The microstructure and formation of duplex and black plessite in iron meteorites

Zhang, J.; Williams, David B.; Goldstein, Joseph I.

doi:10.1016/0016-7037(93)90151-l

Cited by 29 publications

(49 citation statements)

References 15 publications

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“…Metal with a Ni concentration higher than N8 wt% is metastable and would decompose into kamacite (lowNi, bcc phase; <~8 wt% Ni) and taenite (high-Ni, fcc phase; >-20 wt% Ni) upon even mild thermal metamorphism [e.g., Zhang et al, 1993]. It is therefore a particularly sensitive indi- …”

Section: Iron-nickel Metal Condenses With Ni Concentrations Betweenmentioning

confidence: 99%

Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition

Meibom¹,

Petaev²,

Krot³

et al. 1999

J. Geophys. Res.

101

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Section: Iron-nickel Metal Condenses With Ni Concentrations Betweenmentioning

confidence: 99%

Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition

Meibom¹,

Petaev²,

Krot³

et al. 1999

J. Geophys. Res.

101

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“…1), further decomposition of α 2 → α + γ occurs on a very fine scale (Zhang et al 1993). Therefore, the reaction γ → α 2 → α + γ (mechanism IV) is responsible for the formation of the nanoscale decomposed martensite structure in high Ni regions of residual taenite in irons, stony-irons, and chondrites.…”

Section: Martensite Decomposition: Plessite Structure In Irons Mesosmentioning

confidence: 99%

“…As a result, the formation of duplex plessite most likely occurs by mechanism V γ → α 2 + γ → α + γ in which some of the martensite α 2 decomposes during cooling below M s . In these low Ni plessite regions, the microstructure can be observed in the optical microscope (Zhang et al 1993) and often represents a microWidmanstätten pattern. In this microstructure, the residual γ phase surrounding α retains the crystallographic orientation of the parent γ − taenite.…”

Section: Martensite Decomposition: Plessite Structure In Irons Mesosmentioning

confidence: 99%

“…Mechanism IV γ → α 2 → α + γ most likely is responsible for the formation of the sub-micron scale or fine plessite micro-structure such as black plessite (Zhang et al 1993) with Ni contents much above the bulk Ni content of the host meteorite in irons, stony-irons, and chondrites. Martensite forms at lower temperatures because of the higher taenite Ni content.…”

Section: Martensite Decomposition: Plessite Structure In Irons Mesosmentioning

confidence: 99%

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The formation of the Widmanstätten structure in meteorites

Yang

Goldstein

2005

Meteoritics &Amp; Planetary Science

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Abstract-We have evaluated various mechanisms proposed for the formation of the Widmanstätten pattern in iron meteorites and propose a new mechanism for low P meteoritic metal. These mechanisms can also be used to explain how the metallic microstructures developed in chondrites and stony-iron meteorites.The Widmanstätten pattern in high P iron meteorites forms when meteorites enter the threephase field α + γ + Ph via cooling from the γ + Ph field. The Widmanstätten pattern in low P iron meteorites forms either at a temperature below the (α + γ)/(α + γ + Ph) boundary or by the decomposition of martensite below the martensite start temperature. The reaction γ → α + γ, which is normally assumed to control the formation of the Widmanstätten pattern, is not applicable to the metal in meteorites. The formation of the Widmanstätten pattern in the vast majority of low P iron meteorites (which belong to chemical groups IAB-IIICD, IIIAB, and IVA) is controlled by mechanisms involving the formation of martensite α 2 . We propose that the Widmanstätten structure in these meteorites forms by the reaction γ → α 2 + γ → α + γ, in which α 2 decomposes to the equilibrium α and γ phases during the cooling process.To determine the cooling rate of an individual iron meteorite, the appropriate formation mechanism for the Widmanstätten pattern must first be established. Depending on the Ni and P content of the meteorite, the kamacite nucleation temperature can be determined from either the (γ + Ph)/(α + γ + Ph) boundary, the (α + γ)/(α + γ + Ph) boundary, or the M s temperature. With the introduction of these three mechanisms and the specific phase boundaries and the temperatures where transformations occur, it is no longer necessary to invoke arbitrary amounts of under-cooling in the calculation of the cooling rate.We conclude that martensite decomposition via the reactions γ → α 2 → α + γ and γ → α 2 + γ → α + γ are responsible for the formation of plessite in irons and the metal phases of mesosiderites, chondrites, and pallasites. The hexahedrites (low P members of chemical group IIAB) formed by the massive transformation through the reaction γ → α m → α at relatively high temperature in the twophase α + γ region of the Fe-Ni-P phase diagram near the α/(α + γ) phase boundary.

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“…Santa Catharina is fragmented and chemically homogeneous, with major fragments having Ni contents up to 35%. The fragments are severely corroded but, nevertheless, it has been possible to establish (Zhang et al, 1993) that the original structure consisted of the products of spinodal decomposition in the miscibility gap in the Fe-Ni system. This indicates that, due principally to the high Ni content, the original austenite decomposed completely when it reached a temperature of -400 "C, so precluding the formation of martensite at lower temperatures.…”

Section: A252 Birch Et Almentioning

confidence: 99%

Willow Grove: A unique nickel‐rich ataxite from Victoria, Australia

Birch

Samuels

Wasson

2001

Meteorit & Planetary Scien

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Abstract-The Willow Grove meteorite is a new ungrouped iron meteorite from Victoria, Australia, with an extremely high Ni content (28%). It has a lath martensite structure, in contrast to the plate martensite shown by Tishomingo, a compositionally similar ungrouped iron meteorite with a slightly higher Ni content (32%). Despite the meteorite's unique texture, it has not been possible to estimate the rate at which the meteorite cooled, either before or after the transformation to martensite. Willow Grove also shows a pervasive network of microcracks most likely caused by terrestrial stress corrosion.Like Tishomingo, Willow Grove is depleted in volatiles. It is suggested that the volatiles were lost during asteroid processing, the high temperature possibly produced by an impact event.

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The microstructure and formation of duplex and black plessite in iron meteorites

Cited by 29 publications

References 15 publications

Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition

Primitive FeNi metal grains in CH carbonaceous chondrites formed by condensation from a gas of solar composition

The formation of the Widmanstätten structure in meteorites

Willow Grove: A unique nickel‐rich ataxite from Victoria, Australia

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