  The inner part of protoplanetary disks can be threaded by strong magnetic
fields. In laboratory levitation experiments, we study how magnetic fields up
to 7 mT influence the aggregation of dust by observing the self-consistent
collisional evolution of particle ensembles. As dust samples we use mixtures of
iron and quartz in different ratios. Without magnetic fields, particles in all
samples grow into a bouncing barrier. These aggregates reversibly form larger
clusters in the presence of magnetic fields. The size of these clusters depends
on the strength of the magnetic field and the ratio between iron and quartz.
The clustering increases the size of the largest entities by a factor of a few.
If planetesimal formation is sensitive to the size of the largest aggregates,
e.g., relying on streaming instabilities, then planetesimals will
preferentially grow iron-rich in the inner region of protoplanetary disks. This
might explain the iron gradient in the solar system and the formation of dense
Mercury-like planets.
