A 1.5 mm clast of a highlands troctolite attached to a glassy, alkali-rich soil breccia was identified in a thin section of 1--2 mm particles from Apollo 12 soil sample 12033. The troctolite consists of euhedral olivines (Fo89) in a groundmass of granulitic plagioclase (An95); each olivine grain is rimmed by a thin selvage of olivine-plagioclase glass containing vesicles and minute crystallites of spinel, olivine, and plagioclase. This unique texture records a complex history, beginning with the formation of the troctolite as a plutonic cumulate, followed by subsolidus metamorphism, partial remelting, vesculation, crystallite growth, and quenching. Experimental runs on plagioclase-olivine melting rates show that, at temperatures above incipient melting, the grain-boundary selvages in the lunar troctolite would have formed very rapidly (in less than about 10 min). Preservation of selvage glass required rapid cooling. The troctolite was subject to a heat pulse, most likely accompanying an impact event. The same event, or more probably a later one, welded the troctolite to the soil breccia. Inasmuch as this particle was found on Oceanus Procellarum at a site crossed by a ray from Copernicus, and because olivine has been identified by remote sensing experiments as the dominant mafic mineral in the central peak of Copernicus, we suggest that the troctolite is a fragment of Copernicus ejecta.