine octahedrite, was found in 1910 near the now abandoned village of Rica Aventura, Chile (69° 37′W, 21° 59′S). Based on chemical evidence it appears to be a unique new find.

Analytical electron microscopy study of the plessite structure in the Carlton iron meteorite

In order to gain a better understanding of the formation of plessite in iron meteorites, various electron optical techniques were employed to study the range of plessite structures observed in the Carlton fine octahedrite. Compositional and structural studies of twins in clear taenite and the cloudy zone were made. Transmission electron microscopy studies of martensitic and duplex α + γ plessite regions show the presence of γ-taenite rods, 10–200 nm wide, in an α-kamacite matrix. Scanning transmission electron microscope X-ray analyses showed Ni contents in the y rods of ≥43 wt% and Ni contents in the a matrix of 3 wt% Ni. The reaction path involves the decomposition of α 2 martensite into α + γ and these reactions occur below 200°C and possibly below 100°C. Apparently the formation of plessite is intimately related to the formation of martensite and the further decomposition of martensite during the cooling history of the meteorite. It is quite probable that the martensite decomposition reaction has occurred in a large number of iron meteorites and is responsible for many of the observed plessite structures.

Cooling rates of 27 iron and stony-iron meteorites

A method for the precise determination of the cooling rates of iron and stony-iron meteorites has been developed and applied to 18 irons, 8 pallasites, and the siderophyre Steinbach. The cooling rates are determined by comparing the measured Ni gradients in several kamacitetaenite areas which result from the growth of the Widmanstätten structure between 700 and 300°C with gradients calculated by a theoretical growth analysis. The theoretical diffusion-growth analysis model shows quantitatively that the imperfect correlation between kamacite band width and bulk Ni content is due to variations in cooling rates. It is suggested that a classification system based on both kamacite band width and bulk Ni content should be used so that variations in cooling rates from one meteorite to another can be clearly seen. A very wide range of cooling rates was found for the 27 meteorites studied (0.4–40°/10 6 yr). Three approximate groupings were found: (1) fine octahedrites, Ge (IVa), 9–40°/10 6 yr, (2) fine and medium octahedrites, 1–5°/10 6 yr, (3) pallasites, 0.4–1.0°/10 6 yr. It can be seen from these groupings that various iron and stony-iron meteorites developed their Widmanstatten structure in different thermal environments, although each meteorite was insulated within a parent body to a temperature less than 350°C. The differences in cooling rates may be due to either varying depth of burial within one or several parent bodies or within the core of several parent bodies. However, not all of the iron meteorites could have cooled within the core of one parent body.

Evidence for the formation of an iron meteorite at 3.8 × 10 9 years

Rb Sr isotopic analyses were made on silicate inclusions from Kodaikanal, a brecciated fine octahedrite. Twelve analyses of different mineral fractions and separate inclusions were made. The silicates in this iron meteorite are highly enriched in alkalis and show 87Sr/ 86Sr ratios ranging up to 8.8. The samples lie on a well defined isochron on the Sr-Rb evolution diagram and indicate an age of 3.8 ± 0.1 × 10 9y and an initial 87Sr/ 86Sr ratio between 0.69 and 0.73. These data provide unambiguous evidence for the ‘formation’ of younger solid objects in the solar system in a process which demands extensive chemical differentiation. This is evidence for an intermediate history for some meteoritic objects of a different sort than has been previously observed.

