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The process of turbulent radial mixing in protoplanetary disks has strongrelevance to the analysis of the spatial distribution of crystalline dustspecies in disks around young stars and to studies of the composition ofmeteorites and comets in our own solar system.  A debate has gone on in the recent literature on the ratio of the effectiveviscosity coefficient $\nu$ (responsible for accretion) to the turbulentdiffusion coefficient $D$ (responsible for mixing). Numericalmagneto-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 $\nu/D=1/3$ whichare based on elegant, though strongly simplified assumptions. We argue thatwhichever of these numbers comes closest to reality may be determined {\emobservationally} by using spatially resolved mid-infrared measurements ofprotoplanetary disks around Herbig stars. If meridional flows are present inthe disk, then we expect less abundance of crystalline dust in the surfacelayers, a prediction which can likewise be observationally tested withmid-infrared interferometers.

Dust crystallinity in protoplanetary disks: the effect of diffusion/viscosity ratio