I was doing an experiment over the last couple of days to try to crystallize alum using a thermal gradient. The idea was that solute at the bottom of my container would be dissolved at a higher percentage (I used a coffee mug warmer) because of the solubility curve...
Then because the local solution is warmer than the solution above it, it would become buoyant, rise to the top, cool, become super saturated, and then deposit its excess solute preferably where I want it. This setup worked fine for me with epsom salt and ordinary table salt. But in this case, using alum, a salinity gradient, or halocline formed and the solution at the bottom, though warm enough to be uncomfortable to the touch, did not become buoyant. My container was only 9 inches tall. At the top, the solution was almost room temperature. I remembered this effect when reading about solar ponds:
But typically a solar pond is several feet deep.
My question is, why does a halocline form in the first place? Is this simply the action of gravity on the solute? Is alum's greater solubility the reason I observed this effect in only 9 inches of container height?