The aqueous aragonite to calcite transformation: rate, mechanisms, and its role in the development of neomorphic fabrics

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Virginia Polytechnic Institute and State University


The rate of the aqueous transformation of aragonite to calcite was measured at 50°, 77°, and 101°C. The observed mole fraction calcite versus time relationship can be fit by the integrated rate model:

    t = [(3/C₂)(1-X)2/3 + (3/C₁)(X2/3)/[K₂-K₁]

The constants C₁ and C₂ combine geometric factors, especially relative surface areas of the solids, K₁ and K₂ are the thermodynamic equilibrium constants for aragonite and calcite respectively. Apparent activation energies (EA’) and absolute rates were calculated from Arrhenius plots of data from this study and others:

                                               E<sub>A</sub>’        Conditions        Material          Time-50% CAL, 25°C

Metzger and Barnard, 1968 58 kJ mol⁻¹ wet cm cubes 2.25X10² yr Taft, 1967 67 wet syn. powder 2.0X10⁻¹ This study 55 wet syn. powder 5.7X10⁻² Brown et al.,1962 373 dry 4.7X10³³

The EA’ for this study is comparable with that of Metzger and Barnard indicating a similar mechanism, but absolute rates differ dramatically because of the different geometries of the run material. The dry transformation rates are so slow at diagenetic temperatures that this mechanism is of no importance geologically.

Because the rate of the transformation is dependent on the geometry of the reacting system it is not surprising that most studies of neomorphic calcites find that the calcite textures are related to the original aragonite textures. Three transformation regimes, macroscale (passive dissolution), mesoscale (chalk zone), and microscale (thin film) dissolution-precipitation, are proposed to explain the variability in observed diagenetic calcite textures. These are differentiated by the surface area/solution ratio in the reaction zone. In general the smaller the geometric factor in the rate equation. i.e. the smaller the surface area/solution ratio, the slower the transformation rate and the higher the degree of precursor fabric retention in the neomorphic calcite.