  Observations of galaxy clusters show that the intracluster medium (ICM) is
likely to be turbulent and is certainly magnetized. The properties of this
magnetized turbulence are determined both by fundamental nonlinear
magnetohydrodynamic interactions and by the plasma physics of the ICM, which
has very low collisionality. Cluster plasma threaded by weak magnetic fields is
subject to firehose and mirror instabilities. These saturate and produce
fluctuations at the ion gyroscale, which can scatter particles, increasing the
effective collision rate and, therefore, the effective Reynolds number of the
ICM. A simple way to model this effect is proposed. The model yields a
self-accelerating fluctuation dynamo whereby the field grows explosively fast,
reaching the observed, dynamically important, field strength in a fraction of
the cluster lifetime independent of the exact strength of the seed field. It is
suggested that the saturated state of the cluster turbulence is a combination
of the conventional isotropic magnetohydrodynamic turbulence, characterized by
folded, direction-reversing magnetic fields and an Alfv\'en-wave cascade at
collisionless scales. An argument is proposed to constrain the reversal scale
of the folded field. The picture that emerges appears to be in qualitative
agreement with observations of magnetic fields in clusters.
