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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
1 Departamento de Cristalografía y Mineralogía, Universidad Complutense de Madrid, Madrid 28040, Spain
2 Department für Geo- und Umweltwissenschaften, Ludwig-Maximilians-Universität, Munich 80333, Germany
Correspondence: * E-mail: lfdiaz{at}geo.ucm.es
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ABSTRACT |
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 Top Abstract IntroductionExperimental methodsResultsPseudomorphismDiscussionConclcluding remarksAcknowledgmentsReferences cited |
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Key Words: Carbonatation • dissolution-crystallization • mineral replacement • gypsum • calcium carbonate • pseudomorphism
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INTRODUCTION |
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TopAbstractIntroductionExperimental methodsResultsPseudomorphismDiscussionConclcluding remarksAcknowledgmentsReferences cited |
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Deep in the Earth’s crust and in the mantle, under high temperatures and/or pressures, transformations can take place via solid-state reactions. However, on the Earth’s surface and at typical sediment temperatures, solid-state processes are kinetically hindered and their contribution to most mineral transformations can be considered negligible. Under these conditions, the presence of aqueous solutions provides an efficient mass-transfer medium. Thus, mineral transformations can occur at a substantially accelerated rate (Wang and Morse 1996) through the combination of two processes: the dissolution of the reactant phase and the crystallization of the new one (Carmichael 1969; Cardew and Davey 1985; Wang et al. 1995; Pina et al. 2000). These transformations frequently show the characteristics of mineral replacements (Putnis 2002), i.e., the volume and the shape of the reactant crystals are preserved during the transformation. This implies that the dissolution of the reactant and the crystallization of the product phase (or phases) necessarily occur simultaneously and at coupled rates (Harlov et al. 2007; Putnis et al. 2007a; Oelkers et al. 2007). Moreover, the formation of pseudomorphs normally occurs (Putnis et al. 2006, 2007b; Putnis and Putnis 2007; Sánchez-Pastor et al. 2007). Under certain circumstances, pseudomorphization can be extremely faithful to both the shape and the surface features of the original crystal, which are precisely preserved.
The early work by Flörke and Flörke (1961) reported for the first time the characteristics of the transformation of gypsum into vaterite/calcite under laboratory controlled conditions. Despite the fact that the transformation of sulfates into carbonates has evident implications for carbonate sedimentology and, furthermore, is involved in the global geochemical cycle of carbon, few efforts have been made to experimentally approach this type of process. Thus, the microscopic mechanisms involved in the carbonatation of gypsum remain poorly understood. In this paper, we present an experimental study of the transformation of gypsum into CaCO3 as a result of reaction with carbonate-bearing aqueous solutions. Since the dissolution-crystallization reactions involved in the carbonatation process occur at the crystal-fluid interface, we have focused our research on the gypsum (010) face, which is the most important one morphologically, and contributes about 75% of the total surface area of gypsum crystals grown from pure aqueous solutions of CaSO4 (Simon and Bienfait 1965). Our research follows the progress of the carbonatation process by monitoring the movement of the reaction front perpendicular to an (010) gypsum surface and studies the evolution of the mineralogy and the textural characteristics of the transformed region. Furthermore, we relate the kinetics of the carbonatation to both the thickness and texture of the transformed layer and the evolution of basic physicochemical parameters. As such, we have established general conclusions about the factors controlling the progress of the carbonatation of gypsum.
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EXPERIMENTAL METHODS |
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TopAbstractIntroductionExperimental methodsResultsPseudomorphismDiscussionConclcluding remarksAcknowledgmentsReferences cited |
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·cm). Independent experimental runs were carried out for each Na2CO3 aqueous solution concentration. Gypsum crystals were removed from the solutions after different reaction periods, ranging from 1 min to 15 days. In all cases, partially to totally replaced crystals were dried rapidly by blowing pressurized air on their surfaces. All experiments were carried out in a thermostated room at 25 ± 0.5 °C and atmospheric pressure. Each run was repeated at least three times to quantify the variability of the measurements.
