  We present a study on connectivity percolation in suspensions of hard
platelets by means of Monte Carlo simulation. We interpret our results using a
contact-volume argument based on an effective single--particle cell model. It
is commonly assumed that the percolation threshold of anisotropic objects
scales as their inverse aspect ratio. While this rule has been shown to hold
for rod-like particles, we find that for hard plate-like particles the
percolation threshold is non-monotonic in the aspect ratio. It exhibits a
shallow minimum at intermediate aspect ratios and then saturates to a constant
value. This effect is caused by the isotropic-nematic transition pre-empting
the percolation transition. Hence the common strategy to use highly
anisotropic, conductive particles as fillers in composite materials in order to
produce conduction at low filler concentration is expected to fail for
plate-like fillers such as graphene and graphite nanoplatelets.
