Primary and Secondary Structures - Meteorites
New England Meteoritical Services
 

Graphite and Troilite nodules
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Meteorites - Shawnee, Mount Dooling, Pizhanka, Cape York, Odessa
 
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Figure 1. Scale bar 2.5 mm.

Meteorite - Shawnee, IAB iron

Graphite/troilite nodule - Can be either Primary or Secondary.
G - Graphite
T - Troilite
S - Schreibersite
C - Cohenite
K - Kamacite
.SK - Swathing Kamacite
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Graphite and Troilite Nodules

Nodules containing graphite and troilite represent some of the most informative minor phases in iron meteorites. These features provide critical insights into the chemical and thermal evolution of their parent bodies, including differentiation, cooling histories, and post-formation shock or alteration events. In iron meteorites, nodules are heterogeneous aggregates of minerals that formed either during the primary solidification and cooling of the metallic core or through subsequent secondary processes.

This segment briefly examines their classification, mineralogy, textures, and petrogenesis, drawing on representative examples from several well-studied falls and finds.

Classification of Nodules: Primary versus Secondary

Nodules in iron meteorites are broadly classified as primary or secondary based on their timing of formation relative to the initial solidification of the parent melt. Primary nodules crystallize during the early to intermediate stages of cooling of the metallic parent body, typically from a largely molten Fe-Ni alloy containing trace to minor amounts of phosphorus (P), carbon (C), sulfur (S), cobalt (Co), chromium (Cr), and other elements. As the temperature decreases, these incompatibles concentrate in the residual liquid and nucleate distinct mineral phases. Common primary constituents include troilite (FeS), graphite (C), schreibersite ((FeNi)3P), cohenite ((FeNi)3C), and associated kamacite (α-FeNi) or taenite (γ-FeNi). These nodules often exhibit a core-rim structure, with a rounded or elongated troilite-graphite core enveloped by schreibersite and cohenite rims. The presence of such phases reflects the limited solid solubility of S, C, and P in the Fe-Ni metal at lower temperatures.

Secondary nodules, by contrast, form or are significantly modified after the primary cooling phase. They may result from shock metamorphism (e.g., hypervelocity impacts), thermal annealing, recrystallization, or metasomatic alteration in space. Shock events can melt and shear pre-existing troilite, mobilize metals, or induce phase transformations, while recrystallization may produce new grain boundaries or Neumann lines (shock-induced twinning in kamacite). Although secondary nodules often inherit the mineral suite of primary ones, their textures record later events, such as brecciation, melting, or gravitational settling within a partially molten parent body.

Nodules range in size from millimeters to tens of centimeters and frequently display layered structures. Sulfides like troilite may become trapped in elongated interstices during metal solidification, providing nucleation sites for subsequent schreibersite and cohenite precipitation. Cohenite rims, in particular, often form through carbon exsolution during continued cooling. This framework applies generally to magmatic (e.g., IIAB, IIIAB) and non-magmatic (e.g., IAB) iron meteorite groups, though bulk composition—especially sulfur content—strongly influences schreibersite abundance and nodule mineralogy.

Case Studies

Shawnee Meteorite

The Shawnee iron displays a classic graphite-troilite nodule (Figure 1) that may represent either a primary or secondary structure, depending on context. Key phases include graphite (G), troilite (T), schreibersite (S), cohenite (C), kamacite (K), and swathing kamacite (SK)—a rim of low-Ni kamacite that forms around nodules due to local Ni depletion. The core typically consists of troilite and graphite, with successive rims of schreibersite and cohenite reflecting sequential precipitation as the system cooled.

Mount Dooling Meteorite (Iron, IC, 6.26 wt% Ni)

Mount Dooling is a partially recrystallized coarse octahedrite. The illustrated troilite nodule (Figure 2) is interpreted as primarily primary in origin, but it exhibits clear secondary overprints: shearing and shock-melting that partially dissolved adjacent metal. Recrystallization textures indicate post-formation thermal or impact processing.

Pizhanka Meteorite (Iron, IAB-Mg, 7.40 wt% Ni)

This relatively recent Russian find (2016) has limited published data. The nodule shown (Figure 3) combines primary and secondary features. Patches of recrystallized kamacite surround a troilite-graphite core. Diagnostic secondary indicators include Neumann lines in kamacite (lower middle) and serrated edges on cohenite (C), the latter reflecting shock or strain-induced modification.

