Dissolved minerals

Question

I have a question that I am confused about.

If a mineral (say limestone) was added to a groundwater system, say like an aquifer and is completely dissolved in solution, can it ever be a solid again without evaporation happening? My thinking is if there are other sources of calcium to the system then that might promote oversaturation of the solution and so the mineral calcite would precipitate out again. Am I right?

Thanks.

Answer 1

There are several processes that can cause a mineral to precipitate from solution. You mention one which is adding elements to the solution This could occur in a number of ways. For example, aragonite is a form of calcium carbonate with a higher solubility at low temperatures than the mineral calcite that makes up limestone. So it can dissolve and cause calcite to precipitate. Ions adsorbed on the surface of minerals like clay minerals can exchange with ions in solution changing the concentrations and causing precipitation. Two different compositions of water can mix to cause minerals to precipitate.

Changing temperature can also make minerals precipitate.

Changing the pH or oxidation state affects the solubility of various minerals. Increasing the pH will cause precipitation of calcite from a saturated solution. One way this can occur is to degas carbon dioxide from the water. That's what causes the formation of stalactites in caves.

Answer 2

A very important factor is fluid pressure. The capacity of a fluid to hold dissolved minerals is changing with pressure (usually it increases with increasing pressure). Examples (from nice-to-know to significant):

1. A bottle of carbonated water: You open the lid, the pressure drops, degassing occurs.
2. In the formation of stalactites: Water leaves the pressureized ground water aquifer, enters a cavity and precipitates minerals that make the stalactites (ok, other things go on, too, see answer above).
3. Veins: A rock fractures (maybe due to tectonic stress), void is generated, fluid pressure drops, mineral (often quartz or calcite) precipitates as veins. Studying veins tells us something about the (paleo)(tectonic) stress regimes that acted while the veins formed. Super nice to unravel the evolution of a fault or the generation of a mountain chain.
4. Look for Carbonate-Compensation-Depth. It is like an orographic snow-line but under water. Defined by the pressure of depth, calcite will either precipitate or dissolve into the ocean.

Answer 3

Precipitation can be formed by the heterogeneous nucleation by adding little amount of sample which we want to precipitate. Also term precipitate depends on pH, temperature and concentration. by varying the variable parameter one can easily precipitate out the respective phases.