1. Martensitic Formation in the Tishomingo iron meteorite

Tishomingo is a chemically and structurally singular ungrouped iron meteorite. With a bulk nickel content of about 32.5 wt% Ni, it records one of the most distinctive solid-state transformations known in meteoritic metal: a diffusionless martensitic shear of parent taenite that took place only after the metal had cooled to cryogenic temperatures in space.

The microstructure is dominated by coarse, plate-like martensite (approximately 80 vol%) intergrown with residual taenite (approximately 20 vol%). The plates originated from a single crystal of taenite. Because nickel strongly depresses the martensite-start temperature (M-s), transformation did not begin until the metal had fallen well below 0 °C. Published estimates place the onset between about −25 °C and −65 °C, with continued plate nucleation down to −75 °C and, in some reconstructions, as low as −115 °C to −200 °C. That range is among the lowest temperatures recorded by any meteoritic microstructure.

The transformation is athermal. Once temperature dropped below M_s, new plates nucleated discontinuously. Growth proceeded without long-range diffusion, producing the observed coarse lenticular morphology. Typical plate widths are 20–50 µm; lengths range from tens of micrometers to millimeters. The habit is distinct from the Widmanstätten kamacite–taenite intergrowth that defines most octahedrites. Midribs and internal twin striations are coarse enough to be resolved optically. An austenite twin several millimeters wide can cross an entire etched section and abruptly reorient the martensite plates that traverse it.

A later mild shock-reheating event partially decomposed some of the martensite. Electron microscopy and EBSD show that reheating to roughly 320–400 °C for a time on the order of one year produced nanometer-scale taenite precipitates (~50 wt% Ni), low-Ni kamacite (~4 wt% Ni), and local reversion of martensite to taenite of bulk composition. The dominant optical fabric, however, still records the original low-temperature shear. Vickers hardness of the martensite is about 425 HV.

Tishomingo contains neither graphite, carbides, phosphides, nor silicates. Accessory phases are metal–sulfide intergrowths: irregular troilite grains with taenite–troilite cores and daubréelite-bearing rims.

In carbon steels, martensite forms when austenite is quenched rapidly from the face-centered cubic field. Diffusional reconstruction is suppressed; the product is a supersaturated body-centered tetragonal (or cubic) phase that typically appears as fine laths or needles. Hardness is high, ductility is low, and industrial practice almost always follows with tempering.

Tishomingo followed a different path. The parent body cooled so slowly that taenite remained a single crystal through the temperature interval in which kamacite would normally nucleate in lower-Ni irons. Only after the metal reached cryogenic temperatures did the shear-driven, diffusionless transformation occur. The result is unusually coarse lenticular plates set in residual taenite—an optical fabric unknown in any other meteorite and coarser than terrestrial Fe–Ni martensite of comparable composition. A subsequent brief reheating produced only localized decomposition; it did not erase the primary plate network.

Polished and nital-etched sections display the plates as an irregular dark network against brighter residual taenite. The contrast with laboratory quench martensite is therefore both thermal and morphological: cosmic cooling to very low temperature versus industrial quench from high temperature.

Recent work on other irons has sharpened the same point. In carbon-poor meteoritic Fe–Ni alloys, M_s is controlled primarily by local nickel content inherited from prior kamacite–taenite growth, not by cooling rate. Martensite in meteorites is therefore expected at sub-zero (often cryogenic) temperatures; models that treat martensite as a high-temperature transient that later decomposes into plessite require re-examination. Tishomingo remains the clearest natural experiment of that cryogenic path.

Why the Structure Matters

Tishomingo isolates a single variable that most irons never display so cleanly: nickel high enough to suppress kamacite nucleation, yet still able to drive a complete martensitic burst once temperature fell far below the terrestrial quench range. The plates therefore function as a thermometer for the coldest part of the parent body’s cooling curve and as a clock for the brief later reheating. Together they argue for early disruption of a differentiated planetesimal, exposure of metal to space temperatures, and a later collisional pulse of modest intensity and duration.

For the reader working through a suite of etched irons, the practical recognition criteria are straightforward: no Widmanstätten lamellae; coarse lenticular plates in retained taenite; high bulk Ni; hardness near 425 HV; and an occasional through-going austenite twin that reorients the plates.

Figure 3. Tishomingo. An austenite twin crossing the entire section. Scale bar 800 µm.

References

Buchwald, V.F. (1975). Handbook of Iron Meteorites. University of California Press. Describes Tishomingo as an anomalous (ungrouped) ataxite with ~32.5 wt% Ni, coarse martensitic structure, and hardness of ~425 HV.

Ives, L.K., Kasen, M.B., Schaefer, R.J., Clark, A.F., and Gehring, J.S. (1978). A microstructural study of the Tishomingo meteorite. Geochimica et Cosmochimica Acta 42, 1051–1066. Foundational optical and electron-microscopic analysis: ~80% martensite + 20% residual taenite; transformation from −25 °C to −65 °C, continuing to −75 °C to −115 °C; subsequent shock and mild thermal aging.

Russell, K.C., et al. (1991). Characterization of the Tishomingo meteorite. Field-ion and TEM evidence for mixed fcc and bcc products plus small fcc precipitates in the transformed regions.

Yang, J., Goldstein, J.I., Scott, E.R.D., Michael, J.R., Kotula, P.G., Pham, T., and McCoy, T.J. (2014). Thermal and collisional history of Tishomingo iron meteorite: More evidence for early disruption of differentiated planetesimals. Geochimica et Cosmochimica Acta 124, 34–53. SEM, TEM, AEM, and EBSD; 80% martensite plates formed at −75 to −200 °C; reheating to 320–400 °C for ~1 year; orientation relationships (K–S / N–W).

Kaufman, L., and Cohen, M. (1956). The martensitic transformation in the iron–nickel system. Progress in Metal Physics 7, 165–246. Classic M_s framework for Fe–Ni alloys.

Goldstein, J.I., and Michael, J.R. (2006). The formation of plessite in meteoritic metal. Meteoritics & Planetary Science 41, 553–570. Context for martensite versus plessite pathways in irons.

Rubin, A.E., and Ma, C. (2021). Meteorite Mineralogy. Cambridge University Press. Troilite–daubréelite intergrowths in Tishomingo; absence of graphite, carbides, phosphides, and silicates.

Goldschmidt 2024 abstract (session on Fe–Ni phase transformations). Correlative EBSD–APT work arguing that meteoritic martensite forms at sub-zero to cryogenic temperatures and that M_s is controlled by local Ni rather than quench rate.

Additional useful sources: Buchwald’s plate descriptions and twin geometry; MetSoc abstracts on Tishomingo CRE age and reheating; comparative martensite morphology papers on Chelyabinsk, Odessa, Seymchan, and IVB ataxites (lenticular versus packet/lath types).