The crystals were studied by glancing incidence X-ray diffraction (GIXRD) and scanning electron microscopy (SEM). X-ray diffraction patterns were used to identify the different CaCO3 phases formed during the replacement process. The diffractometer used was a PANalytical X’Pert PRO MRD equipped with a CuK
X-ray source, an X-ray parabolic mirror in the incident beam, and a parallel plate collimator with flat graphite monochromator in the diffracted beam (Xe proportional detector). The small angle of incidence (0.5°) allows one to obtain information corresponding to the shallower layers, minimizing the presence of peaks coming from the bulk of the gypsum crystal. This information allowed us to determine the sequence of CaCO3 phases formed at different stages of the transformation. Both the (010) surface and sections cut perpendicular to this face of the partially transformed gypsum crystals were imaged by SEM (JEOL JSM 6400, 40 kV). SEM images of the gypsum (010) surfaces provided information about textural and mineralogical features. Measurements on images of the perpendicular sections of the samples removed from the Na2CO3 solutions at different times allowed us to quantify the rate of advancement of the carbonatation front. The volume, as well as the number of moles, of gypsum that have passed to the solution at any time along the process can be approximately estimated from the thickness of the transformed layer, considering an average area for the (010) surface of the gypsum crystal exposed to the solution of 8 ± 0.5 mm2. These data were used to model the evolution of the saturation index with respect to gypsum (Kspgypsum = 10–4.58) with time, as well as the evolution of the concentration of CO32– and SO42– ions in the aqueous solution, considering times subsequent to the first appearance of calcite. This modeling was done using the geochemical code PHREEQC (Parkhurst and Appelo 2000). The initial Na2CO3 concentration of the aqueous solution, equilibrated with atmospheric CO2, was used as input datum. Then, the tool "Reaction," considering gypsum as the reactant, was applied. This tool operates through reaction steps. In each step, the solution was equilibrated with both atmospheric CO2 and calcite, using the "Equilibrium" tool of PHREEQC. A total number of 70 reaction steps were defined.
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RESULTS |
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TopAbstractIntroductionExperimental methodsResultsPseudomorphismDiscussionConclcluding remarksAcknowledgmentsReferences cited |
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) with SEM images (Figs. 2–6) obtained after different periods of exposure of the gypsum (010) surface to the Na2CO3 solutions allow us to monitor the mineralogical and textural evolution of the transformed layer. Reactions on the gypsum (010) surface begin immediately after immersion of crystals into an aqueous solution of Na2CO3, independent of its initial concentration. However, the results obtained show that the sequence of reactions depends on the initial concentration of the aqueous solution. In the experiments carried out using 0.5 M Na2CO3 solutions, SEM images show that after 1 min a thin layer (1 µm thick), resulting from the coalescence of nanometric spheres (diameter: 300–400 nm), completely carpets the gypsum (010) surface (Fig. 2). This layer progressively becomes thicker and more compact, with formation of "bottle necks" between the spheres. The corresponding GIXRD diffractogram only shows peaks that match those of gypsum, indicating that this first layer most probably consists of the amorphous calcium carbonate (phase ACC). In contrast, the diffractograms corresponding to slightly longer reaction periods show several peaks that match either vaterite (5 min) or vaterite and calcite (15 min). For reaction periods longer than 45 min, these two phases are clearly distinguishable in the SEM images (Figs. 3a and 3b). Whereas vaterite appears as rounded aggregates with an equatorial cleft and a very rough surface, calcite crystals show the common rhombohedral habit, with signs of dendritic growth. As can be seen in Figure 3, ACC still remains as the main phase in the transformed region for a reaction period of 45 min. However, the ACC layer has clearly dissolved around vaterite aggregates and calcite crystals, which indicates that a secondary dissolution-recrystallization process between CaCO3 phases occurs locally. Moreover, the dissolution rate of gypsum seems to be increased by the presence of vaterite and/or calcite, as evidenced by the surface depressions formed around the points where crystals of these phases have developed. Similarly, in those areas where vaterite and calcite are in contact, vaterite aggregates have dissolved, whereas calcite crystals have grown (Fig. 3b). For reaction periods over 180 min, the original ACC layer has completely disappeared and the transformed region consists of an aggregate of calcite rhombohedra, with a minor amount of vaterite spherulites. After 24 h, the whole volume of the original gypsum fragment has completely transformed into CaCO3. Although SEM images show that calcite is the main phase, XRD patterns indicate that vaterite is still present. Finally, after a week in contact with the solution, only calcite is detected.
