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.