Slow evaporation yields few large crystals while quick evaporation yields many smaller crystals. Crystals can also form if the solution or vapor is cooled or subjected to a decrease in pressure. In general, hot solutions are able to accommodate more atoms per unit of solution. As the solution cools, the atoms have less and less room to move and when the solution has cooled sufficiently, the atoms link together precipitating the mineral. This is the same for changes in pressure.
High pressures allow more atoms per unit volume.
If the pressure is lowered, the atoms bond together forming the solid mineral.