Transformation Kinetics of Monohydrocalcite to Aragonite in Aqueous Solutions

Journal of MineralogicalTransformation and Petrological kinetics of monohydrocalcite Sciences, Volume to 103�� aragonite,� page 345─ 349, 2008 345

LETTER Transformation kinetics of monohydrocalcite to aragonite in aqueous solutions

* ** Takashi Munemoto and Keisuke Fukushi

*Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan **Institute of Nature and Environmental Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan

Monohydrocalcite (CaCO3·H2O; MHC) is a rare mineral in geological settings. It is metastable with respect to calcite and aragonite. This metastability of MHC is considered to make it a rare mineral in geological settings. Alteration experiments of MHC in aqueous solutions in a closed system were conducted at temperatures between 10 and 50 °C in order to measure its metastability quantitatively. In the present study, monohydrocalcite transformed to aragonite with time. There are two rate-limiting steps in the transformation of monohydrocalcite to aragonite: the nucleation and crystal growth of aragonite. On the other hand, the dissolution of monohydrocalcite is a faster process than the nucleation and crystal growth of aragonite. The amounts of aragonite were calculated from the X-ray diffraction (XRD) intensity to evaluate the rate of both the processes at different temperatures. The induction times for the nucleation of aragonite were estimated to be 2.7 ± 0.9×103, 5.4 ± 1.8×103, 3.2 ± 0.4×104, and 7.3 ± 0.4×105 s at 50, 40, 25, and 10 °C, respectively. The conventional rate constants by as- –5 –5 suming a zero-order reaction of aragonite crystal growth were estimated to be 1.0 ± 0.3×10 , 6.1 ± 0.8×10 , 1.0 ± 0.3×10–5, and 1.6 ± 0.3×10–6 mmol·s–1 at 10, 25, 40, and 50 °C, respectively. From Arrhenius plots, the apparent activation energies were estimated to be 108.1 kJ·mol–1 and 80.7 kJ·mol–1 for the nucleation and crystal growth steps, respectively.

Keywords: Monohydrocalcite, Transformation, Metastability, Apparent activation energy, Aragonite

INTRODUCTION of aragonite (Kinsman and Holland, 1969). MHC is known to be metastable with respect to calcite and arago- Monohydrocalcite (CaCO3·H2O; MHC) is a rare mineral nite (Dahl and Buchardt, 2006). This metastability of in geological settings. It has been found in several saline MHC is considered to make it a rare mineral in geological lakes; its presence was first reported in Lake Issyk-Kul in settings. However, there have been few studies that have Kirgizia by Sapozhnikov and Tsvetkov (1959), Lake Kivu focused on the metastability of MHC quantitatively. In the in Africa by Stoffers and Fischbeck (1974), and Lake present study, the mechanism and rate of transformation Fellmongery and Lake Butler in Australia by Taylor of MHC to aragonite are examined by performing altera- (1975). Its presence has also been reported in a seawater tion experiments in the laboratory by using a synthesized environment (Dahl and Buchardt, 2006). Recently, Fuku- specimen. shi et al. (2007) found MHC from deep sediments formed at least 50 thousand years ago in Lake Hovsgol in Mon- MATERIALS AND METHODS golia. MHC was first prepared by Brooks et al. (1950) in a laboratory, and some synthesis studies on it have been Preparation of monohydrocalcite reported (Kinsman and Holland, 1969; Kralj and Brece vic, 1995; Dejehet et al., 1999). In the laboratory, MHC The formation of MHC requires a high Mg/Ca ratio in easily precipitates from artificial seawater as a precursor mother solutions (Dejehet et al., 1999). In the present doi:10.2465/jmps.080619 study, MHC was synthesized by a similar procedure as T. Munemoto, [email protected] Corresponding au- that used by Dejehet et al. (1999). A mixing solution con- thor taining 0.06 M CaCl2 and 0.06 M MgCl2 was prepared at 346 T. Munemoto and K. Fukushi

