Aragonite apparent solubility

We have chosen to calculate the effect of pressure on the apparent solubility of aragonite using the Avalues of Hawley and Pytkowicz (36), and equations 28 and 29... 

At this time it is not possible to make an accurate estimate of the absolute uncertainty in the situ values of the apparent solubility constants for calcite and aragonite. An approximate maximum uncertainty can be determined for at 5000 m water depth and 2°C by using two different data sets. The maximum value for is obtained by using value and temperature coefficient calculated by Berner (26) and the effect of pressure of Ingle (32). The value for obtained by this method is 19.8 X 10" mole kg . The minimum value is obtained by using the v ue and temperature coefficient of Ingle. (25),... 

The apparent solubility products of calcite and aragonite have been determined repeatedly... 

The apparent solubility products of calcite and aragonite have been determined repeatedly in seawater solutions. We adopt the values of Mucci (1983) fC sp for calcite and aragonite = 4.35 0.20 x 10 and 6.65 0.12 X lO moPkg, respectively, at 25 °C, 35ppt and one atmosphere. These data agree within error to measurements determined previously and represent many repetitions to give a clear estimate of the reproducibility (c. 5%). 

Two lines of evidence suggest that the apparent solubilities of the two phases are in the thermodynamic ratio. First if the lysocline for aragonite depth is predicted from the curve in Fig. 

The difference in depth of the calcite and aragonite lyso-clines in the Atlantic is consistent with the l.ifT solubility ratio. (2) The apparent solubilities of calcite and aragonite as measured iji situ saturometry are consistent with the thermodynamic ratio of 1. 7 0.02. (3) The response times for calcite... 

Neither group included phosphate. It is possible that the same calcite (or aragonite) exposed to different amounts of sea water, or the same amount of sea waters with different trace contaminant levels could yield different apparent solubilities due to different states induced into the outermost several molecular layers of the crystal surface. It must be noted that the crystal/sea water volume ratio employed in Berner s experiments is considerably smaller than that employed by Ingle et al. (1973) and for the saturometer experiments. This points out the need for experiments involving greatly different ratios of sea water volume to calcite siurface area and experiments comparing real and artificial sea... 

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. 

Before continuing, it is important to note that the solubility behavior of aragonite in seawater has proven rather strange, in that for equilibration periods of less than about a month, different solubility values can be obtained in an apparently reversible manner. This result points to some sort of short-term, solubility controlling surface phase and may have implications for processes such as ooid formation which is discussed in Chapter 5. 

Calcium has been recognized as a constituent of gallstones since the earliest chemical studies (see reference 118). It occurs as calcium carbonate (either calcite or aragonite), calcium phosphate (apatite), calcium bilirubinate, or the salts of bile acids and fatty acids. In normal hepatic bile, the calcium concentration is about 21 mg %. It is about 45 mg % in normal gallbladder bile and about twice that in the gallbladder bile of patients with cholesterol stones (119). The concentration of calcium is said to vary with the concentration of bile salts (3). Apparently, calcium in bile is bound in part to bile salt micelles, and under normal conditions this complex remains stable and soluble (120). However, under appropriate conditions of pH, calcium concentration, and anion concentration, calcium salts may precipitate. 

---