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Rhabdites and Schreibersite |
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| Meteorite - Abai, IAB Iron |
| Figure 1. Scale bar 60 µm. |
| Meteorite - Abai, IAB - Mg. |
| Classification - Iron, IAB-Mg, 6.11% Ni,
0.14% P Rhabdites - Secondary. |
| Rhabdites and Schreibersite in Iron Meteorites Rhabdites and schreibersite are tetragonal iron-nickel phosphide minerals with the general formula (FeNi)3P. They constitute important accessory phases in many iron meteorites, offering valuable insights into phosphorus behavior, subsolidus cooling histories, and the kinetics of phase transformations during the prolonged thermal evolution of asteroidal parent-body cores. Mineralogy and Occurrence Schreibersite and rhabdite are compositionally identical but distinguished primarily by their morphology, size, and nucleation sites. Schreibersite typically refers to larger, often anhedral to subhedral grains, plates, or veinlets that occur at kamacite-taenite interfaces, grain boundaries, or associated with other phases such as troilite. Rhabdites (from the Greek for “rod-like”) describe smaller, euhedral to subhedral prismatic, needle-like, or plate-shaped crystals that nucleate homogeneously within kamacite grains. Both phases are common in iron meteorites with moderate to high bulk phosphorus contents (typically >0.1–0.2 wt% P), but they are absent or rare in low-P groups such as many IVA irons (e.g., Gibeon). When bulk P exceeds ~0.4 wt%, phosphides are particularly prominent and often appear as angular, plate-shaped inclusions. Formation and Petrogenesis These phosphides precipitate in the solid state during slow cooling of the Fe-Ni metal, after the onset of the Widmanstätten pattern (kamacite exsolution from taenite). Phosphorus solubility in kamacite and taenite decreases markedly with falling temperature, leading to supersaturation and nucleation of (FeNi)3P. • Schreibersite can form over a broader temperature range and may exhibit both primary (early subsolidus) and secondary characteristics. Larger schreibersite grains often nucleate heterogeneously at kamacite-taenite boundaries or grain boundaries, typically at higher temperatures (potentially from ~850 °C down to ~500 °C or lower). They are enriched in Ni relative to the host metal and can influence local diffusion and microstructure. • Rhabdites are secondary features formed by homogeneous nucleation within P-supersaturated kamacite at lower temperatures, generally in the 600–400 °C range (with microrhabdites often <500 °C). Their formation reflects limited diffusion of Ni and P at these conditions, resulting in the characteristic prismatic or plate-like habits. Nucleation and growth are strongly controlled by bulk composition (Ni and P content), cooling rate, and the availability of diffusion pathways. In the Fe-Ni-P system, entry into multi-phase fields (e.g., α + γ + phosphide) facilitates these reactions. The process is diffusion-limited, with P flux and Ni partitioning at growth interfaces determining crystal size and composition. Case Studies Abai Meteorite (Iron, IAB-Mg, 6.11 wt% Ni, 0.14 wt% P) (Figure 1, scale bar 60 µm) This specimen displays rhabdites interpreted as secondary precipitates. The relatively low bulk P content results in modest phosphide development within the recrystallized or annealed kamacite matrix typical of many IAB-complex irons. Cosby’s Creek Meteorite (Iron, IAB-Mg, 6.57 wt% Ni, 0.41 wt% P) (Figures 2–3, scale bar 20 µm) Higher phosphorus abundance promotes well-developed plate-shaped rhabdites. Side lighting in Figure 3 highlights Neumann bands (shock-induced mechanical twins) in the host kamacite, illustrating the superposition of secondary shock features on the primary Widmanstätten microstructure and later phosphide precipitation. Walker County Meteorite (Iron, IAB-Mg, 5.46 wt% Ni, 0.28 wt% P) (Figure 4, scale bar 30 µm) Plate-shaped rhabdites are prominent within kamacite, consistent with homogeneous nucleation under slow-cooling, low-temperature conditions. Broader Implications The distribution, size, composition, and morphology of schreibersite and rhabdites serve as sensitive recorders of thermal history. Larger schreibersites constrain higher-temperature cooling paths, while rhabdites provide constraints on lower-temperature regimes where diffusion is sluggish. Combined with Widmanstätten bandwidth measurements and microprobe traverses, these phases help refine metallographic cooling rates and reconstruct the multi-stage evolution of iron meteorite parent bodies—including differentiation, slow core crystallization, and later impact events. Advanced studies using electron probe microanalysis (EPMA), scanning electron microscopy (SEM), and crystallographic techniques continue to elucidate the complex interplay between phosphide growth and the surrounding Fe-Ni metal matrix. |
| Figure 2. Scale bar 20 µm. |
| Meteorite - Cosby's Creek |
| Classification - Iron, IAB-Mg, 6.57 Ni, 0.41% P |
| Plate-shaped rhabdites. |
| Figure 3. Scale bar 20 µm. |
| Meteorite - Cosby's Creek |
| Classification - Iron, IAB-Mg, 6.57 Ni, 0.41% P |
| Plate-shaped rhabdites, side lighting to accent Neumann bands (lines). |
| Meteorite - Walker County |
| Figure 4. Scale bar 30 µm. |
| Meteorite - Walker County |
| Classification - Iron, IAB-Mg, 5.46 Ni, 0.28% P |
| Plate-shaped rhabdites. |
| References • Buchwald, V.F. (1975). Handbook of Iron Meteorites. University of California Press. • Clarke & Goldstein (1978). Smithsonian Contributions to the Earth Sciences (and related works on schreibersite growth). • Meteoritical Bulletin Database entries for classified specimens. |
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