room temperature, and Na2CO3 was added to the mixing trolyte solutions. Before conducting the experiments, the 2– solution to yield a 0.08 M CO3 solution. Immediately af- electrolyte solutions were preheated or cooled in order to ter Na2CO3 was added to the solution, a whitish suspen- bring their temperatures to the experimental temperatures. sion was formed in the reaction vessel. The resulting sus- A series of vessels containing the solid samples and elec- pension was stirred and aged for 48 h. The aged sus‑ trolyte solutions were prepared to perform batch alteration pension was filtered through a 0.2 µm membrane. The experiments at each temperature. After the appropriate ag- white paste collected on the filter paper was washed sev- ing time, one of the vessels was selected to conduct the eral times with sufficient amount of ion-exchanged water, experiment. For conducting the experiment at 25 °C, the and the resulting solid was air-dried. Ten milligrams of pH of the suspensions was measured using an automated the specimen was dissolved in 10 ml of 1.2 wt% HNO3 pH meter (EUTECH INSTRUMENTS, pH 510). The sus- solution and analyzed with an Inductive Coupled Plasma- pensions were filtered through a 0.2 µm membrane and Optical Emission Spectrometer (ICP-OES, VARIAN air-dried at room temperature. The air-dried samples 710-ES) to measure the amounts of Ca and Mg. The were analyzed by X-ray diffraction (XRD, Rigaku RINT‑ amounts of Ca2+ and Mg2+ contents in the unit mass were 1200, CuKα, 40 kV, and 30 mA). The amounts of MHC estimated to be 8.31 mmol·g–1 and 0.46 mmol·g–1, respec- and aragonite obtained in millimoles in the dried samples 2– tively. The CO3 and H2O contents were calculated from were evaluated by the external standard method (Cullity, the charge balance and mass balance of the solid, respec- 1956). The XRD patterns of the synthesized MHC speci- tively. Thus, the chemical composition of the specimen men (starting material) and aragonite specimen which is was estimated to be Ca0.95Mg0.05CO3·0.82H2O. completely altered from MHC after the experiments at each temperature were used for the external standards. Alteration experiments From the repeated XRD measurements of pure MHC and aragonite specimens, we estimated an error of ±10% in Alteration experiments were conducted in closed polycar- the relative abundances of each of these specimens in bonate vessels at 10, 25, 40, and 50 °C, where the temper- each measurement. This quantification method cannot ac- ature was controlled by a temperature-controlled incuba- count for the aqueous components released from the solid tor (SANYO MIR-153), and the resulting mixture was phases. The solubility of MHC (Hull and Turnbull, 1973) stirred with mix rotor. Each vessel contained 80 mg (0.70 revealed that the maximum dissolution amount of MHC mmol) of MHC powder and 40 ml of 0.01 M NaCl elec- must be as low as 2 mg, which is 3% of the total amount

Figure 1. (a) XRD patterns of collected samples after 14400, 43200, 72000, and 86400 s at 25 °C. (b) Changes in amounts of aragonite formed as functions of time up to 1200000 s. The figure in the inset shows the changes in the formation of aragonite up to 22000 s. The line indicates the regression for the pseudo-zero-order reaction for aragonite crystal growth (see text). The open circles indicate the periods when both aragonite and MHC coexist from XRD patterns. The closed circles indicate the periods when aragonite and MHC rarely occur. Transformation kinetics of monohydrocalcite to aragonite 347 of the initial MHC (80 mg) used. The process of transfor- Table 1. Mineralogy, amounts of MHC and aragonite calculated mation of MHC to a more stable phase is expected to lead by the external standard method to a decrease in the released aqueous components. There- fore, we assumed that the contribution of the aqueous components to the total system is negligible in the current batch experiments.

RESULTS AND DISCUSSION

MHC transformed to aragonite with time for all temperatures under the experimental conditions (Figure 1a and Table 1). Calcite did not appear during the experiments

