Acapulcoites are primitive achondrites, derived from a partially differentiated parent body that experienced maximum temperatures above the Fe-Ni-S solidus [1, 2]. Their textures reflect annealing driven by the minimization of surface energy, yielding smoothly curved silicate grain-boundaries, common 120° triple junctions, and an equiaxial shape of Fe-Ni and troilite (FeS) pockets [e.g. 3]. However, Fe-Ni and FeS pockets are often spatially separated and rarely share boundaries, contrary to the expected minimum energy configuration, which would predict Fe-Ni grains embedded in FeS.
To understand the origin of these textures, we investigated a model system composed of olivine with 5 wt% gold, the higher surface energy phase (representing Fe-Ni), and 5 wt.% FeS, the lower energy phase. Samples were enclosed in graphite capsules and compressed to 2.5 GPa using a multi-anvil apparatus. In an attempt to mimic the slow cooling of planetesimals, samples were equilibrated at 1300 °C (above the liquidi of Au and FeS) for 3 h before rapidly cooling to 1000 °C (below the Au and FeS solidi) and maintaining temperature from 1 week to 1 month.
Our experiments simulate the phase separation and the progressive decrease of Fe-Ni–FeS boundary length observed in H3-H6 chondrites and acapulcoites [4]. The most likely mechanism is passive solid-state re-adjustment of the opaque phases during grain growth. Since FeS has a higher mobility than the high surface energy phase (Au and Fe-Ni in experiments and meteorites, respectively), the phases are dragged for varying distances by migrating grain-boundaries, leading to spatial separation. The accompanied increase of the surface energy of the opaque phases is likely compensated by the decrease of surface energy gained by silicate grain growth.

[1] Palme et al., GCA (1981). [2] Zipfel et al., GCA (1995). [3] Keil & McCoy, Geochemistry (2018). [4] Guignard & Toplis, GCA (2015).