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Palmate-shaped cohenite crystals |
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Meteorite, Santa Rosa, IC iron. |
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| Figure 1. Scale bar 200 µm. |
| Meteorite - Santa Rosa. |
| Classification - Iron, IAB-Mg, 6.11% Ni, 0.14% P |
| Palmate-shaped cohenite structures - Secondary. |
| The mineralogy and structures in the Santa Rosa iron meteorite are
challenging to model. Santa Rosa is unusual in that it's a polycrystalline iron with decomposed cohenite and graphite structures. It has been cosmically reheated and displays recrystallized kamacite, distorted Neumann lines, and no clearly identifiable Widmanstatten structure. Chemically, it has only one known near-relative, Chihuahua City. Further study of this meteorite reveals palmate-shaped cohenite crystals that are almost skeletonized in appearance. Palmate-shaped crystals of this nature are uncommon in meteorites. Palmate refers to a "hand" or "fan"-shaped figure. Research suggests a complex formation process for the Santa Rosa iron meteorite. Initially, the metal likely existed at high temperatures as a polycrystalline aggregate of austenite grains, as evidenced by multiple crystal boundaries and twinning. The cooling grains then nucleated into kamacite, and the residual austenite (taenite) grains, slowly by diffusion, became carbon enriched. This led to the decomposition of taenite into intricate composite intergrowths of cohenite, taenite, kamacite, and schreibersite, forming the 0.5 to 2.0 mm-sized palmate structures. Later shock events produced troilite melts and cohenite decomposition. |
| Figure 2. Scale bar 1 mm. |
| Palmate-shaped cohenite structures. |
| Figure 3. Scale bar 150 µm. |
| Palmate-shaped cohenite structure. |
| Figure 4. Scale bar 300 µm. |
| Palmate-shaped cohenite structures. |
| Note, Buchwald, 1975, "Handbook of Iron Meteorites" addresses the cohenite structures in Santa Rosa in detail. |
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