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Two Generations of Rhabdites
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 Meteorite - Gressk, IIAB Iron.
 

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Figure 1. Scale bar 500 µ.
Meteorite: Gressk, IIAB iron.
First generation rhabdites.
 
 
Two Generations of Rhabdites in Iron Meteorites

Figure 1.
First-generation rhabdites in the Gressk IIAB iron meteorite. Scale bar=500 µm.

Figure 2.
Second-generation rhabdites in the Gressk IIAB iron meteorite. Scale bar=50 µm.

Rhabdites are morphologically distinct, rod-like or needle-shaped crystals of schreibersite (FeNi)3P, a common phosphide mineral in iron meteorites. The term “rhabdite” historically distinguished these small, prismatic or acicular forms from larger schreibersite plates or massive occurrences, although both share the same mineral species and tetragonal crystal structure.
In the Gressk IIAB iron meteorite, two distinct generations of rhabdites record different stages of the parent body’s thermal evolution.

First-Generation Rhabdites (Primary/Subsolidus) Larger first-generation rhabdites (typically 1–3 mm plates or coarser needles) form during the initial slow cooling of the metallic core or pool following differentiation of the parent asteroid. These nucleate and grow subsolidus—primarily in the temperature range of ~700–500 °C—via diffusion-controlled exsolution of phosphorus from supersaturated kamacite or taenite as the Widmanstätten pattern develops. Their relatively large size, euhedral to subhedral habit, and more uniform distribution reflect ample time for phosphorus and nickel diffusion in a slowly cooling environment (cooling rates on the order of 1–10 °C per million years, characteristic of many IIAB irons). This generation precipitates under near-equilibrium conditions deep within the parent body, where thermal gradients were minimal and long-term annealing allowed significant crystal growth.

Second-Generation Rhabdites (Microrhabdites)
Smaller second-generation rhabdites or microrhabdites (commonly 5–10 µm or finer) form later, at lower temperatures (~500–400 °C or below), often through homogeneous nucleation within kamacite. These can result from:
• Residual phosphorus supersaturation after depletion by earlier massive schreibersite or first-generation rhabdites.
• Localized reheating events, such as impacts, which temporarily mobilize P and Ni before renewed slow cooling.
• Continued slow cooling into regimes where grain-boundary or volume diffusion becomes highly restricted, leading to fine-scale precipitation.

In the Gressk meteorite, these finer crystals often appear concentrated along grain boundaries, within plessite fields, or in localized zones, reflecting more heterogeneous nucleation conditions during later thermal perturbations. Their smaller size indicates shorter diffusion distances and lower temperatures, consistent with kinetic limitations at advanced stages of cooling.

Broader Implications The presence of multiple generations of schreibersite/rhabdite provides valuable constraints on the metallographic cooling history, phosphorus budget, and post-accretion processing of iron meteorite parent bodies. IIAB irons, including Gressk, are magmatic group members thought to sample a fractionally crystallized metallic core. Their structures record not only primary differentiation and slow core cooling but also potential secondary events such as impact-induced reheating, which are common in the asteroid belt. Combined with Widmanstätten bandwidth measurements, taenite Ni profiles, and other exsolution features, rhabdite petrography helps determine cooling rates across different temperature intervals and reconstruct the size and thermal insulation of the parent body. Such multi-stage phosphide growth is observed across many iron groups and underscores the complex, protracted histories of differentiated planetesimals.
 
 
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Figure 2. Scale bar 50 µm.
Meteorite: Gressk, IIAB iron.
Second generation rhabdites.
 
 
References
• Buchwald, V. F. (1975). Handbook of Iron Meteorites. University of California Press.
• Clarke, R. S., Jr., & Goldstein, J. I. (1978). Schreibersite growth and its influence on the metallography of coarse-structured iron meteorites. Smithsonian Contributions to the Earth Sciences, No. 21. (Key reference on generations and growth temperatures).
• Goldstein, J. I., et al. (various works on Fe-Ni-P phase relations and diffusion).
• Meteoritical Bulletin Database entry for Gressk (IIAB iron).
• Scott, E. R. D., & Wasson, J. T. (1975). Classification and properties of iron meteorites. Reviews of Geophysics and Space Physics.
• Wasson, J. T. (1985). Meteorites: Their Record of Early Solar-System History. W. H. Freeman.
 
 
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