Pb2+âCalcite Interactions under Far-from-Equilibrium Conditions

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Pb -Calcite Interactions under Far-from-Equilibrium Conditions: Formation of Micro Pyramids and Pseudomorphic Growth of Cerussite Ke Yuan, Vincent De Andrade, Zhange Feng, Neil C. Sturchio, Sang Soo Lee, and Paul Fenter J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11682 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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The Journal of Physical Chemistry

Pb2+-Calcite Interactions under Far-from-Equilibrium Conditions: Formation of Micro Pyramids and Pseudomorphic Growth of Cerussite

Ke Yuan1*, Vincent De Andrade2, Zhange Feng1, Neil C. Sturchio3, Sang Soo Lee1, Paul Fenter1

Affiliation: 1. Chemical Sciences and Engineering Division, Argonne National Laboratory. 9700 South Cass Avenue, Argonne, IL 60439; 2. Advanced Photon Source, Argonne National Laboratory. 9700 South Cass Avenue, Argonne, IL 60439; 3. Department of Geological Sciences, University of Delaware. Newark, DE 19716.

Corresponding Authors: Ke Yuan, Chemical Sciences and Engineering Division, Argonne National Laboratory. 9700 South Cass Avenue, Argonne, IL 60439. Tel: 630-252-1787; Email: [email protected]

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Abstract The presence of impurity ions is known to significantly influence mineral surface morphology during crystal growth from aqueous solution, but knowledge on impurity ion-mineral interactions during dissolution under far-from equilibrium conditions remains limited. Here we show that calcite (CaCO3) exhibits a rich array of dissolution features in the presence of Pb. During the initial stage, calcite exhibits non-classical surface features characterized as micro pyramids developed spontaneously in acidic Pb-bearing solutions. Subsequent pseudomorphic growth of cerussite (PbCO3) was observed, where nucleation occurred entirely within a pore space created by dissolution at the calcite/substrate interface. Uneven growth rates yielded a cerussite shell made of lath- or dendritic-shaped crystals. The cerussite phase was separated from the calcite by pores of less than 200 nm under transmission X-ray microscopy, consistent with the interfacecoupled dissolution-precipitation mechanism. These results show that impurity metal ions exert significant control over the microscale dissolution features found on mineral surfaces and provide new insights into interpreting and designing micro structures observed in naturallyoccurring and synthetic carbonate minerals by dissolution. In addition, heterogeneous microenvironments created in transport limited reactions under pore spaces may lead to unusual growth forms during crystal nucleation and precipitation.


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1. Introduction Adsorption of metal ions and organic molecules on the calcite surface can significantly influence its surface morphology during crystal growth1-7, whereas studies on the role of metalmineral interactions during mineral dissolution reactions is limited. Calcite (CaCO3) is a ubiquitous and relatively soluble mineral in earth’s crust. Lead (Pb) is a common pollutant found in the environment, especially in the acid mine drainage sites8-10. At near-equilibrium conditions, Pb-calcite interaction is dominated by adsorption, where the formation of metal-surface complexes has been observed11. With increasing acidity, mild dissolution/reprecipitation of calcite surface layers can lead to incorporation and co-precipitation of Pb carbonate phases12-15. Far-from-equilibrium conditions can dramatically influence the reaction mechanisms8, 16. For example, highly acidic environments at acid mine drainage sites promote the dissolution of soluble metal sulfide/oxide minerals, resulting in the enrichment of metal ions in ground water910

