Aragonite — Calcite Transformation

Katz, A., Sass, E., Starinsky, A. and Holland, H.D. (1972) Strontium behavior in the aragonite-calcite transformation An experimental study at 40-98°C. Geochim. Cosmochim. Acta, 36, 41-496. 

Carlson W.D. (1983) Aragonite-calcite nucleation kinetics an application and extension of Avrami transformation theory. /. Geol. 91, 57-71. 

Bischoff, J. L. and Fyfe, W. S. Catalysis inhibition and the calcite-aragonite problem. I. The aragonite-calcite transformation. Amer. J. Sci. 266, 65 (1968)... 

These values indicate that calcite is more stable than aragonite and that with increase in temperature aragonite should alter into calcite. Under dry conditions, rates are slow below 100 °C and complete transformation will take millions of years. In the presence of water, however, the aragonite -> calcite transformation takes only a few hours in the temperatures regime between 50° and 100 °C93,94 ... 

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... 

Bischoff J.L. (1969) Temperature controls on aragonite-calcite transformation in aqueous solution. Amer. Mineralogist 54, 149-155. 

Carlson, W.D., 1983, The polymorphs of CaCOs and the aragonite - calcite transformation. Chap. 6 in Carbonates Mineralogy and Chemistry, R.J. Reeder, ed.. Reviews in Mineralogy, 11 191-225. 

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). 

Vaterite is a rare mineral that only forms from solutions highly supersaturated with respect to calcite and aragonite, Ogino et al. (1987) precipitated vaterite with calcite between 14 and 30°C from laboratory solutions in which the fCa ]lCOa ] product equaled about 10" . The vaterite was transformed to calcite in several hours. 

C endotherm, aragonite is transformed to calcite 930 °C endotherm, calcite decomposes to CaO and releases CO, 930 °C endotherm, strontianite undergoes a polymorphic transformation. 

Kinetically driven crystallization often involves an initial amorphous phase that may be non-stoichiometric, hydrated, and susceptible to rapid phase transformation. Amorphous calcium carbonate (ACC) for instance is highly soluble, has a low density of almost half of the crystalline mineral indicating a high hydration [62], and rapidly transforms to calcite, vaterite, or aragonite unless kinetically stabilized. In aqueous solution, this transformation into vaterite or calcite takes place within seconds or less even if additives are present, as shown by recent SAXS/WAXS measurements of ACC transformation in the presence of a DHBC [63]. 

Calculate ArG gg in the unit (J/mol) for transformation of calcite into aragonite at... 

Of course, freezing of a liquid - or its inverse - are themselves phase transformations, but the scientific study of freezing and melting was not developed until well into the 20th century (Section 9.1.1). Polymorphism also links with metastability thus aragonite, one polymorphic form of calcium carbonate, is under most circumstances metastable to the more familiar form, calcite. 

Simple Phase Transition Aragonite to Calcite Complex Phase Transformation Complex Phase Transformation Magma to Rock Volcanic Eruption ... 

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