Transport reactions are classified according to the thermodynamics of the reaction between the solid and the transporting agent.  When the reaction is exothermic, then the solid of interest is transported from the cooler end (which can be quite hot) of the reactor to a hot end, where the equilibrium constant is less favorable and the crystals grow.  The reaction of molybdenum dioxide with the transporting agent iodine is an exothermic process, thus the MoO migrates from the cooler end (700 °C) to the hotter end (900 °C):
:MoO  +  I    MoOI  ΔH < 0 (exothermic)
Using 10 milligrams of iodine for 4 grams of the solid, the process requires several days.  
Alternatively, when the reaction of the solid and the transport agent is endothermic, the solid is transported from a hot zone to a cooler one. For example:
:FeO  +  6 HCl    FeCl+ 3 HO  ΔH > 0 (endothermic)
The sample of iron(III) oxide is maintained at 1000 °C, and the product is grown at 750 °C.  HCl is the transport agent.  Crystals of hematite are reportedly observed at the mouths of volcanoes because of chemical transport reactions whereby volcanic hydrogen chloride volatilizes iron(III) oxides.