Bestrahlungsalter und Herkunft der Eisenmeteorite

Cosmic ray exposure ages of more than 60 iron meteorites have been determined using the 41K/40-K-method. Isotope fractionation processes and the reproducibility of age values have been studied. Iron meteorites with ages of 900, 700, 600, 400 × 106 y (and hexahedrites with ages of 10 to 100 × 106 y according to WÄNKE) are abundant, such with ages exceeding 1 × 109 y are rare. Correlations have been found between exposure ages and structural classes as well as Ga—Ge-groups. These prove that the remarkable groupings of iron meteorites according to some of their structural and compositional features are indeed due to the fact that it were only a few distinct parent iron masses which essentially contributed to the production of iron meteorites. Some of these parent masses supplied the meteorites mainly in one or two “catastrophic” collisions and break-up events, others in a quasi-continuous series of events over a period of many hundreds of million years. More specified, the results are as follows: Medium and fine octahedrites of the Ga—Ge-group III have, essentially, been produced in two major collisions 600 and 700 × 106 y ago, fine octahedrites of the Ga—Ge-group IV a in one collision about 400 × 106 y ago. These events alone supplied already about 50% of all iron meteorites. Hexahedrites, coarse octahedrites of the Ga—Ge-group I and nickel-rich ataxites reveal, on the other hand, age distributions which appear rather continuous. Mean interplanetary life times deduced from these distributions are about 100 × 106 y for hexahedrites (according to WANKE), 500 × 106 y for nickel-rich ataxites and 700 × 106 y for coarse octahedrites of the Ga — Ge-group I. The differences in the statistics of meteorite production events for different meteorite classes indicate that fundamental dissimilarities may exist in the parent iron masses and parent celestial bodies. Accordingly, one model is proposed for explaining the data on hexahedrites, coarse octahedrites of the Ga—Ge-group I, and ataxites, another one for explaining the data on the medium and fine octahedrites of the Ga—Ge-groups III and IV a: The hexahedrites, coarse octahedrites and ataxites are fragments of a multiplicity of nickel-iron inclusions (“pools” according to URET) within the surface layers of Moon, Mars and asteroids (Mars-asteroids), respectively. These fragments, together with stone meteorites, have been produced in a multiplicity of crater-forming impacts by comets on Moon, Mars or asteroids. Rather compact and large nickel-iron masses, perhaps fragments of asteroidal nickel-iron cores are considered as being the parent masses of the fine octahedrites and medium octahedrites of the Ga—Ge-groups III und IV a. The meteorites of each group have been produced either when such a nickel-iron mass as a projectile hit another asteroid or when a comet hit the nickel-iron mass. Mainly surface material is expected to survive the catastrophe in form of structurally undisturbed meteorites, of which a considerable portion may have been exposed to cosmic rays to a measurable degree prior to the collision. Possible conclusions from a comparison of 41K/40K-age values with 36Ar/36Cl- and similar ages are reconsidered.

The Woodbine meteorite, with notes on silicates in iron meteorites

SummaryThe Woodbine meteorite, ploughed up in Illinois in 1953, is a fine octahedrite with numerous inclusions of silicate minerals. The minerals present include kamacite, taenite, troilite, schreibersite, orthopyroxene (Fs6), diopside, olivine (Fa4), plagioclase (An9), and graphite. The chemical analysis is: Fe 73·06, Ni 9·08, Co 0·15, P 0·47, C 0·21, FeS 1·65, SiO2 8·55, TiO2 0·02, Al2O3 0·53, Cr2O3 0·03, FeO 0·49, MnO 0·03, MgO 5·19, CaO 0·34, Na2O 0·29, K2O 0·02, sum 100·11. Woodbine is compared with other meteorites of similar composition. The silicate material has chondritic composition, and is quite different from the silicate material of the mesosiderites, with which some of these meteorites have been classed.

A trace-element study of iron meteorites

The technique of neutron activation and γ counting on a lithium-drifted germanium semiconductor detector has been used in measuring concentrations of cobalt, copper, arsenic, gallium, iridium, and gold in 33 iron meteorites. Similar data on 25 other iron meteorites are available in the literature. These meteorites are grouped according to similarities in their trace-element content. Out of 23 samples classified as hexahedrites and coarse octahedrites, 22 can be assigned to three groups corresponding to the structural classifications as normal hexahedrites, coarse octahedrites, and a group intermediate between these two. Within these groups, as Ni increases, the elements Co, Cu, As, Ga, and Au increase while Ir decreases. This is thought to represent trace-element behavior during crystallization and crystal settling from a melt of a homogeneous chemical composition. A discontinuity involving the elements As, Au, Ga, and Ir exists between the coarse octahedrites and medium octahedrites. The medium octahedrites are also characterized by their heterogeneity. Only 8 out of 17 can be assigned to groups. Out of 10 fine octahedrites, 5 have uniform elemental compositions. This group differs from the medium octahedrites in Ni, Co, and Ga. These differences in trace-element abundance are ascribed to differences in composition of the Fe-Ni melt from which the meteorites crystallized. At least three melts, varying chiefly in Ni and Ga, are required to account for the compositions of all the iron meteorites. Six Ni-rich ataxites show no uniformity in their trace-element abundances. They probably represent the end stage of crystallization from several sources. Two structurally anomalous meteorites, Nedagolla and N'Goureyma, are unusually low in trace elements. This may have resulted from a remelting of a normal iron meteorite during break-up of the meteorite parent body.

