The form of the mineral may very well change; what it is bound to. Calcium bicarbonate is prevalent in our water. I have no clue if this form is bioavailable but it is so prominent that our pH is usually about 7.8-8.1, very alkaline. It is very heavy when it dries out and leaves a white film on the soil. We have to run sulfuric acid through our lines to drop the pH to a "life sustaining" level for our trees. We don't drink the sulfuric water. Calcium bicarbonate is a calcium atom bound to two carbons, and the two carbons make it a very dense molecule. I would imagine it would take a lot of heat to break them down, or even if boiling our water would affect the pH. That would be an interesting experiment, though sadly I don't have the resources (and only some of the knowledge) to correctly complete an experiment to see if it was the calcium bicarbonate breaking down or something else.
From an interesting little page I found searching for heat related stuff;
http://water.me.vccs.edu/courses/ENV115/lesson9.htmIn addition to having different removal methods, carbonate and noncarbonate hardness can cause different problems. Carbonate hardness is the most common and is responsible for the deposition of calcium carbonate scale in pipes and equipment. The equation below shows how this deposition is formed in the presence of heat:
Calcium bicarbonate ? Calcium carbonate + Water + Carbon dioxide
Ca(HCO3)2 ? CaCO3 + H2O + CO2
Heat does seem to break down the bicarbonate into carbonate. Is one more bioavailable than the other?
http://en.wikipedia.org/wiki/Calcium_carbonateEggshells, snail shells and most seashells are predominantly calcium carbonate and can be used as industrial sources of that chemical. [6] Oyster shells have enjoyed recent recognition as a source of dietary calcium, but are also a practical industrial source.[7][8] While not practical as an industrial source, dark green vegetables such as Broccoli and Kale contain dietarily significant amounts of calcium carbonate.[9]
Calcium bicarbonate is many times more soluble in water than calcium carbonate—indeed it exists only in solution.
Calcium carbonate will react with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.
CaCO3 + CO2 + H2O ? Ca(HCO3)2
This reaction is important in the erosion of carbonate rocks, forming caverns, and leads to hard water in many regions.
So bicarbonate is the form in solution, carbonate is outside of solution, as a solid mineral rock or precipitate of water.
Tough to say for certain the effect of heat on minerals but for this one, heat seems somewhat irrelevant to overall effect desired from the mineral, that is getting calcium.