. However, in the acidic metal-rich fluids, the interactions of heavy metal ions with calcite, a

common mineral phase present in the mine tailings, are not well understood. Understanding these fundamental processes provide the basis for developing heavy metal sequestration techniques and for designing materials of novel structures17-19. Here, we used Pb-calcite interaction as a model system to investigate metal ion-mineral interactions at an unstable (rapidly dissolving) solid-liquid interface at far-from-equilibrium conditions. We studied the role of Pb during calcite dissolution by reacting synthetic micronsized calcite crystals with acidic Pb-bearing solutions, building on our previous observations of the pseudomorphic replacement of calcite by cerussite (PbCO3)20 and studies on mineral replacement of phosgenite (Pb2Cl2CO3) by cerussite21-22. In particular, we focus on two distinct features observed during the early and late stages of this reaction. The initial stage of the reaction 3 ACS Paragon Plus Environment


involved significant surface morphology changes of calcite during dissolution in Pb-rich solutions, where the potential mechanism on metal-mineral interactions leading to surface feature (micro pyramids) formation were proposed. The second stage of reaction involved precipitation of cerussite having a form that was strongly modulated by the orientation and size of the calcite seed crystals. Transmission X-ray microscopy (TXM) revealed the three-dimensional pore structures of the reaction products and numerical models were built to investigate interfacecoupled dissolution-precipitation reactions and mass transport effects at the pore scale. The connection between the observed nano-porous structures to the formation of the pseudomorphic cerussite crystals were discussed. These results shed light on how metal ions could influence dissolution occurring at the mineral-water interface and aid our understanding of dynamic mineral-fluid interactions under far-from-equilibrium conditions observed both in natural and synthetic environment. 2. Experimental and simulation methods Micron-sized (20-100 µm) calcite crystals were grown on Kapton® film substrates having dimensions of 10 mm × 10 mm × 0.05 mm by using the ammonium carbonate diffusion method23. The reactant solution composition was 5 mM Pb(NO3)2 at pH 2.8 (Accumet Basic AB15 pH meter) in equilibrium with atmospheric CO2. Each piece of the solid support with calcite crystals was reacted in 4.2 mL of the reactant solution for 30 min to 5 hours. The solid to liquid mass ratio was around 1:500020. The control experiments were conducted by reacting calcite crystals in a Pb-free solution of the same pH adjusted by 0.1 M HCl. The reaction was quenched by removing the samples from the solution and gently rinsing in deionized water followed by drying under flowing N2. Samples were Au-coated prior to imaging with a Hitachi S4700 SEM. For the ex situ atomic force microscopy (AFM) experiments, the reaction time was 4 ACS Paragon Plus Environment

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reduced to 1 min in order to maintain a relatively flat (104) surface on the micron-sized calcite crystals appropriate for AFM measurements, where the initial dissolution on the surface steps was resolved. Transmission X-ray microscopy measurements were performed on beamline 32-ID-C at the Advanced Photon Source of Argonne National Laboratory. The monochromatic X-rays had an energy of 8 keV with a field of view of 51 µm × 51 µm. The sample was imaged in air with 721 projections spanning over 180° rotation with an exposure time of 1s per image. The acquired data were reconstructed by TomoPy24. The Fresnel zone plate resolution was 60 nm and the voxel size of the reconstructed images was 44.5 nm after a 2 × 2 binning. Image segmentation was completed by Drishti 2.625 and visualized in Fiji ImageJ26. A one-dimensional numerical model of the chemical reactions in a thin fluid layer confinement defined by dissolving calcite and growing cerussite was constructed in COMSOL Multiphysics 5.2a. The dissolution kinetics of calcite under experimental pH conditions was described by a first order rate equation. The growth kinetics of cerussite was represented by a simplified linear rate law derived from transition state theory (See SI for details). Only the chemical reactions that had major influences on the concentration of carbonate species were considered (Table S1 & S2). Mass transport and chemical exchange between species in the thin fluid layer and bulk solution was simplified as the parameter α which represents the fractional loss of dissolved carbonate species from the dissolution of calcite at each simulated time step (1s). 3. Results 3.1. Initial reaction: Formation of calcite micro pyramids



