6.2: Carbonate Precipitation

Chemistry of Carbonate Precipitation

The chemistry of carbonate precipitation can be summarized by the following, somewhat counterintuitive, statements:

• Anything that adds CO₂ to water increases the acidity of the water which discourages carbonate precipitation and increases its solubility.
• Anything that removes CO₂ from the water increases carbonate precipitation and decreases its solubility

We can back those statements up with a bit of chemistry. In the four following equations, the arrow points in the direction that the reaction proceeds and the k values are equilibrium constants (smaller values are slower reactions; note the negative exponent):

1. CO₂ + H₂O → H₂CO₃ (fast, k=10^-1.43)

(Carbon dioxide plus water creates carbonic acid)

 

2. H₂CO₃ → H⁺ + HCO₃^- (med., k=10^-6.40)

(Carbonic acid dissociates into hydrogen and bicarbonate)

 

3. HCO₃^- → H⁺ + CO₃^2- (slow, k=10^-10.33)

(Bicarbonate dissociates into hydrogen and carbonate)

 

4. Ca²⁺ + CO₃^2- → CaCO₃ (med., k=10^-8.33)

(Dissolved calcium combines with carbonate to form calcium carbonate)

If you just looked at these four equations without paying attention to the k value, you’d think that CO₂ would be a good thing for carbonate precipitation. But the k value in that third equation is slow, so much slower that the H⁺ produced in Reaction 2 overwhelms the system (it makes the water acidic) and causes Reaction 3 to reverse. Its because of this fact that having lots of CO₂ in the system is bad for carbonate precipitation.

Controls on Carbonate Deposition

There are seven major factors that have an important influence on whether significant amounts of carbonate are deposited. Several of them are derived from the chemistry described above and might be intuitive to those of you that drink carbonated beverages.

Water temperature: All other factors being equal, warm water holds less CO₂ than cold water. If you’ve got a soda stream, you might know that you can make cold water much fizzier than warm water. Thus, warm water encourages carbonate precipitation.

Agitation: Increasing agitation allows excess CO₂ to go from the water to the atmosphere. Just think about what happens if you shake a can of soda. Thus, increased agitation encourages carbonate precipitation.

Water depth: You can keep more CO₂ in solution at higher pressures (deeper water) and much less in solution at lower pressures. Think about what happens when you open a bottle of soda. Thus, shallow water is friendlier to carbonate deposition.

This variable becomes very apparent in the deep ocean. Shallow waters are frequently supersaturated in calcite and many microorganisms build calcareous skeletons. When the organisms dies, their skeletons settle and move into the cold, calm, waters of the deep ocean. If the water is deep enough, they will eventually reach the Carbonate Compensation Depth (CCD) which is the point beyond which carbonate cannot accumulate because dissolution overcomes supply.

Life: The activity of organisms can encourage the accumulation of carbonate by building calcareous skeletons, photosynthesis (removal of CO₂), bacterial activity, and the decay of organic material. Respiration adds CO₂ to the water and this has a negative impact on carbonate precipitation.

Salinity: Increasing salinity increases the number of dissolved cations in the water which has a negative impact on the precipitation of carbonates because the introduction of foreign cations tends to make calcium carbonate less stable.

Light: Many calcite-producing organisms are photosynthetic. That, combined with some of the other variables described above means that the majority of carbonate is produced at shallow depths.

Clastic sediment input: Clastic sediment input can overwhelm carbonate production and thus dilute the amount of carbonate. Additionally, clastic sediment can clog the gills of many shell-building organisms and decrease the amount of light available for photosynthesis.

Dolomite Formation

Primary Dolomite

The chemical formula for the creation of true stoichiometric dolomite is:

Ca²⁺(aq) + Mg²⁺(aq)+ 2CO₃^2-(aq) → CaMg(CO₃)₂

But, chemical conditions are suitable for the precipitation of primary dolomite only at temperatures above 60°C or in cases where the ratio of Mg to Ca is much higher than it is in the open ocean which makes primary dolomite formation unlikely to be responsible for much of the dolomite present in the geologic record.

Dolomitization

Dolomitization is the process by which CaCO₃ is transformed to CaMg(CO₃)₂. This reaction can occur at normal atmospheric temperature and pressures and the reaction is:

Mg²⁺(aq) + 2CaCO₃(solid) → CaMg(CO₃)₂ +Ca²⁺ (aq)

In order to make this reaction happen, existing carbonates must be flushed with Mg-rich brine. The “hypersaline model” of dolomitization postulates that these brines can form in areas where evaporation of seawater causes the precipitation of gypsum (CaSO₄) and aragonite (CaCO₃) and that the remaining brine has an Mg:Ca ratio >8:1, which is much greater than that of seawater (~1:3). Although this model has its problems, its be best explanation we have for this poorly understood process.

Figure \(\PageIndex{1}\): Plot of magnesium to calcium ratio versus salinity with overlays showing the chemistry of natural waters (colors), calcite and dolomite stability (dark lines and labels). After Figure 1 in Folk and Land (1975) available from Michael C. Rygel via Wikimedia Commons; CC BY-SA 4.0.

Additional Readings and Resources

• Folk, R. L., & Land, L. S., 1975, Mg/Ca ratio and salinity: two controls over crystallization of dolomite; AAPG Bulletin, v. 59 (1), p. 60-68.
• García-Ruiz, J. M., 2023, A fluctuating solution to the dolomite problem. Science, 382 (6673), 883-884.
• Kim, J., Kimura, Y., Puchala, B., Yamazaki, T., Becker, U., & Sun, W., 2023, Dissolution enables dolomite crystal growth near ambient conditions. Science, 382 (6673), 915-920.