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Taenite, martensite and dense plessite fields |
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| Meteorite: Bella Roca, IIIAB iron |
| Figure 1. Scale bar 250 µm. |
| Meteorite: Bella Roca, IIIAB, Iron |
| Taenite, martensite, plessite fields. |
| This is #50, the last topic (for now) of this
visual presentation of Primary and Secondary structures – Meteorites
(mostly iron sturctures). The Bella Roca IIIAB iron is a structure-rich meteorite covering a lot of what has been previously presented. Like many iron meteorites, Bella Roca likely originated from the core of a differentiated asteroid. These asteroids formed early in the solar system’s history and underwent melting and differentiation, separating into a metallic core and a silicate mantle. As the molten metal in the asteroid’s core cooled, it began to crystallize. This process led to the formation of different iron-nickel minerals. Taenite and kamacite are the primary iron-nickel phases. Taenite has a higher nickel content, while kamacite has a lower nickel content. The cooling rate and nickel content determine the formation of these minerals. A finer-grained intergrowth of these two is plessite, a mix of taenite and plessite. It is often found as “fields” in small areas between taenite and kamacite lamellae. Another mineral that forms later in the cooling process is schreibersite. This is an iron-nickel phosphide that forms as phosphorus becomes concentrated in the remaining melt during crystallization. Bella Roca experienced significant shock events, likely from collisions in space. The evidence for this in Bella Roca is severalfold: The kamacite grains were deformed and hardened due to the intense pressure and heat. Martensite, a hard, brittle phase of iron formed from the rapid cooling of taenite during shock events, is present within the plessite fields of Bella Roca. Shock events can cause the redistribution of elements, leading to the formation of phosphorus-rich areas and precipitates. The kamacite in Bella Roca was converted by shock to a hatched e-structure (e-phase iron is a high-pressure phase of iron that can form under extreme pressure and heat from severe impacts.) Additionally, schreibersite residues (phosphides) are present throughout the e-phase. (Buchwald, 1975). These tiny precipitates form during the slow cooling of a meteorite in space. |
| Figure 2. Scale bar 250 µm. |
| Meteorite: Bella Roca, IIIAB, Iron |
| Taenite, martensite, plessite fields. |
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