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For solutions with lower starting carbonate concentrations (0.1 and 0.05 M) the sequence of reactions observed shows several distinctive features: (1) a less developed ACC layer, which appears as very thin patches randomly distributed on the gypsum surface and that rapidly dissolves in favor of the growth of calcite crystals from the very beginning of the reaction (reaction time 
5 min). Moreover, calcite crystals have formed either on ACC or directly on the gypsum surface (Fig. 4a
). (2) After the formation of calcite, carbonatation may progress without the formation of any new phase. However, the formation of aragonite, vaterite, or both has been observed (in about a third of the experimental runs) after short reaction periods (15 min) (Fig. 4b). When these phases form, aragonite spherulites and vaterite aggregates coexist with calcite crystals during the first 24 h of the process. After a week in contact with the solution, calcite is the main phase in the transformed layer, vaterite is absent, and aragonite, although still distinguishable in both the X-ray diffraction diagrams and the SEM images, is not abundant.
Figure 5 schematically summarizes the sequence of formation of CaCO3 phases vs. time for the different starting concentrations of carbonate in aqueous solution. As illustrated, the ACC lasts longer when the initial concentration of the solution is higher. In addition, the stable CaCO3 phase calcite forms later in highly concentrated solutions. In the case of solutions with lower carbonate concentrations, calcite occasionally coexists with other metastable phases (vaterite and/or aragonite) for longer periods.
Some specific textural aspects of the replacement of gypsum crystals by calcite can be better observed on sections cut perpendicular to the (001) original surface. SEM images show that the transformed region consists of two layers. The outer-layer is very thin (15–20 µm) and consists of fine-grained calcite. In contrast, the inner-layer is thick and consists of columns of calcite rhombohedra oriented with their 
-axes approximately perpendicular to the original surface of the sample (Figs. 6a and 6b). Most interesting is the existence of a small gap between the inward-penetrating layer of CaCO3 and the surface of the gypsum crystal, which is much more evident when aqueous solutions with a low carbonate concentration are used. Such a gap is indicated by arrows in Figures 6a and 6b. A clearer image of the relationship between the CaCO3 layer and the gypsum "relict" is observed in Figure 7. The CaCO3 layer appears continuous, encapsulating the gypsum crystal, and highly porous (Fig. 7a). In Figure 7b, a detail of the CaCO3 layer, the gypsum surface and the small gap in between are shown. Note that the thickness of the CaCO3 layer is not constant. Moreover, the differences in thickness are specially marked for different surfaces, due to their different reactivity. As mentioned above, we have focused on the advance of the replacement parallel to (010) because this is the face that dominates the morphology of most natural gypsum crystals.
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PSEUDOMORPHISM |
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TopAbstractIntroductionExperimental methodsResultsPseudomorphismDiscussionConclcluding remarksAcknowledgmentsReferences cited |
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). When less concentrated solutions are used, the pseudomorphs are less perfect and the transformation affects only up to 30% of the whole volume of the original gypsum crystal. Thus, in the case of 0.25 M Na2CO3 solutions, both the external shape and the volume of the original gypsum crystals are preserved, but not the surface detail. Moreover, the surface of the pseudomorphs is not smooth. Finally, when 0.1 or 0.05 M Na2CO3 solutions are used, the development of the replacement process is limited to the outermost region of the original gypsum crystal and the pseudomorphs are much less perfect than in either of the two previous cases considered (Fig. 8b). Therefore, it can be concluded that the higher the starting carbonate concentration of the solution, the more faithful the pseudomorphs obtained.
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. As illustrated, the width of the CaCO3 layer rapidly increases with time during the early stages of the process. Later, in all cases, the rate of advance of the carbonatation front decreases. When 0.5 M Na2CO3 aqueous solutions are used, the entire gypsum crystal transforms into CaCO3 in a relatively short period of time. However, in the remainder of the cases, the time elapsed between the beginning of the process and the virtual end of the transformation depends on the starting carbonate concentration in the solution. Thus, the higher the concentration, the longer a high carbonatation rate is maintained. When the starting carbonate concentration in the solution is 0.25 M, the replacement front moves forward rapidly for ~300 min, but almost no advancement is observed for longer reaction periods. Finally, in the case of 0.1 and 0.05 M Na2CO3 solutions, the rapid advancement of the carbonatation front is restricted to the very initial stages of the process, almost completely stopping for reaction periods over 200 min.
