Aragonite stabilization

Stability. AH calcitic and dolomitic limestones are extremely stable compounds, decomposing only in fairly concentrated strong acids or at calcining temperatures of 898°C for high calcium and about 725°C for dolomitic stones at 101.3 kPa (1 atm). A very mild destabilizing effect is caused by C02-saturated water, as described in the preceding section on solubihty. Aragonite, however, is not as stable as calcite. In sustained contact with moisture,... 

Aragonite is decompressed to calcite stability field Magma is cooled to below the liquidus Gas-bearing magma is decompressed to oversaturation... 

There is an extensive literature that exists concerning carbonate diagenesis in the vadose and phreatic meteoric environmental settings (see e.g., Bathurst, 1975, 1980 James and Choquette, 1984 and Moore, 1989 for reviews). Much of this work has dealt with the stabilization of aragonite and magnesian calcite to calcite in the present meteoric diagenetic realm associated with Pleistocene limestones. As... 

In contrast, Lafon and Vacher (1975) estimated the half-life of aragonite in the vadose zone of Bermuda as about 230,000 years, and the half-life of high-magnesian calcite as only 60,000 years. Minor corrections need to be made to these estimates, because of new estimates of limestone ages for Bermuda, but the fact still remains that stabilization in the vadose freshwater zone is much slower than in the phreatic zone (Land, 1970 Steinen and Matthews, 1973 Steinen, 1974 Vacher et al 1989). 

Although cementation is a process that can occur throughout the life of a sedimentary carbonate body, the dominant processes and types of cements produced generally differ substantially between those formed in the shallow-meteoric and deep-burial environments. Mineralogic stabilization (i.e., dissolution of magnesian calcites and aragonite, see Chapter 7) commonly drives cement formation during the early shallow-burial period, whereas the previously discussed processes of pressure solution and neomorphism are more important in the deep-burial environment. The pore waters in which cementation takes place also tend to differ substantially between the two environments. In shallow subsurface environments, cementation usually takes place in dilute meteoric waters that are oxic to only... 

Vacher H.L., Bengtsson T.O. and Plummer L.N. (1989) Hydrology of meteoric diagenesis - Ground-water residence times and aragonite-calcite stabilization rates in island fresh-water lenses. Geol. Soc. Amer. Bull, (in press). 

The small stability difference in favor of calcite with respect to aragonite ... 

The reader will observe that equilibrium constants for the same reactions may differ depending on the source of the data. (Often, different stability constants for ion pairs are the reason for the difference.) For the solubility of calcite and aragonite in seawater, see Table A6.4. 

An example of an intensively studied set of polymorphs whose decompositions are of great theoretical and practical importance (see Chapter 12) is CaCOj which may exist (in order of decreasing thermodynamic stability) as calcite, aragonite or vaterite [18]. Vaterite can be prepared by precipitation from aqueous solutions under carefully controlled conditions. A DTA curve for the vaterite calcite transition is shown in Figure 2.3. The transition is exothermic AH = -34.3 J g ) with onset at 704 K. Isothermal extent of conversion against time curves were described [18] by the Johnson, Mehl, Avrami, Erofeev model (see Chapter 3) with n = 2. The measured Arrhenius parameters were F, = 210 kJ mol and A = 1.15x10 min. The decomposition of vaterite and its concurrent transformation to calcite under various conditions were compared [18] with the decomposition of calcite xmder the same conditions (see Chapter 12). 

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