Aragonite vs. Calcite

Written By Dr. Mother Earth

Welcome, beautiful souls; today, we will discuss aragonite and calcite and why they are important yet different, especially regarding the environment and climate.

Calcite and aragonite are two calcium carbonate polymorphs. When exposed to seawater, calcium carbonate forms aragonite instead of its preferred state, calcite (Chandler, 2015). Aragonite can convert into calcite with time or when heated (Chandler, 2015). Calcite is the most stable and abundant of the carbonates (Tsai et al., 2011). Regarding solubility, aragonite is more soluble than calcite (White, 2019). The solubility of calcite depends on magnesium concentration. The higher the concentration, the higher the solubility (Plath et al., 1980). Isomorphic substitution is common for calcite and aragonite. The elements that tend to substitute for Ca in calcite are magnesium, iron, manganese, cobalt, zinc, cadmium, and nickel (Skinner et al., 2005). The elements substituting Ca in aragonite are strontium and lead (Skinner et al., 2005). Calcite crystallizes in the hexagonal/rhombohedral space (due to smaller cations) (Figure 1). While aragonite is crystallized in the orthorhombic space group (larger cations) (Figure 2).

Figure 1: Calcite crystal structure (Skinner et al., 2005)

Figure 2: Aragonite crystal structure (Skinner et al., 2005)

There is a difference between aragonite and calcite in its solubility. Therefore, the aragonite saturation state can be used to indicate ocean acidification. Carbonate ion concentration is measured by aragonite's saturation state (Friedrich, 2022).

Calcite is soluble in dilute acids but is barely soluble in water. As a result, calcite becomes significantly more soluble in water containing CO2 since carbonic acid is formed, which reacts to form calcium bicarbonate, Ca (HCO3)2 soluble in water (Butler, 1999).

A reaction shifts from left to right when CO2 is introduced into the system, resulting in calcite dissolution. Because of temperature, pressure, and acidity, calcite precipitates in the system when CO2 decreases (Butler, 1999).

Temperature: Since CO2 is less soluble in warm solutions than cold solutions, increasing temperatures significantly produce CO2. Calcite precipitates when equation [1] shifts to the left. CO2 dissolves more readily at lower temperatures, resulting in equation [1] shifting to the right and calcite soluble (Butler, 1999).

Pressure: Gases become more soluble as the pressure increases. Consequently, equation [2] will increase the production of carbonic acid, and equation [1] will shift to the right, leading to calcite dissolution. The opposite is true. Gas solubility decreases with decreasing pressure, which forces equation [2] to reduce carbonic acid production. Calcite precipitates because equation [1] shifts to the left (Butler, 1999). 

Acidity: Hydrogen atoms increase as pH decreases. In response to increased acidity, equation [1] shifts to the right, causing calcite to dissolve. The reverse is true as well. By reducing the production of carbonic acid, equation [2] shifts to the right, and calcite precipitates as a result (Butler, 1999).

To summarize:

High Temperature= precipitation, Low temperature= dissolution

High pH = precipitation, Low pH = dissolution

High pressure= dissolution, Low pressure = precipitation

References

Dr. Mother Earth
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