Fractional crystallization is an essential process proposed to explain
worldwide compositional abundances of igneous rocks. It requires
crystals to precipitate from the melt and segregate from its residual
melt, or crystal fractionation. The compositional abundances of volcanic
systems show a bell-curve distribution suggesting that the process has
variable efficiencies. We test crystal fractionation efficiency in
convective flow in low to intermediate crystallinity regime. We simulate
the physical segregation of crystals from their residual melt at the
scale of individual crystals, using a direct numerical method. We find
that at low particle Reynolds numbers, crystals sink in clusters. The
relatively rapid motion of clusters strips away residual melt. Our
results show cluster settling can imprint observational signatures at
the crystalline scale. The collective crystal behavior results in a
crystal convection that governs the efficiency of crystal fractionation,
providing a possible explanation for the bell curve distribution in
volcanic systems.