Cape York Meteorite (Iron, IIIAB, 7.34 wt% Ni)

Cape York, with a total known weight (TKW) of approximately 51 tons, has yielded abundant large specimens ideal for detailed study. It contains elongated troilite nodules (Figures 4–9), some exceeding 11 cm in length, interpreted as secondary in the context of parent-body processes. Buchwald (1975) proposed that these nodules formed or were modified in a parent body possessing a significant gravitational field. During a molten or partially molten stage on the asteroid, dense accessory minerals (particularly chromite) settled through the low-viscosity troilite melt. As the cooling front advanced, these minerals were trapped in clusters at one end of the elongated nodules (see arrows in Figure 6). Chromite occurs as 0.2–3 mm euhedral crystals that remain stable and undissolved in troilite even at high temperatures. Minor solid-solution elements (V, Mn, Zn) form smaller (0.1–0.2 mm) grains. Internal metal blebs within troilite can display martensitic microstructures (Figure 9), recording rapid cooling following localized melting.

Odessa Meteorite (IAB coarse octahedrite, 7.2 wt% Ni)

The troilite-graphite nodule in Odessa (Figure 10) closely resembles the Shawnee example in overall structure, featuring a similar core-rim organization consistent with primary formation modified by later events typical of the IAB complex.

Broader Implications

The mineralogy and textures of graphite-troilite nodules encode the differentiation history of iron meteorite parent bodies, which are thought to represent the cores of disrupted planetesimals. Primary nodules track the initial partitioning of volatiles and incompatibles during core crystallization, while secondary features record the dynamic environment of the early solar system—impacts, reheating events, and gravitational settling.
 
 
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Figure 2. Scale bar 1 mm.

Meteorite - Mount Dooling
Classification - Iron, IC, 6.26% Ni.
Troilite nodule - Primary structure, mostly.
Mount Dooling is a partially recrystallized coarse octahedrite. This image is of a sheared troilite nodule that has been shock-melted dissolving part of the surrounding metal in the process. The troilite nodule is (likely) primary but the shearing and recrystallization are secondary effects..
 
 
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Figure 3. Scale bar 700 µm.

Meteorite - Pizhanka
Classification - Iron, IAB - Mg, 7.40% Ni.
Troilite nodule - Primary / secondary.
Very little research has been published on this meteorite found in Russia in 2016. In this photograph, patches of recrystallized kamacite surround a troilite/graphite nodule. Note the Neumann lines, lower middle, and the serrated cohenite edges (C).
 
 
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Figure 4. Scale bar 8 mm.

Meteorite - Cape York
Classification - Iron, IIIAB, 7.34% Ni.
Troilite nodule - Secondary.
Elongated troilite nodule with 1cm cube for size assesment (lower right).
 
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Figure 5. Scale bar 5.3 mm.

Cape York

Cape York, TKW is around 51 ton, with several large individuals recovered presenting researchers with an abundance of sample material for study. The troilite nodule shown above is 11 cm in length. Buchwald, (1975), suggests a gravity field in the parent body - see next image.
 
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Figure 6. Scale bar 5.3 mm.

Cape York
The troilite in Cape York contains several accessory minerals. One of the most important is chromite appearing as 0.2 -3 mm crystals. It is undissolvable in troilite even at high temperatures.

A few other accessory minerals in minor quantities - vanadium, manganese, and zinc in solid solution - are also occasionally seen attaining sizes of 0.1 to 0.2 mm in Cape York. Buchwald (1975) noted that chromite and these accessory minerals are concentrated at one end of the elongated troilite nodules

Cape York went through a molten stage on an asteroid with a gravity field. These accessory minerals moved through the molten troilite settling just outside of the nodule as the cooling front passed through the metal where they were trapped in clusters.
The black arrows mark inclusions of chromite and other accessory minerals.
 
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Figure 7. Scale bar 300 µm.

Cape York, inclusions of chromite, other, that settled out of the cooling troilite.
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Figure 8. Scale bar 300 µm.

Cape York, inclusions of chromite, other, that settled out of the cooling troilite

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Figure 9. Scale bar 100 µm.

Cape York, internal metal bleb with a martensitic interior. This inclusion can be located in the left side of the troilite nodule seen in Figure 6.
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Figure 10. Scale bar 1.5 mm.

Meteorite - Odessa
Classification - IAB coarse octahedrite, 7.2% Ni.
Structurally, the troilite/graphite nodule is similar to Figure 1.
 


References

Buchwald, V.F. (1975). Handbook of Iron Meteorites. University of California Press.
Essential for nodule descriptions, including elongated troilite in Cape York and secondary structures.

Scott, E.R.D. & Goldstein, J.I. (2012). “Occurrence of Carbides and Graphite in Iron Meteorites.” Lunar and Planetary Science Conference (LPSC) abstract/proceedings.

Kontny, A.M. et al. (2011). “Magnetic Mineralogy of Troilite-Inclusions and their Fe-Ni Host Alloys in IAB Iron

 
 
 
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