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DISCUSSION |
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TopAbstractIntroductionExperimental methodsResultsPseudomorphismDiscussionConclcluding remarksAcknowledgmentsReferences cited |
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30 min for 0.5 M and 5 min for 0.25, 0.1, and 0.05 M), only ending when the ACC layer has completely disappeared. The different crystallization sequences observed, depending on the starting carbonate concentration in the solution, can be related to differences in the supersaturation level for the polymorphs of CaCO3 during the initial stages of the transformation processes. It is evident that at the initial stages, supersaturation is higher when more concentrated carbonate solutions are used (Na2CO3 0.5 and 0.25 M). Thus, when the carbonate concentration is high, the progress of gypsum dissolution maintains a high supersaturation with respect to other CaCO3 phases even after the formation of the ACC layer. This promotes the nucleation of metastable vaterite. Finally, a further reduction of supersaturation resulting from the growth of vaterite favors the nucleation of the stable phase calcite. In contrast, when less concentrated solutions are used (0.1 and 0.05 M), the supersaturation decrease, a consequence of the formation of the ACC layer, readily leads to the nucleation of calcite. Although the differences in the mineralogical sequence observed as a result of the starting carbonate concentration are evident, in all the cases several dissolution-crystallization loops, characteristic of solvent-mediated transformations, are simultaneously operating during the carbonatation process. As has been explained previously, vaterite and/or aragonite crystals frequently form after the nucleation of calcite when low carbonate concentrations are used. This seems to indicate that the very high SO42–/CO32– ratios in the solution contribute to promote the crystallization of both vaterite and aragonite with respect to calcite. The formation of aragonite and/or vaterite after calcite nucleation always occurs very soon (reaction time ~15 min), when the carpeting of the gypsum surface is still very limited. It can be assumed that under these conditions the solution film directly in contact with the crystal surface approaches saturation with respect to gypsum. This means that the SO42–/CO32– ratio will be higher in the case of aqueous solutions with lower carbonate concentrations. Our observation is consistent with the results reported by Bischof and Fyfe (1968) and Bischof (1968), who found that SO42– ions promote the formation of aragonite with respect to calcite and have a retarding or inhibiting effect on the transformation of vaterite into aragonite and of aragonite into calcite via solution. Moreover, a connection between the formation of metastable vaterite in nature and the presence of high concentrations of SO42– ions in the crystallization media has also been proposed by several authors (Lippmann 1973; Grasby 2003). On the other hand, it is worth noting that aragonite lasts longer than vaterite in the transformed layer. This may be related to the slight difference in solubility between calcite and aragonite, which leads to a very low driving force and, consequently, more sluggish kinetics for the solvent-mediated transformation of aragonite into calcite.
Textural characteristics and pseudomorphism
The textural characteristics of the transformed layer, shown in Figure 6, are consistent with the advance of a carbonatation front into the gypsum crystals. There is particularly strong evidence of this in the orientation of the columns of calcite rhombohedra within the transformed layer. Such columns can be identified as soon as calcite becomes the dominant phase in the replacement and, in them, the rhombohedra show their threefold axis oriented perpendicular to the original surface of the reactant crystal. Moreover, these calcite crystals show signs of dendritic growth pointing to the inward-moving surface of the gypsum crystal. Therefore, it indicates that the carbonatation can be described as a process of mass-transfer through an interfacial fluid film from the gypsum surface to the CaCO3 layer.
Another aspect that deserves consideration is the faithfulness of the pseudomorphs formed after the replacement of the original gypsum crystals by a CaCO3 aggregate. A higher degree of faithfulness is obtained when gypsum crystals react with solutions with a higher carbonate concentration. When the concentration is high, the nucleation of the replacing phase ACC occurs immediately, forming a thin layer that completely carpets the gypsum surface, reproducing most of its original features. Moreover, the subsequent transformation of the ACC into vaterite and calcite via solvent also occurs under high supersaturation conditions, which causes the formation of a high number of small crystals. Measurements of the calcite crystals’ size formed on the original gypsum surface after extended reaction periods (longer than 24 h) give an average value of 5 µm when the starting carbonate concentration in the aqueous solution is 0.5 M. The small size of the crystals is the reason that the pseudomorphs obtained in this case show smooth surfaces and that a higher degree of faithfulness in the reproduction of the details of the original surface is achieved.
In contrast, when the starting carbonate concentration in the solution is lower, a more significant volume of the original gypsum crystal needs to dissolve before the critical supersaturation for the nucleation of the new phase is reached. In fact, for 0.1 and 0.05 M Na2CO3 aqueous solutions, ACC never forms a homogeneous layer, but appears as discontinuous patches that do not completely cover the surface of the gypsum crystal. This causes a partial loss of the original textural information of the substrate. Moreover, the nucleation occurs under lower supersaturation conditions and a smaller number of nuclei of calcite form on the gypsum surface. The growth of these nuclei leads to the development of crystals much larger (calcite crystals’ average size 90 µm) than in the case described above. This explains the reduction of the pseudomorphs’ faithfulness.