Calcite crystals reacted in both Pb-bearing and Pb-free solutions having initial pH = 2.8 were imaged by SEM (Figure 1). These solutions are highly under-saturated with respect to calcite, but gradually saturated the adjacent fluid layer with respect to cerussite upon release of carbonate. The comparison was conducted for two orientations of calcite crystals: “lying down” crystals with one rhombic (104) surface attached to the solid support (Figure 1a, b) and “standing up” crystals with a growth plane truncated through a corner or edge (Figure 1c, d). Dissolution of calcite dominated the initial stage of the reaction while minor growth of cerussite was identified at corners of the calcite crystals in Pb-bearing solution (Figure 1a, c). For both orientations, calcite crystals reacted with the Pb-bearing solution showed rough “spiky” surfaces, and a zoomed-in view indicated formation of micron-sized pyramidal features (Figure1a, c, inset). The formation of these micro pyramids was identified on most calcite crystals (Figure S1). Elemental mapping using energy dispersive X-ray spectroscopy (EDS) showed mainly Ca and O signals on area covered by micro pyramids and any Pb-signals were below detection limits (Figure S2). In contrast, calcite crystals reacted in acidic Pb-free solution had rounded corners due to the dissolution but relatively smooth surfaces compared with calcite crystals reacted in Pb-bearing solutions (Figures. 1b, d; See Figure S3 for comparison with pristine calcite crystals). Apparently, the formation of micro pyramids is related to the presence of Pb and its interaction with calcite surfaces. The micro pyramids tended to form as arrays orientated parallel to the calcite edge directions in a “standing up” crystal (Figure 1c). Inhomogeneous distribution of the micro pyramids was observed on the (104) surface of the “lying down” crystal (Figure 1a). A large number of micro pyramids were mainly distributed on half of the (104) surface (e.g., above the dashed line in Figure 1a). In comparison, micro pyramids below the diagonal line were fewer and smaller. 6 ACS Paragon Plus Environment

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Figure 1. “Spiky” vs. smooth calcite crystals. SEM images of calcite crystals of two different orientations (a, b “lying down” vs. c, d “standing up”) reacted in (a, c) 5 mM Pb2+ solution having initial pH = 2.8 and in (b, d) Pb-free solution having initial pH = 2.8 for 1 h. Scale bars are both 1 µm on the inserted images in (a) and (c). The structure of the micro pyramids was further characterized by TXM. The reacted calcite (104) surfaces consisted of arrays of micro pyramids (Figures. 2a, b), similar to those observed by SEM (Figure 1c). These pyramids were typically 100 nm to several µm wide and less than 2 µm tall (Figure 2b). The internal angles between the faces and bases of the pyramid had a wide range (20°-70°) (Figures. S4b & S5), indicating that the side surfaces of the micro pyramids were 7 ACS Paragon Plus Environment


composed of a diverse range of crystallographic planes. Cross-sectional views revealed that the bulk density of the micro pyramids was indistinguishable from that of calcite (Figure 2c), indicating that the chemical composition of the pyramids was primarily CaCO3. This confirms that the pyramids result from anisotropic dissolution of calcite in the presence of Pb rather than from precipitation of a secondary phase that contains Pb. In addition, previous X-ray diffraction results on similar samples also show that calcite is the only CaCO3 phase, rather than aragonite and vaterite20. We carefully examined a well-developed micro pyramid on the calcite surface (Figure S6 and S7). The crystallographic orientations of the lateral surfaces of this micro pyramid can be estimated using angles measured from TXM cross sectional images. We found that the four lateral surfaces are approximately perpendicular to the [-1 2 0], [1 0 -8], [0 1 8], and [-2 1 0] directions of calcite (Figure 2d). The four lateral surfaces might not be composed of crystallographic planes of calcite and further studies are needed to determine if these are real crystallographic planes or not.


