  There is evidence that protoplanetary disks--including the protosolar
one--contain crystalline dust grains on spatial scales where the dust
temperature is lower than the threshold value for their formation through
thermal annealing of amorphous interstellar silicates. We interpret these
observations in terms of an extended, magnetocentrifugally driven disk wind
that transports grains from the inner disk--where they are thermally processed
by the stellar radiation after being uplifted from the disk surfaces--to the
outer disk regions. For any disk radius $r$ there is a maximum grain size
$a_\mathrm{max}(r)$ that can be uplifted from that location: grains of size
$a$$\ll$$a_\mathrm{max}$ are carried away by the wind, whereas those with
$a$$\lesssim$$a_\mathrm{max}$ reenter the disk at larger radii. A significant
portion of the reentering grains converge to--and subsequently accumulate in--a
narrow region just beyond $r_\mathrm{max}(a)$, the maximum radius from which
grains of size $a$ can be uplifted. We show that this model can account for the
inferred crystallinity fractions in classical T Tauri and Herbig Ae disks and
for their indicated near constancy after being established early in the disk
evolution. It is also consistent with the reported radial gradients in the mean
grain size, crystallinity, and crystal composition. In addition, this model
yields the properties of the grains that remain embedded in the outflows from
protoplanetary disks and naturally explains the inferred persistence of small
grains in the surface layers of these disks.
