  The process of turbulent radial mixing in protoplanetary disks has strong
relevance to the analysis of the spatial distribution of crystalline dust
species in disks around young stars and to studies of the composition of
meteorites and comets in our own solar system.
  A debate has gone on in the recent literature on the ratio of the effective
viscosity coefficient $\nu$ (responsible for accretion) to the turbulent
diffusion coefficient $D$ (responsible for mixing). Numerical
magneto-hydrodynamic simulations have yielded values between $\nu/D\simeq 10$
(Carballido, Stone & Pringle, 2005) and $\nu/D\simeq 0.85$ (Johansen & Klahr,
2005}). Here we present two analytic arguments for the ratio $\nu/D=1/3$ which
are based on elegant, though strongly simplified assumptions. We argue that
whichever of these numbers comes closest to reality may be determined {\em
observationally} by using spatially resolved mid-infrared measurements of
protoplanetary disks around Herbig stars. If meridional flows are present in
the disk, then we expect less abundance of crystalline dust in the surface
layers, a prediction which can likewise be observationally tested with
mid-infrared interferometers.