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Figure 2. TXM 3D reconstructed calcite crystal with micro pyramids. (a) 3D view of a reacted calcite rhomb in 5 mM Pb2+ solution having initial pH = 2.8 for 1 h. (b) A magnified view of the aligned micro pyramids highlighted by the yellow box in (a). (c) Cross-sectional view of the micro pyramids cut along the plane indicated by the dashed black line in (a). Dark boundary at the calcite-air interface in (c) was due to a reconstruction artifact (See SI, Figure S4a), and not due to adsorption of Pb. (d) A calcite micro pyramid model based on TXM measurement composed of four lateral surfaces that are perpendicular to the calcite [-1 2 0], [1 0 -8], [0 1 8], and [-2 1 0] directions.



The morphology of the calcite (104) surface was imaged by AFM after ~1 min of reaction with 5 mM Pb2+ solution in order to gain insight into the formation mechanism of the calcite micro pyramids. A “lying down” calcite micro crystal was imaged (Figure 3a, optical image insert). Distinct surface features were observed on two areas (a) and (b) (marked on Figure 3a, inserted image) on the calcite (104) surface. Characteristic rhombic etch pits were observed (Figure 3a). The orientations of the edges of these etch pits were consistent with those of the calcite micro-crystal. Islands about 30-nm tall (Figure 3a) without any well-defined shape were found outside of the etch pits, suggesting that these were features preserved from calcite dissolution. However, taller islands having a more distinct pyramidal shape were found in area (b) (Figure 3b, inset). A magnified scan of the area revealed that these islands exhibited a rhombic base plane and four side surfaces (Figure 3b). The measured triangular height profile resembled the TXM cross-sectional view of the micro pyramids (Figure 2c). Considering the shorter reaction time of the AFM sample, these ~200 nm tall rhombic islands appear to reflect the early stage development of micro pyramids during calcite dissolution.


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Figure 3. Formation of early stage micro pyramids on the (104) surface of a calcite microcrystal. Images were recorded after 1 min of reaction in 5 mM Pb2+ solution of pH = 2.8. AFM amplitude images (up) and corresponding height profiles (down) measured along the horizontal dashed lines. The inserted optical image of the calcite in (a) marked the two spots where images (a) and (b) were scanned. Rhombic black boxes highlighted the shape of the etch pits and the red box highlighted the base plane of one micro pyramid. 3.2. Longer term reactions: Effects of calcite orientation on the growth of lath-shaped vs. dendritic cerussite Further calcite reaction times increased both solution pH and dissolved carbonate concentration, leading to growth of cerussite on calcite surfaces20. The majority of the cerussite



crystals observed on “lying down” calcite were lath-shaped and aligned along the c-glide direction (Figure 4a). For “standing up” calcite crystals, dendritic growth of cerussite was favored (Figure 4b). From the branching directions of the dendrites, the roots of the dendrites appeared to originate underneath the calcite crystals (Figure 4b). In addition, micro pyramids were observed on the areas that were not covered by cerussite (Figure 4), consistent with the previous observation that the micro pyramids likely formed prior to large-scale precipitation of cerussite.

Figure 4. Lath-shaped and dendritic cerussite crystals. SEM images of (a) “lying down” and (c) “standing up” calcite crystals reacted with 5 mM Pb2+ solutions of initial pH = 2.8 for 5 h. TXM data on a “standing up” calcite crystal confirmed that the root of the dendrite was located at the intersection between the calcite and Kapton substrate (Figure 5). In this sample, a single cerussite dendrite was found with a relatively uniform thickness of ~2 µm (Figures. 5a, b, c, & e). The volume left by the dissolved calcite was filled by the precipitated cerussite (Figure 5a). The cerussite dendrite grew in two directions, parallel and perpendicular to the calcite (104) surface. A bright region between the calcite and cerussite was resolved, indicative of a low density pore space between the two phases (Figure 5a). The reconstructed 3D pore structure 12 ACS Paragon Plus Environment

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resembled the shape of the cerussite dendrite (Figure 5d, e). These pores were flaky or tabular and mostly less than 200 nm thick (Figures. 5d and S8). Some pores were interconnected to form a larger pore (Figure 5d, indicated by the arrow). Other isolated pores spread along the growth direction of the dendrite (Figure 5e).

