Primary and Secondary Structures - Meteorites
New England Meteoritical Services
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Martensitic structure
 
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Figure 1. Scale bar 300 µm.
 
Meteorite - Tishomingo
Multiple groups of martensite (black) in retained taenite (white).
Martensitic structure - Secondary.
 


In meteoritics, martensite, metallic Fe-Ni with 8-15% Ni, is typically associated with a shock-heating event.

It forms through a process of rapid cooling (quenching) of taenite, which effectively traps carbon atoms that lack the time to diffuse out of the crystal structure, a key detail in understanding its unique characteristics.

This process is known as the martensitic transformation. It is a diffusionless process involving a series of temperature declines that develop new plates and structures. Since it is diffusionless, any early-developed martensitic structures or plates will not grow with time.

Martensite is also very hard, typically ranging from 600 to 700 Vickers Hardness, which makes it brittle. When acid etched, martensite generally appears as an irregular series of dark brown/black, thin plates overlying the parent taenite. The meteorite used for this example of martensite is Tisomingo.

Buchwald's 1975 Handbook of Iron Meteorites intriguingly described Tishomingo as "a unique meteorite, unlike any other in its characteristics and composition."

Chemically anomalous with 32.5 percent (by weight) Ni, it is among the most Ni-rich irons known. It is also structurally anomalous, with 20% residual taenite and 80% coarse martensite plates.

Examination of Tishomingo shows no Widmanstätten pattern, no plessitic fields, minor levels of kamacite, and an occasional sub-mm-sized, shock melted troilite nodule. It is a field of martensite and the only known meteorite to display the martensitic structure throughout its entire mass (164 kg).
 

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Figure 2. Scale bar 150 µm.
Etched sections of Tishomingo display a coarse martensitic structure.
 
 
 
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Figure 3. Scale bar 800 µm. 
Tishomingo. Austenite twin crossing the entire section.
 
 
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 Figure 4. Scale bar 800 µm.
Tishomingo. Shock melted troilite nodule, upper left. Scale bar 800 µm.
 
Tishomingo contains no graphite, carbides, phosphides, or silicates (Buchwald, 1975). However, it does have metal-sulfide intergrowths consisting of irregular troilite grains. The cores of these grains are composed of taenite and troilite. Also reported is daubréelite within the rims of these grains (Ruben, Meteorite Mineralogy, 2021).
 
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