Why couldn't "evaporation" be a possibility here? We usually think of many igneous crystals forming during the cooling of magma, but evaporation too is a cooling process, we know. So if these molecules know how to arrange themselves from being in liquid form with say a wall of points growing out of a milky quartz matrix, for instance, why couldn't a much gentler and slower process of cooling happen with minerals suspended in water?
Back in the early 90s I spend a couple of days mining quartz points near Mt. Ida, Arkansas. I mostly worked on a single vein with two facing walls of large, optical-quality points growing toward one another. In between the two facing walls was packed about 6-8" that had filled in with a dense, rubbery red clay that had obviously settled into the vein over many many years and dried out. Suspended in that red clay at various points were, not double-terminated points, but tabular crystals that were not touching either wall of the vein, but had been apparently floating in their as it filled with clay-particle sediment. I have always wondered how they formed.
Thanks for the previous comments. I like the idea of a tiny nucleus begining the expansive growth into a double-termination, by contrast with the broad cooling of a wall of points, etc.