Figure 5. TXM data of a calcite crystal covered by a cerussite dendrite. Crystal was reacted in 5 mM Pb2+ solution of initial pH = 2.8 for 3 h. (a) Cross-sectional view shows the distribution of calcite (grey), cerussite (black) and interstitial pore space (white). (b, c) 3D views of the reconstructed calcite crystal (grey) covered partially by a growing cerussite dendrite (cyan). (d) 3D pore distribution and (e) its spatial relation with the cerussite dendrite. 3.3. Simulation of pore-scale reactions: Coupled dissolution and growth reactions A one-dimensional reaction model was developed in order to understand the apparent presence of pores between the dissolving calcite and the growing cerussite (Figure S15). The porosity is observed after substantial reaction, suggesting that it is stable during growth, and therefore, it



may provide a constraint on understanding the growth behavior. In this case, the evolution of the thickness of a fluid-filled pore space bounded by calcite and cerussite was modeled as a function of time under a varying parameter, α, that described the fraction of the dissolving carbonate lost from the pore (varied from 0% ≤ α ≤ 50%, i.e., α = 0 corresponds to a closed system). The model is constrained using the experimentally measured temporal evolution of pH of the bulk fluid phase. In this calculation, the dissolution front retreats by 18.7 µm in 1 h (Figure S9a, dashed line), which is about 3 times larger than the observed amount of dissolved calcite estimated from the area replaced by cerussite in Figure 5a (~ 2 µm in thickness and ~19 µm in length after 3 h of reaction). The simulation results indicate that the amount of calcite dissolution was overestimated when using experimentally measured pH of the bulk solution. When using a constant pH = 4.5 (steady state pH, Figure S16), the amount of dissolved calcite reduced to ~11 µm in 3 h (Figure S9b). The dependence of the model results on the parameter, α, showed a large influence on the thickness of the fluid layer. When there is no carbonate loss (α = 0%), the simulated pore space sealed within 30 min at constant pH = 4.5 and the pore closed even faster (less than 7 min) when experimental pH data were used. However, the thickness of the fluid layer showed only a small variation when 10% of dissolved carbonate species was released from the system (Figures. S9 and 10a & b). The observation of a constant pore size with time implies that the reaction followed a synchronized dissolution/precipitation reaction. When the carbonate loss was larger than 10%, the thickness of the fluid layer expanded up to 9 µm in 5 hrs (Figure S10b). By comparing our observations to the modeling results, we infer that the local pH inside the pore space was less acidic than that in the bulk phase, and that a net loss of ~10% of the carbonate species from the confined fluid layer is required to maintain the openness and near


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constant thickness of the fluid layer. That is, the coupled dissolution/precipitation behavior is controlled by local variations in pH, and dissolved ion concentrations inside pore spaces. 3.4. Effects of calcite crystal size on the growth of cerussite: Cerussite shell vs. formation of a complete pseudomorph The extent of replacement of calcite by cerussite also depended on the size of the original calcite crystal. For a calcite crystal of ~40 µm in diameter, a large portion of the crystal remained as calcite after 3 h of reaction (Figure 6). Cerussite crystals completely covered the four side surfaces of calcite and partially covered the top (104) surface (Figures. 6a and 6d). This morphology represented the process where a cerussite shell was formed on the surfaces of the dissolved calcite crystal as observed previously20. Noticeably, the cerussite crystals were identified at the intersection region between the calcite and the Kapton substrate in the crosssectional side view (Figure 6c, highlighted inside the boxes). The 3D structure of the shell close to the intersection can be viewed in the movie (Movie. M1) The lath shaped cerussite crystals were partially buried underneath the original calcite (See an additional example of a buried cerussite crystal in Figure S11). On the top (104) surface, the distribution of cerussite indicates that the growth likely started from the two acute corners and gradually moved towards the middle area (Figure 6b, indicated by arrows). In addition, similar to the “standing up” crystal (Figure 5), a ~180 nm gap between calcite and cerussite shell was resolved (Figure 6d). The full replacement of calcite by cerussite was observed in smaller calcite crystals. For example, a crystal of ~20 µm in diameter was fully replaced by polycrystalline cerussite within 5 hours of reaction in the acidic Pb solution (Figure S12).



