Chapter 1 The Geochemistry of Continental Carbonates

When solutions reach high Mg2+/Ca2+ ratios (>10), aragonite (rather than calcite or dolomite) begins to precipitate.

“When the solutions reach high Mg 2+ /Ca 2+ ratios (>10), aragonite (rather than calcite or dolomite) begins to precipitate.”

The precipitation of Ca and Mg carbonate minerals is enabled by supersaturation conditions, mainly explained by the ionic activities in solution when precipitation occurs and the kinetic effects inhibiting/favoring precipitation.

“The precipitation of Ca and Mg carbonate minerals is accepted to be enabled by supersaturation conditions, being mainly explained by the ionic activities in solution when the precipitation occurs, and the kinetic effects inhibiting/favoring the precipitation.”

Dolomite often precipitates in lacustrine waters despite the small concentrations of Mg2+, and modern dolomite formation is found in lakes with high Mg2+/Ca2+ ratio (>10) and elevated carbonate alkalinity concentration (>5 × 10^-3 g L-1).

“Dolomite often precipitates in lacustrine waters despite the small concentrations of Mg 2+ , and modern dolomite formation are found in lakes with high Mg 2+ /Ca 2+ ratio (>10) and elevated carbonate alkalinity concentration (>5 × 10 -3 g L -1 ).”

Calcite is the easier mineral carbonate to precipitate due to its lower solubility at 25 °C (K calcite = 10^-8.48, K aragonite = 10^-8.34), and dolomite is the most difficult, being nearly insoluble (K dolomite = 10^-17.2).

“Calcite is the easier mineral carbonate to precipitate due to its lower solubility at 25 °C (K calcite = 10 -8.48 , K aragonite = 10 -8.34 ), and dolomite the most difficult one, being nearly insoluble (K dolomite = 10 -17.2 ).18”

Several studies have been conducted attempting to explain the conditions of dolomite saturation at earth-surface conditions, but no synthetic dolomite has been obtained at such low temperatures and pressures.

“Several studies have been conducted attempting to explain the conditions of dolomite saturation at earth-surface conditions, but no synthetic dolomite has been obtained at such low temperatures and pressures.”

Experimental observations supported by the geological timescales indicate that dolomite is ideally precipitated in solutions with Mg2+/Ca2+ ratios ranging from 2 to 7.

“However, experimental observations supported by the geological timescales indicate that the dolomite is ideally precipitated in solutions with Mg 2+ /Ca 2+ ratios ranging from 2 to 7. 18”

The inhibitory effect of Mg2+ on calcite precipitation is attributed to Mg2+ having a smaller ionic radii (consequently, wider hydration sphere) than Ca2+, so dehydration of Mg2+ for fixation on nucleation sites is not kinetically favorable and dolomite does not precipitate.

“This is attributed to the inhibitory effect of Mg 2+ on the calcite precipitation: even if present at high concentrations, Mg 2+ has a smaller ionic radii (consequently, wider hydration sphere) than Ca 2+ ; thus, the dehydration of Mg 2+ cations for allowing the fixation on the nucleation sites is not kinetically favorable and dolomite does not precipitate.18”

Living organisms in tufas work as the alkaline engine for precipitation conditions and act as nucleation sites due to extracellular polymeric substances (EPS).

“It has been demonstrated that the living organisms in tufas work as the alkaline engine for the precipitation conditions, acting also as nuclea-tion sites due to the presence of extracellular polymeric substances (EPS).18”

Tufa precipitation is associated with the degassing of CO2, which elevates the pH and leads to supersaturation conditions.

“The tufa precipitation is associated with the degassing of CO 2 , which elevates the pH and leads to supersaturation conditions.”

The degree of supersaturation also plays an important role in the precipitation mechanism.

“Nevertheless, the degree of supersaturation also plays important role in the precipitation mechanism.11,18”

The catalytic effects of biotic activity on the precipitation of carbonates are clearly seen in tufas.

“The catalytic effects of biotic activity on the precipitation of carbonates are clearly seen in tufas, which are constituted of carbonates formed at low temperatures, and living organisms such as microbes, macrophytes and animals.”

The influence of biotic processes on the formation of Mg carbonates can be related to changes in CO2 levels due to respiration and/or photosynthesis of aquatic organisms.

“Thus, the influence of biotic processes on the formation of Mg carbonates can be related to changes in CO 2 levels due to the respiration and/or photosynthesis of aquatic organisms.18”

Temperature control is essential for dolomite precipitation, which is achieved easier at high temperatures.

“Some investigations have demonstrated that temperature control is essential for dolomite precipitation, which is achieved easier at high temperatures.”

The kinetics of calcite formation have a direct effect on the type of Mg mineral formed in sediments, and on the final molar ratios of [Mg2+(aq)]/[SO4 2-(aq)] and [SO4 2-(aq)]/[CO3 2-(aq)] in the residual fluid.

“Therefore, the kinetics of calcite formation have direct effect on the type of Mg mineral to be formed in sediments, and the final molar ratios of [Mg 2+ (aq)]/[SO 4 2-(aq)] and [SO 4 2-(aq)]/ [CO 3 2-(aq)] in the residual fluid.18”

In the presence of sulphate ions, the type of Mg mineral formed varies according to the kinetics of calcite precipitation: fast precipitation of calcite is followed by precipitation of Mg carbonates; slow precipitation of calcite extends the residence time of Mg cations in solution favoring the nucleation of Mg sulphates minerals.

“In the presence of sulphate ions, the type of the Mg mineral to be formed will vary according to the kinetics of the calcite precipitation: the fast precipitation of calcite is followed by the precipitation of Mg carbonates; however, the slow precipitation of calcite extends the residence time of the Mg cations in solution favoring the nucleation of Mg sulphates minerals (Fig. 8).”

In evaporatively concentrated lakes, the precipitation of calcite leads to depletion of Ca2+, and consequent raise of the Mg2+/Ca2+ ratio followed by the precipitation of Mg minerals.

“In evaporatively concentrated lakes, the precipitation of calcite leads to depletion of Ca 2+ , and consequent raise of the Mg 2+ /Ca 2+ ratio followed by the precipitation of Mg minerals.”