Calcite activation energy

Apparent activation energies of acid dissolution of calcite and dolomite and severa. samples of agricultural limestone indicate that the activation energy of dolomite varies... 

Judd and Pope [36] conclude that because the activation energies for decompositions of CaCOj, SrCO, and BaCOj are all close to the corresponding enthalpies of dissociation (apparent values of , are 180,222 and 283 kJ mol and A//, 178, 235 and 269 kJ mol, respectively) the mechanisms of decomposition in all three substances are the same as that proposed by Hills [18] for calcite. Strontium carbonate [37] generally resembles the calcium salt in that an increase in sample size results in a decrease in reaction rate. Differences in behaviour were ascribed [37] to the occurrence of a crystallographic transformation and to fusion. 

The kinetics of reversible decompositions are often highly sensitive to reaction conditions [43]. For example, the values of and E, for the decomposition of CaCOj show unusually wide variations, owing to the sensitivity of reaction rate to the availability of COj [44,45]. The spread of apparent E values is considerable [46] and some values are close to the dissociation enthalpy [1]. However, Beruto and Searcy [47] concluded that, under high vacuum conditions, the constant rate of interface advance in large crystals was probably controlled by the dissociation step in the absence of a perceptible contribution from the reverse process. The decomposition activation energy (205 kJ mol ) was appreciably larger than the dissociation enthalpy (178 kJ mol ). This is probably the most precise kinetic measurement for the calcite decomposition [48]. 

In contrast, the second effect could play an important role because the core energy is highly localized. For example, if we could calculate a molar free energy lor a core by assuming that this core energy disrupts one calcite formula unit of atoms, it follows that the core energy for a mole of such formula units would be - 40 kJ, which is comparable to the activation energy for dissolution. 

The reported activation energy of calcite dissolution varies from 1.5 to 1A kcal/gmol (8,9,10,12,13). The trend of data gives low activation energy at pH 2 - A and high activation energy at pH 8 - 10. 

Table 2.21 Calcite dissolution rate constants and activation energy.

Clay, sand, and gravel deposits were mapped to determine likely sources of cave fill and relative flow system energy (Table 1). The vertical relationships between sediments indicate the sequence of sedimentation in most of the caves of Lime Creek. Older deposits are buried by younger ones, or can be preserved in pockets and cemented by calcite above current base level. Older deposits may have been completely removed by later stream activity, so a complete sediment record may not be preserved. 

Flowstone and calcite cement were used as indicators of relatively low energy. In active stream passages, older calcite-cemented sediments are actively being incised. Presumably, the calcite could have been deposited in the stream paths only during full glacial conditions when streamflow would have been minimal (Ford, 1979). In Alphine Twister, calcite speleothems are currently being deposited in parts of the cave away from active streams. 

Lattice defects may influence the rates of mineral dissolution in two ways (1) by changing the bulk thermodynamic properties and (2) by creating sites of accelerated dissolution on the solid surface. Strain and core energies associated with dislocations contribute insignificantly to the total energy of minerals. For instance, even with the extremely high dislocations density of 1011 cm"2, the free energy of calcite is increased by only 80 J mol", which corresponds to a 25°C activity of 1.04. The same features are observed for quartz and silicate minerals. 

At 25 °C, a saturated, ideal aqueous solution of CaCOs contains a deposit of solid calcite CaC03(s) specify the activity a and free energy G for CaC03(s) ... 

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