Carbonate minerals aragonite solubility

Calcium carbonate solubility is also temperature and pressure dependent. Pressure is a 6r more important fector than temperature in influencing solubility. As illustrated in Table 15.1, a 20°C drop in temperature boosts solubility 4%, whereas the pressure increase associated with a 4-km increase in water depth increases solubility 200-fold. The large pressure effect arises from the susceptibility of the fully hydrated divalent Ca and CO ions to electrostriction. Calcite and aragonite are examples of minerals whose solubility increases with decreasing temperature. This unusual behavior is referred to as retrograde solubility. Because of the pressure and temperature effects, calcium carbonate is fer more soluble in the deep sea than in the surfece waters (See the online appendix on the companion website). 

Marine carbonate minerals have both biotic (dominant) and abiotic (minor) sources. Their formation is often controlled by kinetic factors or biomediated processes in organisms. Surface seawater is most highly supersaturated (the ion activity product (lAP) is much greater than the solubility product) with respect to dolomite ( 50X), followed by pure calcite ( 6 X), then by aragonite ( 4x). It may be close to... 

Aragonite A mineral form of calcium carbonate. It is more soluble than calcite. Some marine organisms, such as pteropods, deposit shells composed of aragonite. This mineral is also a common component of evaporites. 

Calcium carbonate occurs in two forms— hexagonal crystal known as cal-cite, and orthorhombic form, aragonite. Calcite decomposes on heating at 825°C, aragonite melts at 1,339°C (at 102.5 atm). Density 2.71 g/cm (calcite), 2.83 g/cm3 (aragonite) insoluble in water (15mg/L at 25°C) Ksp 4.8x10 soluble in dilute mineral acids. 

Unlike the aluminosilicates and most other minerals, the carbonates have an exothermic heat of dissolution, which means that their solubilities decrease with increasing temperature. For example, for calcite decreases from 10 at 0°C to 10 at 30°C. The effect of temperature on the solubility products of aragonite, calcite, and ordered dolomite is plotted in Fig. 6.8. The figure shows that between 0 and 90°C solubilities of the carbonates decrease by about 6-fold for aragonite and calcite and 14-fold for dolomite. This decreasing solubility with temperature is magnified by the fact that the solubility of CO2 gas also declines with temperature. Kqq, decreases from 10 " to 10 between 0 and 30°C. 

Kinetically driven crystallization often involves an initial amorphous phase that may be non-stoichiometric, hydrated, and susceptible to rapid phase transformation. Amorphous calcium carbonate (ACC) for instance is highly soluble, has a low density of almost half of the crystalline mineral indicating a high hydration [62], and rapidly transforms to calcite, vaterite, or aragonite unless kinetically stabilized. In aqueous solution, this transformation into vaterite or calcite takes place within seconds or less even if additives are present, as shown by recent SAXS/WAXS measurements of ACC transformation in the presence of a DHBC [63]. 

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