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Palmate-shaped cohenite crystals |
| Listing of Structures |
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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 Santa Rosa iron meteorite presents a complex challenge for
mineralogical and structural modeling due to its unique characteristics. Unlike
typical iron meteorites, Santa Rosa is a polycrystalline iron featuring
decomposed cohenite and graphite structures. Evidence of cosmic reheating is
apparent in its recrystallized kamacite, distorted Neumann lines, and the
absence of a distinct Widmanstätten pattern. Chemically, it stands apart,
with Chihuahua City being its only known close relative.
Close examination reveals striking palmate-shaped cohenite crystals, which appear almost skeletal in form. These "hand" or "fan"-shaped structures are rare among meteorites. Prior research points to a multifaceted formation history. Initially, the meteorite's metal likely existed at high temperatures as a polycrystalline aggregate of austenite grains, indicated by numerous crystal boundaries and twinning. As it cooled, these grains nucleated into kamacite, while the remaining austenite (taenite) grains gradually became carbon-enriched through diffusion. This process triggered the breakdown of taenite into elaborate intergrowths of cohenite, taenite, kamacite, and schreibersite, giving rise to the meteorite's 0.5 to 2.0 mm palmate structures. Subsequent shock events further shaped their evolution, which also resulted in troilite melts and additional 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. |
| Listing of Structures |