  In an earlier work, we found that large metallic iron fractions in dust
aggregates and strong magnetic fields boost preplanetary growth. This sets an
initial bias for the formation of Mercury-like planets in the inner part of
protoplanetary disks. We extended these experiments here by adding pure quartz
aggregates to the iron-rich aggregates. Magnetic boost still leads to the
formation of larger clusters of aggregates. These clusters now include silicate
aggregates, which can also be connecting bridges between chains. However, at
least a certain fraction of iron-rich aggregates are needed to trigger magnetic
boost. Without a magnetic field, the sticking properties of the aggregates and
their constituents determine the composition of clusters of a given size. This
introduces a new fractionation and sorting mechanism by cluster formation at
the bouncing barrier.