(Table 1), although it is the most stable CaCO3 polymor ph. Both the nucleation and the growth of calcite were in- hibited by the presence of Mg2+ in the solution (Chen et al., 2004). Because synthesized MHC contains a small amount of magnesium, the amount of magnesium present in MHC will be released to the solution during the dissolution of MHC. The formation of aragonite instead of calcite may be attributed to the inhibition of calcite by magnesium released during the dissolution of MHC. From the results of the XRD analyses, aragonite was formed 1800-3600, 3600-7200, 28800-36000, and 691200- 777600 s after the beginning of the experiments at 50, 40, 25, and 10 °C, respectively (Table 1). After aragonite was formed, the peak intensity of MHC decreased with time, while that of aragonite increased with time (Figure 1a). These results indicate that there are two different rate- limiting processes in the transformation of MHC to aragonite. The first process involves the period before the formation of aragonite (step 1), while the second involves the period after the formation of aragonite (step 2). Ogino et al. (1987) studied the transformation mechanisms of metastable polymorphic calcium carbonate (aragonite and vaterite) to a stable-phase calcium carbonate (calcite). They suggested that the polymorphic transformation occurred through the dissolution of the metastable phase and the formation of the more stable phase. This transformation is a process of recrystallization throu The pH values of suspensions in collected samples with time at - gh nucleation and crystal growth and not a direct solid 25 °C. phase conversion. Assuming that the transformation of MHC denotes monohydrocalcite. Ara denotes aragonite. MHC is also characterized by the dissolution of MHC and precipitation of aragonite, the observed alteration behav- iors were explained effectively. According to the recrys- pected to be a fast process. When MHC is added to the tallization mechanism, the following three potential ele- solution, the solution immediately attains equilibrium due mental processes are involved in this transformation: (1) to the high dissolution rate of MHC. Because the solubili- the dissolution of MHC, (2) the nucleation of aragonite, ty of the unstable phase must be higher than that of more and (3) the crystal growth of aragonite. The pH of the sus- stable phases, the solution that is in equilibrium with pension attained the highest value immediately after the MHC must be supersaturated with respect to aragonite. aging began at 25 °C and gradually decreased with time However, aragonite is not formed in this step; therefore, (Table 1). Because the increase in pH results in the release we consider that step 1 represents the nucleation of arago- of components from MHC, the dissolution of MHC is ex- nite. In step 2, the relative amount of MHC decreased and 348 T. Munemoto and K. Fukushi that of aragonite increased with time (Figure 1a). Because changes in the concentration of all the chemical species the dissolution of MHC must be faster than other process- that participate in the reaction, and the rate constants de- es, step 2 represents the crystal growth of aragonite after rived from this model are conditional constants (Shaw et nucleation, when the dissolved components released from al., 2005). The conditional rate constants in step 2 were MHC are consumed due to the growth of aragonite. estimated to be 1.0 ± 0.3 × 10–5, 6.1 ± 0.8 × 10–5, 1.0 ± 0.3 The rates of both the processes strongly depend on × 10–5, and 1.6 ± 0.3 × 10–6 mmol·s–1 at 10, 25, 40, and 50 the temperature (Table 1). In order to assess this depen- °C, respectively. The errors were estimated from the error dence, we estimated the rates of both the processes at propagations, including the ±10% errors of the relative each temperature. The nucleation rate is simply evaluated abundances from the XRD measurements and the devia- from the induction time for the formation of aragonite tions of the plots from the regression lines. Figure 2 from XRD. Because we could not determine the exact shows the Arrhenius plots of both the processes. Good time of formation of aragonite from the XRD data, we linear relationships between the rates and reciprocals of used both the time immediately before and that immedi- the temperatures were obtained. The regression line for ately after the appearance of aragonite as the induction the nucleation of aragonite was represented by times. The induction times for step 1 were evaluated by averaging these two times. They were 2.7 ± 0.9×103, 5.4 ln k = 13000/T – 32 (r = 0.98) ± 1.8×103, 3.2 ± 0.4×104, and 7.3 ± 0.4×105 s obtained at 10, 25, 40, and 50 °C, respectively. Figure 1b shows the and that for the crystal growth of aragonite was changes in the amounts of aragonite formed with time at each temperature after the elapse of the respective induc- ln k = –9700/T + 21 (r = 0.99). tion times. The closed plots indicate the periods when both aragonite and MHC coexist from XRD patterns. The From the slopes these lines, the apparent activation ener- –1 open plots indicate the periods when aragonite and MHC gies were estimated as Ea(n) = 108.1 kJ·mol and Ea(g) = rarely occur. The time dependent data of step 2 were ana- 80.6 kJ·mol–1 for the nucleation and growth of aragonite, lyzed along with the closed plots in Figure 1b by using a respectively. zero-order reaction model, first-order reaction model, and second-order reaction model. Among these models, the zero-order reaction model provided the best fit to all ex- ACKNOWLEDGMENTS perimental data (Figure 1b). In practice, the model is a “pseudo” zero-order model because only the formation of The quality of this manuscript improved significantly by an aragonite end product is considered rather than the valuable comments of two anonymous reviewers. We would like to thank T. Kawanishi for the use of ICP-OES. Financial support was provided by Grant-in-Aid for Sci- entific Research, Japan Society for the Promotion of Sci- ence (No. 19840022 and 20740315).

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