Mass distribution within planetesimals formed from streaming instability filaments, compared to the composition of Mercury. We assume here that iron pebbles have St = 0.1, enstatite pebbles have St = 0.01, and corundum pebbles have St = 0.001. We consider filtering function power laws ranging from p_SI = 0 (where particles maintain their size distribution when they enter planetesimals) to p_SI = 1.0 (where smaller particles have a reduced presence in planetesimals proportional to their Stokes number). We find a good match to Mercury between p_SI = 0.5 and p_SI = 1.0. This filtering additionally implies that Mercury will be strongly depleted in small corundum particles. This is consistent with the measured low Al/Si ratio in Mercury’s crust (Peplowski et al. 2012).