The distribution of some trace elements in iron meteorites, as determined by neutron activation

The ten trace elements germanium, arsenic, antimony, copper, chromium molybdenum, silver, indium, zinc and palladium have been determined in sixty-seven different meteorites, using neutron activation and radiochemical separations. In addition the meteorites have been analysed for gallium using either neutron activation or colorimetric methods. The sulphur content of twenty-five of the meteorites has also been measured, using an evolution and colorimetric method. The meteorites analysed consist of a reasonably representative selection of all the structural types of irons. The distribution of the determined trace elements is discussed, in relation to the structural classifications of the iron meteorites. It is shown that a number of meteorites belonging to specific structural classes have the same or closely similar trace element composition. Thus, the following five groups can be specified: seven hexahedrites, three coarsest octahedrites, seven coarse octahedrites, twelve medium octahedrites and six fine octahedrites. Particular attention is paid to the behaviour of germanium and gallium following the widely accepted division of iron meteorites into four distinct Ga Ge “quantized” groups. It is also shown that arsenic, antimony, silver, indium and zinc are generally concentrated in the coarse and coarsest octahedrite groups, compared with the remaining structural groups. In the case of indium this behaviour is observed even at the 0·0005–0·01 ppm levels. It is also seen that the distribution of zinc does not appear to be related to that of sulphur. Finally, the five structural groups are discussed in relation to the evolution of iron meteorites, and to possible correlations with cosmic-ray exposure ages.

The Anoka, Minnesota, Iron Meteorite*

Abstract The Anoka, Minnesota, meteorite was found on the Joe Fields Farm at location coordinates 45° 12′ N, 93° 26′ W. It is a fine octahedrite distinguished by large fields of dense plessite. The chemical analysis of the meteorite is 84.9 percent iron, 11.75 nickel and 0.51 cobalt.

Phosphates in iron and pallasite meteorites

The ferrous (manganous) orthophosphates, sarcopside and graftonite (both calcium-free) have been found in four fine octahedrites (Bella Roca, Chupaderos, Sam's Valley, Verkhne Dnieprovsk). They coexist with schreibersite, troilite and metal. Calculations of P( O 2) − T for reactions relating these phases show that the presence of these phosphates presents no inconsistency with respect to the degree of reduction encountered in iron meteorites. Similar calculations for magnesium orthophosphate (farringtonite) indicate that the pallasite in which it occurs is somewhat more reduced than the octahedrites. These phosphate-bearing irons suggest a degree of oxidation close to that of common chondrites.

Thermomagnetic analysis of co-existing nickel-iron metal phases in iron meteorites and the thermal histories of the meteorites

The nickel contents of co-existing kamacite, taenite and plessite in all major structure classes of iron meteorites have been determined by thermomagnetic analysis. The results are in substantial agreement with preliminary data available on the electron probe microanalysis of octahedrites and are consistent with the view that the metal phases in iron meteorites have formed during cooling to temperatures of 300 to 370° while the meteorites were subject to pressures up to about 25,000 atmospheres. There is some evidence that iron meteorites with relatively low nickel contents (i.e. hexahedrites and coarse octahedrites) formed at pressures slightly higher than those with higher nickel contents (i.e. fine octahedrites and nickel-rich ataxites). It is also suggested that iron meteorites with nickel contents greater than about 30 per cent (e.g. Santa Catherina) may have been composed of two γ-phase alloys while they still formed part of the core within a parent meteorite body.