During the melting of the micrometeoroids, the immiscibility of metals causes the formation of beads rich in iron and nickel. These dense phases experience a stronger gravitational pull or centrifugal force from spin than the surrounding melt. Thus, they frequently occur at the leading front or sides of oriented cosmic spherules, sometimes forming a chain of beads. Once exposed on the surface and in contact with the atmosphere during entry, oxidation of the iron may cause it to redissolve into the melt. This can be seen as a discoloration near the bead, where the crystals are lighter or darker in color.

 

As soon as the beads are exposed at the surface, they experience pronounced heating and ablation. As a result, the beads will gradually decrease in size or may flow out across the surface, creating an Fe-rich coating in a so-called wetting event. Alternatively, the bead can be ejected from the particle. 

Iron will be the first metal to ablate, followed by nickel and chromium. If this process continues, what remains are the most refractory elements. These mainly consist of the platinum group elements (PGEs) including platinum, palladium, rhodium, ruthenium, osmium, and iridium, of which the latter element is famous from the worldwide Ir-rich deposits directly following the dinosaur-killing impact 66 million years ago. If these elements get concentrated enough, we can observe them as tiny platinum group nuggets (PGNs), which are quite rare in cosmic spherules. See also Fig. 12 in Suttle et al. (2021).

 

Once on Earth, the beads are subjected to climatic conditions, which often result in the formation of rust, turning the beads a brownish color. Beads rich in nickel are commonly much more resistant to this oxidation process.