Summary: A 250 µm olivine crystal in Sharps contains 1 to 5 µm insets identified by microprobe analysis as ferroan monticellite, spinel, and fassaite with average compositions: olivine (host), (Fe_0·44Mg_1·51Ca_0·05) (Si_0·99Al_0·01)O₄; ferroan monticellite, (Ca_0·89Fe_0·41Mg_0·66) (Si_1·00Al_0·02)O₄; spinel, (Fe0·41Mg0·59)(Fe0·063+Cr0·23Al1·69Si0·01Ti0·01)O4; and fassaite, Ca0.96(Fe0·01Mg0·59)(Al0·18Ti0·07Fe0·163+)(Si1·53Al0·47)O6. Textural and experimental data suggest early crystallization of spinel from a calcic olivine melt, exsolution of ferroan monticellite from the host olivine, and reaction of spinel and ferroan monticellite to form fassaite. The severe depletion of alkalies and silica in this crystal suggests it is a residue from vapour fractionation.