The Transformation of Aragonite to Calcite

1) The rate of reaction is dependent on the nucleation and growth rates of calcite, not the dissolution rate of aragonite. Curiously, it has also been observed that absolute rates are strongly dependent on the aragonitic material used. This observation appears to contradict the generally held conclusion that rates are strictly dependent on calcite nucleation and precipitation rates, not the dissolution rate of aragonite. 

3) The transformation of aragonite to calcite can be catalyzed in solutions containing electrolytes which are not inhibitors of calcite precipitation (e.g., NaCl). This observation has been interpreted to be due to increased concentrations of the calcium bicarbonate ion pair (e.g., Bischoff and Fyfe, 1968) in these solutions. Why this species should catalyze the reaction remains unexplained. 

General considerations The dolomite problem has been one of the most intensely studied and debated topics in geology. The problem is that modem marine sediments contain only relatively rare and minor occurrences of this mineral, whereas it is a major component of sedimentary rocks. Questions about whether major dolomite formation from seawater could have occurred in the geologic past, the conditions necessary for dolomite formation, and many other aspects of the dolomite problem have resulted in a voluminous literature that includes entire books devoted to the topic (e.g., Zenger et al., 1980 Zenger and Mazzullo, 1982). The literature on dolomite formation is typified by the vigor with which contending hypotheses are supported and attacked. Many of the controversies have stemmed from attempts to find the answer to how sedimentary dolomite forms. 

Land (e.g., 198S) has stressed the chemical and structural variations associated with natural dolomites, and has gone so far as to suggest that the name dolomite be used in the same way that the mineral name feldspar is used. The fact that dolomite is relatively unreactive compared to most other sedimentary carbonate minerals has severely limited experimental studies under temperature and pressure conditions that exist during shallow burial. Consequently, most information on the chemical behavior of dolomite must be obtained from observations of complex natural systems. Such observations are all too often open to multiple interpretations. 

The relationship between calcite (or aragonite) and dolomite solubilities has been extensively discussed in terms of reactions for the dolomitization of limestones. Although this relationship is relatively simple, it has been a source of controversy (e.g., Machel and Mountjoy, 1986 Stoessel, 1987). If both calcite and dolomite are in equilibrium with the same solution, they must be in equilibrium with each other. Under these conditions the following relations hold (e.g., Carpenter, 1980, see Chapter 6). 

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Modulated microstructure occurs in calcite, as well as in dolomite, and was first identified in Jurassic oolitic limestone by Gunderson and Wenk (1981). However, chemical analysis of the oolitic limestone showed no compositional differences between areas with the modulation and those without. Since, the modulations could not be the result of compositional fluctuations of Ca-Mg as in dolomite, Gunderson and Wenk (1981) hypothesised that the rotational disorder of CO3 groups that occurred during phase transformation of aragonite to low-magnesium calcite might cause the modulations. 

Under the modest temperature and pressure conditions characteristic of the environments in which meteoric diagenesis typically takes place, many of the most important reactions are slow. This has severely constrained the study of the chemical mechanisms and kinetics involved in such fundamental processes as the aragonite to calcite transformation and dolomite formation. Information on these processes obtained under conditions not typical of the meteoric realm (e.g., elevated temperatures) are of questionable applicability to "real world" carbonate diagenesis. 

A more sophisticated chemical understanding needs to be obtained of the problem of why elevated ionic strength seems to influence so many important reactions such as the aragonite to calcite transformation and dolomite formation. 

The polymorphism of calcium carbonate is still not completely understood despite its very long history and the appearance of many published studies. In addition to calcite, aragonite and vaterite, non-crystalline forms of CaCOs of biolo cal origin exist. An account of the relationships between these solid phases has appeared recently [39] together with a summary of thermodynamic and kinetic data for the transformations of the metastable polymorphs aragonite and vaterite to the stable calcite. These authors describe the preparation of non-crystalline calcium carbonate and report preliminary values of the transition temperature and enthalpy change for its crystallization to calcite. The DSC method, siq)ported by TG and PXRD, was used in this study. 

Nautilus pompilius deposits a shell composed entirely of aragonite. Experiments95-97 conducted at temperatures between 150° and 900 °C showed the shell aragonite to be stable for a few hours at temperatures of up to 300 °C even under aqueous conditions. Above 300 °C a rapid transformation into calcite was observed... 

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