It is worth mentioning that the reproduction of fine morphological details of the original surface is also favored by the random crystallographic orientation of the calcite crystals. Pöml et al. (2007) have pointed out the importance of the lack of epitaxial relationships in the transfer of surface information during a replacement process.
Advancement of the carbonatation front
Some general conclusions about the factors that control the coupling of the dissolution and crystallization reactions involved in the carbonatation of gypsum can be extracted from the kinetic data previously reported in the results section. The fact that in all the cases considered, the carbonatation kinetics is characterized by an initial region in which the transformation front advances rapidly, followed by a progressive slowdown of its front advancement rate, seems to indicate that the general kinetics of the process is independent of the sequence of phase transformations occurring within the transformed region. Moreover, in all cases a reasonably linear relationship between the thickness of the CaCO3 layer and time1/2 exists, as is evidenced by plots presented in Figure 10. Therefore, it must be concluded that, at least during the initial stages of the carbonatation process, the advancement of the carbonatation front is controlled by the diffusion of the reactant from the solution bulk to the interface between the CaCO3 layer and the gypsum crystal (Lasaga 1998). The subsequent slowdown of the carbonatation rate can be interpreted as a consequence of the interplay of at least three main factors: (1) the formation of a thick layer of CaCO3 that partially precludes the percolation of the solution toward the gypsum surface; (2) the depletion of the solution in CO32–, which reduces the concentration of the reactants’ gradient; and (3) the increase in the SO42–/CO32– ratio during the process. This interpretation is supported by the results of the simulations of the physicochemical evolution of the system, carried out using the geochemical code PHREEQC. These simulations show that, for the time at which the carbonatation front advancement stops, in none of the cases considered has the bulk aqueous solution reached equilibrium with respect to gypsum (see Fig. 11). Moreover, the concentration of carbonate ions in the solution is still high enough to guarantee the precipitation of CaCO3 if the dissolution of gypsum progressed, as can be observed in Figure 12. Consequently, the driving force for the progress of carbonatation still persists when it stops.
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). This can be explained considering that when solutions with a low concentration are used, a much higher amount of gypsum needs to be dissolved before the interfacial liquid reaches a supersaturation that guaranties the progress of calcite crystal growth. The development of a similar gap between the inward moving surface of the reactant phase and the product has been previously observed and discussed by Pöml et al. (2007) for the replacement of pyrochlore as a result of hydrothermal alteration. In the case under consideration, the progress of the transformation is also favored by the absence of crystallographic relationships between the gypsum substrate and the product phases. As Prieto et al. (2003) demonstrated, the formation of epitaxial layers can rapidly armor a substrate and lead to the virtual stoppage of the dissolution-crystallization process, while a connected porosity net develops when epitaxial relationships are absent. The existence of both microporosity and the layer of liquid filling the gap at the interface should guarantee the progress of the carbonatation process at a relatively high rate. However, this is not supported by the observations. Therefore, other factors must play an important role in the evolution of the carbonatation kinetics. The most likely explanation for the inconsistency between the expected evolution of the carbonatation rate and our observations is a progressively reduced permeability of the CaCO3 layer as time goes on, as a consequence of both the compositional and textural re-equilibration. The closing of porosity after replacement as a result of textural re-equilibration has been observed in the case of some simple systems (Putnis et al. 2005). Such evolution of the porosity has been interpreted as being driven by the reduction of interfacial area. A similar process could occur in the system under consideration. To achieve a general conclusion on this particular aspect will require carrying out a detailed study of the microtextural characteristics of the CaCO3 layer and their evolution, which will be addressed in a forthcoming work. |
CONCLCLUDING REMARKS |
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TopAbstractIntroductionExperimental methodsResultsPseudomorphismDiscussionConclcluding remarksAcknowledgmentsReferences cited |
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ACKNOWLEDGMENTS |
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Footnotes |
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MANUSCRIPT RECEIVED January 11, 2009; MANUSCRIPT ACCEPTED April 19, 2009
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4} faces. (b) Calcite rhombohedra and aragonite spherulites showing the typical fibrous-radial morphology, formed after a reaction period of 15 min. In both cases, the concentration of the Na2CO3 aqueous solution was 0.05 M.
