Abstract Vigarano 1623-5 is a forsterite-bearing refractory inclusion in which oxygen, magnesium, and silicon show large degrees of mass-dependent isotopic fractionation. The core of 1623-5 consists of forsteritic olivine, fassaite, melilite (Åk 89), and spinel, all of which show isotopic mass fractionation, averaging ~30.6%./amu for magnesium and ~10.0%./amu for silicon; it is enclosed in a mantle consisting of aluminous melilite (Åk 16–60), spinel, perovskite, and hibonite. Hibonite and spinel in the mantle are enriched in heavy isotopes of magnesium by 8–15%./amu relative to the core. 1623-5 originally formed by solidification of a magnesium-rich melt. The endemic isotopic mass fractionation may have been caused by evaporation of that melt or may reflect an earlier evaporation event affecting the reservoir from which the interior of 1623-5 formed. Later flash remelting of the outer part of the core, and volatilization of SiO 2 and MgO from the melt so produced, caused formation of a mantle that is in isotopic and petrologic disequilibrium with the core.