Aragonite solubility

During the past decade there have been four major studies of calcite solubility in dilute solutions, three of which also included aragonite (Jacobsen and Langmuir, 1974 Berner, 1976 Plummer and Busenberg, 1982 Sass et al., 1983). These studies were aimed at more accurate and precise determinations of the thermodynamic solubility product for these minerals. Earlier work on this topic was well summarized by Jacobsen and Langmuir (1974). 

Jacobsen and Langmuir (1974) determined a value for pKSp (25°C) for calcite of 8.42 0.01, whereas Berner s (1976) value was 8.45 0.01. Berner also determined the pKSp for aragonite at 25°C to be 8.28 0.03. An aspect of particular interest of both studies was that to obtain internal consistency for the carbonic acid system or constant values for the solubility products over the range of conditions studied, it was necessary to neglect ion pair formation. The potentially important ion pairs that could have formed in the experimental solutions are CaHCC 3+ and CaC03°. The former is by far the most important species, and a vast body of previous literature supported its existence (see Plummer and Busenberg, 1982, for summary). 

The question then becomes one of why in both the studies of Jacobsen and Langmuir (1974) and Berner (1976), they found it necessary to neglect ion pairing. One of the reasons is that their studies were done in very dilute solutions over a rather narrow compositional range. If a wider range of solution compositions had been used, the need to include ion pairing would have become clear. Sass et al. 

Author Solid pKqp Open System Closed System  

In the previous chapter, the fact that stoichiometric and apparent constants have been widely used in seawater systems was discussed. Berner (1976) reviewed the problems of measuring calcite solubility in seawater, and it is these problems, in part, that have led to the use of apparent constants for calcite and aragonite. The most difficult problem is that while the solubility of pure calcite is sought in experimental seawater solutions, extensive magnesium coprecipitation can occur producing a magnesian calcite. The magnesian calcite should have a solubility different from that of pure calcite. Thus, it is not possible to measure pure calcite solubility directly in seawater. 

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Figure 2.7. The influence of including or not including ion pairing in the calculation of calcite and aragonite solubility (pKc or pKa) as a function of ionic strength. (After Sass et al., 1983.)...

Another problem is the determination of aragonite solubility in seawater. A study of the solubility of calcite and aragonite in seawater... 

The solubility of calcite and aragonite increases with increasing pressure and decreasing temperature in such a way that deep waters are undersaturated with respect to calcium carbonate, while surface waters are supersaturated. The level at which the effects of dissolution are first seen on carbonate shells in the sediments is termed the lysocline and coincides fairly well with the depth of the carbonate saturation horizon. The lysocline commonly lies between 3 and 4 km depth in today s oceans. Below the lysocline is the level where no carbonate remains in the sediment this level is termed the carbonate compensation depth. 

The solubility of gaylussite, however, varies strongly with temperature. Unlike calcite and aragonite, gaylussite grows less soluble (or more stable) as temperature... 

Let s consider first a formal (equilibrium) approach to the solubility of Mg-calcite and compare its solubility with that of CaC03 (caicite or aragonite)... 

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). 

Table 15.1 Solubility as (S = 35) Stoichiometric Equilibrium Constants (-logKip) for a Function of Temperature and Pressure Where K p Calcite and Aragonite Has Units of (mol/kg) ...

The degree of satiuation decreases with increasing depth for two reasons. First, the solubility of biogenic calcite and aragonite increases with depth due to increasing... 

All surface seawater is presently supersaturated with respect to biogenic calcite and aragonite with Cl ranging from 2.5 at high latitudes and 6.0 at low latitudes. The elevated supersaturations at low latitude reflect higher [COj ] due to (1) the effect of temperature on CO2 solubility and the for HCO3, and (2) density stratification. At low latitudes, enhanced stratification prevents the upwelling of C02-rich deep waters. 

North Atlantic to 500 m in the North Pacific. This reflects an increasing addition of CO2 to deep waters as meridional overturning circulation moves them from the Atlantic to the Indian and then to the Pacific Ocean. Thus, as a water mass ages, it becomes more corrosive to calcium carbonate. Since aragonite is more soluble than calcite, its saturation horizon lies at shallower depths, rising from 3000 m in the North Atlantic to 200 m in the North Pacific. 

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. 

Many observations have shown that calcium carbonate deposition in wells is only troublesome when the first level of boiling is in the well and that the deposition is strongest just above this level. The deposit is most often calcite. However, aragonite, which is slightly more soluble than calcite, has also been identified. It seems that aragonite forms at the expense of calcite, when C02 degassing causes strong oversaturation (Arnorsson 1989). 

Each compartment of the cell contains a mixture of solid PbC03 ( sp = 7.4 X 10 l4) and either calcite or aragonite, both of which have Ksp 5 X 10-9. Each solution was buffered to pH 7.00 with an inert buffer, and the cell was completely isolated from atmospheric C02. The measured cell voltage was — 1.8 mV. Find the ratio of solubility products, Kif> (for calcite)/ (for aragonite)... 

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