Aragonite-calcite transition

Konigsberger, E., j. Buoajski, and H. Gamsjager. 1989. Solid-solute phase equilibria in aqueous solution. II. A potentiometric study of the aragonite-calcite transition. Geochim. Cosmochim. Acta 53 2807-10. 

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). 

The conversion of one phase of a substance to another phase is called a phase transition. Thus, vaporization (liquid —> gas) is a phase transition, as is a transition between solid phases (such as aragonite —> calcite in geological processes). With a few exceptions, phase transitions are accompanied by a change of enthalpy, for the rearrangement of atoms or molecules usually requires or releases energy. 

From the more recent reports cited below, further references to the extensive literature concerned with calcite decomposition may be traced. Other modifications of CaC03 (aragonite and vaterite) undergo solid phase transitions to calcite at temperatures of 728 K and 623—673 K respectively [733], below those of onset of decomposition (>900 K). There is strong evidence [742] that the reaction... 

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

CaC03 (c, aragonite). For the heat of transition of aragonite to calcite the following values were reported Favre and Silbermann,2 2.4 LeChatelier,7 —0.3 Laschtschenko,5 —2.7 Backstrom,4 —0.33 Backstrom,5 —0.04. 

Figure 2.1. Generalized phase relations in the unary system CaCC>3. "A" -aragonite "I" through "V - calcite polymorphs with metastable fields indicated by (). Dash-dot line at 800°C represents transition encountered on cooling runs solid line at lower T represents transition encountered on heating runs. (After Carlson, 1980.)...

Manganese Numerous investigations have been made of the coprecipitation of transition and heavy metals with calcite and aragonite. The two elements that have received the most attention are Mn2+ and Zn2+. This attention probably stems from their potential importance for studying diagenetic processes (e.g., Pingitore, 1978). 

Unfortunately, there is generally a large scatter in the values obtained for these partition coefficients. A possible reason for this scatter, as shown by the study of Lorens (1981), is probably the major effect of precipitation rate on the values of the partition coefficients. It is interesting to note that the partition coefficients for the transition and heavy metals in calcite, studied by Lorens (Cd2+, Mn2+, Co2+), have a negative linear log partition coefficient- log precipitation rate relation, whereas Sr2+ has a positive relation. This behavior may be explained by the fact that the transition metal carbonates are isostructural with calcite, whereas strontium carbonate is isostructural with aragonite. Also, as precipitation rates increase, partition coefficients tend towards unity. 

The phenomenon of polymorphism was noted by Martin Heinrich Klaproth °i in 1798, when he proposed that the minerals calcite and aragonite must have the same chemical composition, CaCOa. Calcite forms a rhombohedral uniaxial crystal and is the stable form under normal conditions, with a density of 2.71 g/ml. Its metastable polymorph, aragonite, is an orthorhombic biaxial crystal with a density of 2.94 g/ml. This work was continued by Louis Jacques Thernard, Jean Baptiste Biot, and Eilhard Mitscherlich. Mitscherlich, for example, reported on it in his studies of phosphates and arsenates. The transition from calcite to aragonite has been studied at different pressures. ... 

Figure 1. The calcite-aragonite phase boundary in P/T space is curved, due to the temperature-dependence of the entropy of calcite. The entropic stabilisation of calcite at high-T arises from the order-disorder phase transition from orientationally ordered R3c calcite at low temperatures to the orientationally disordered R3c stmcture, stable above 1260 K. Figure after Redfern etal. (1989a).

Redfern, S.A.T., Salje, E., Navrotsky, A. (1989a) High-temperature enthalpy at the orientational order-disorder transition in calcite implications for the calcite/aragonite phase equilibrium. Contrib Mineral Petrol 101 479-484... 

An additional possible reason for differences in behaviour is that CaCOj exists in three crystalline forms. Calcite is the most stable and aragonite and vaterite undergo transitions to calcite at 728 K and between 623 and 673 K, respectively. Consequently, at decomposition temperatures (above 900 K) the reactant CaCOj would normally be in the calcite structure [2]. Salvador et al. [3] concluded that the... 

Davies et al. (174) found that the polymorph transition kinetics of the changes of vaterite to calcite and aragonite to calcite could be followed using DTA and the equation... 

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. 

One final aspect of the above carbonate model that is worth noting is that the transition pressure for conversion of calcite through to aragonite is accurately predicted to be 2.4 kbar, as compared to experimental estimates of 2.5 kbar (Crawford and Hoersch 1972). This transition pressure is very sensitive to the relative energies of the two polymorphs and requires a good description of the polarization contribution for the two materials. 

Corrales LR (1999) Dissociative model of water clusters. J Chemical Physics 110 9071-9080 Curtiss LA, Halley JW, Hautman J, Rahman A (1987) Nonadditivity of ab-initio pair potentials for molecular dynamics of multivalent transition metal ions in water. J Chem Phys 86 2319-2327 de Leeuw NH, Parker SC, Catlow CRA, Price GD (2000) Proton-containing defects at forsterite (010) tilt grain boundaries and stepped surfaces. Am Min 85 1143-1154 de Leeuw NH, Parker SC (1998) Surface stracture and morphology of calcium carbonate polymorphs calcite, aragonite, andvaterite An atomistic approach. JPhys ChemB 102 2914-2922... 

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