| Primary and Secondary Structures - Meteorites |
Graphite and Troilite nodules |
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| Meteorite - Shawnee, Mount Dooling, Pizhanka, Cape York, Odessa |
| Figure 1. Scale bar 2.5
mm. Meteorite - Shawnee Graphite/troilite nodule - Can be either Primary or Secondary. |
| G - Graphite |
| T - Troilite |
| S - Schreibersite |
| C - Cohenite |
| K - Kamacite |
| .SK - Swathing Kamacite |
| . |
| . |
| Nodules in iron meteorites can be either
primary or secondary structures. Primary nodules form during the initial solidification of the meteorite’s parent body and can consist of kamacite, taenite, troilite and occasionally schreibersite. Secondary nodules develop after primary cooling, often from shock metamorphism (impact) or the alteration of primary phases. These nodules could have initially been primary structures altered metamorphically (heat-generated shock) into secondary structures, or they could have formed from other in-space events. They contain most of the same mineralogy found in the (then) parent meteoroid, often in low concentrations. While the parent mass was originally a large single or twinned austenite crystal, it was likely not a pure alloy of Fe/Ni. Containing low to trace levels of P, C, Co, S, and Cr, these would come into play late in the primary cooling phase as a new series of minerals would nucleate and be precipitated, e.g., phosphates, sulfides into troilite and shreibersite**, cohenite and others. Nodules formed after the molten metal phase would be secondary structures. This series of minerals can be found in most meteoritic nodules although sometimes occurring as only single grains. Physically, ranging in size from a millimeter to 10 cm or more, nodules are often individually layered in the sense of a rounded or elongated core of troilite/graphite, surrounded with layers or rims of schreibersite and often followed by cohenite precipitating from carbon and forming more massive rims. Sulfides like troilite can be trapped in elongated interstices in the cooling metal. From there, the nucleation of schreibersite and cohenite would precipitate into rims around the troilite/graphite "core". The above description is coarsely written to be helpful with type I, II, and III irons. Further refinement would be needed for each type. ** Depending upon bulk sulfur content. |
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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 disolving 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), argues for 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 undissolable 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 |
| 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. |
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