Aragonite equilibrium

Carlson W.D. (1980) The calcite-aragonite equilibrium effects of Sr substitution and anion orientational disorder. Amer. Mineral. 65, 1252-1262. 

Figure 17. Log of the dissolution rate vs. total carbonate ion concentration for synthetic aragonite, pteropods, calcitic Pacific Ocean sediment, and foraminifera in the 125-500 iim size fraction. (A) indicates ihe aragonite equilibrium total carbonate ion concentration at 25°C, 1 atm (26). (C) indicates the calcite equilibrium total carbonate ion concentration at 25°C, 1 atm (25).

MacDonald, G. J. F. Experimental determination of calcite-aragonite equilibrium relations at elevated temperatures and pressures. Amer. Mineralogist 41, 744-756 (1956). 

The ArCp = 0 approximation is an obvious choice of options if there are no data whatsoever available for ArC°, and can give acceptable results in favorable circumstances. The next possibility is that ArC° is known at 25°C but not at higher temperatures. In this case it is generally better to assume that A C° is constant as temperature increases. We have already seen the effect of this assumption on the variation of G with T in the calculation of the calcite-aragonite equilibrium in Chapter 8. Because we have an expression for as a function of T that includes the... 

So we can solve the equation for Peqbm> pressure of the calcite-aragonite equilibrium at 25 "C. 

An increase in carbonate-ion concentration moves the equilibrium in favour of calcium carbonate deposition. Thus one secondary effect of cathodic protection in seawater is the production of OH , which favours the production of CO, , which in turn promotes the deposition of CaCOj. Cathodically protected surfaces in seawater will often develop an aragonite (CaCOj) film. This film is commonly referred to as a calcareous deposit. 

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)... 

Although equilibrium was not established, it was more closely approached in the KCl-KBr-H20 system than in carbonate systems. For example, in a similar analysis of the strontianite-aragonite solid solution system (4 ), it was found that the experimental distribution coefficient for Sr substitution from seawater into aragonite is 12 times larger than the expected equilibrium value. Most of the distribution coefficients for the KCl-KBr-H20 system are within a factor of two of the equilibrium value, but clearly not at equilibrium. Considerable caution should be exercised before reaching the conclusion that equilibrium is established at relatively low temperatures in other solid solution-aqueous solution systems. 

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) ...

Most water masses are not at equilibrium with respect to either calcite or aragonite. The degree to which a water mass deviates from equilibrium for a particular mineral type can be expressed as its degree of saturation (fi), which is defined as ... 

Kim ST, O Neil JR (1997) Equilibrium and nonequUibrium oxygen isotope effects in synthetic carbonates, Geochim Cosmochim Acta 61 3461-3475 Kim S-T, Mucci A, Taylor BE (2007) Phosphoric acid fractionation factors for calcite and aragonite between 25 and 75°C, Chem Geol 246 135-146... 

A key question is then the relationship between the isotopic composition of the C02(g) in the soil and the CO2 dissolved in the mineral species growing in the soils. The equilibrium carbon isotope fractionations between C02(g) and carbonate minerals (CaC03 (both calcite and aragonite polymorphs), MgC03 (magnesite)) are known experimentally and can be accurately calculated from quantum chemistry (75). Overall, carbon in carbonate minerals is isotopically heavier than carbon in C02(g) by about... 

Roughly sketch a plot of G vs. T for the two forms of CaC(>3. Sketch G vs. T for the two forms under very high pressure. At what temperature at 1 bar are aragonite and calcite in equilibrium ... 

Increase in water depth will shift the equilibrium and between 3.000 and 4.000 m the ocean starts to become unsaturated with respect to aragonite and calcite due to pressure effects122-124. In contrast, the majority of river and lake waters are unsaturated at all depths, because precipitation of CaC03 will readily take place the moment saturation is exceeded. The solubility products are92 ... 

Unfortunately, equations (3.19) and (3.20), although the best currently available for the data, can only be considered tentative. The weakness lies in the temperature dependence. As mentioned earlier, it is not known whether the results reflect a temperature dependence or a change in the proportion of aragonite to calcite. Another shortcoming in this relationship is that the temperature range of the experiments, 20° to 63°C (if Baertschi s data are included), is well above the temperature at which many organisms secrete carbonate. Because benthic foraminifera have proved very useful in carbon isotopic studies, it is important to have carbon isotopic equilibrium defined over their temperature range. 

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