The presence of two generations
of rhabdites in the Gressk meteorite provides information about its cooling and
impact history. (Reminder - rhabdites are needle-like or plate-like inclusions
of schreibersite.)
The first generation forms during the meteorite's
initial solidification (primary cooling). These are usually larger, having more
time for the schreibersite to nucleate. They are generally well-distributed
throughout the meteorite, ranging from 1 to 3mm in plates.
As it cools, the meteorite undergoes thermal
metamorphism from the reheating and subsequent cooling from impacts or passage
near other heat sources.
During this time, smaller schreibersite inclusions
form. These are known as second-generation rhabdites and are typically found
within specific areas or along grain boundaries. The reheating redistributes
phosphorus and nickel, fostering the nucleation and growth of smaller, 8 –
10 µm rhabdites.
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