Figure 6. The formation of cerussite shells on calcite. Two different perspectives viewed from the side (a, c) and top (b, d). (a, b) 3D reconstructed structure of the cerussite shell (calcite core was not shown). (c, d) Cross-sectional views of the reacted crystal (cerussite in black and calcite in grey with pore space in-between). Scale bars represent 5 µm. 4. Discussion 4.1. Calcite micro pyramids: Implications of Pb2+ interaction with a dissolving calcite surface The dissolution of calcite in a Pb-bearing solution shows significant surface morphological differences from that in Pb-free solution of the same initial pH (Figure 1). Classical calcite dissolution occurs through formation of etch pits and step retreat27-29. Here, instead, calcite dissolution in the presence of dissolved Pb2+ led to the formation of micro pyramids on the (104) surfaces. The micro pyramids have a morphology similar to calcite growth hillocks2, 5, 30. Since the reaction solution was acidic and undersaturated with respect to all CaCO3 phases, it is 16 ACS Paragon Plus Environment

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unlikely that the micro pyramids were growth hillocks formed by precipitation of the dissolved carbonate species. Preserved dissolution features containing calcite nano shoots/rods have been observed during calcite dissolution in the presence of formamide, NH4I, and (NH4)2SO417-19, but formation of calcite micro pyramids in metal ions-bearing solutions has not been reported previously. The shape and orientation of the base planes of the micro pyramids are similar to those of the etch pits (Figure 3b, red and black boxes). This indicates that the steps observed on the lateral surfaces of the micro pyramids can be assigned to the known obtuse ([48-1]+, [-441]+) and acute ([48-1]-, [-441]-) steps of a calcite etch pit. Previous studies on Pb-calcite interactions have revealed Pb inner-sphere complexes on calcite (104) surfaces through X-ray absorption spectroscopy and incorporation of Pb in calcite at the Ca sites through X-ray standing waves and X-ray reflectivity13, 31. Pb adsorption and desorption on steps of the etch pits likely occurred during calcite dissolution. Since PbCO3 (Ksp =10-13.13) is less soluble than calcite (Ksp = 10-8.48), formation of PbCO3-like steps could slow down the rate of step retreat, therefore, preserving these steps and leading to the formation of micro pyramids as preserved dissolution features (Figure 7(I)). We infer that one possible formation mechanism of a pyramid was achieved through the dissolution at four adjacent etch pits (Figure S13). The steps highlighted in red were an example of the micro pyramids, where the protected steps were preserved by interaction with Pb to form the rhombic base plane. The protected steps likely retreated thereafter, leading to the formation of a series of slowly dissolving steps in decreasing sizes, which became the lateral surfaces of a micro pyramid. Formation of arrays of parallel micro pyramids (Figure 1c) was likely caused by the regular distribution of etch pits, often observed in straight chains parallel to the or direction28-29. The inhomogeneous distribution of micro pyramids observed on the calcite (104) surfaces (Figures. 1a and S1) is not well understood. Given the



significant differences in the evolution with and without dissolved Pb, it appears that the parts of the (104) surface where the micro pyramids formed were significantly influenced by Pb, presumably by adsorption at steps, while other parts having smooth terrace areas had less interaction with Pb. Given the rounding of the calcite in the absence of Pb, we can infer that the local step density will preferentially expose acute vs. obtuse steps (i.e., at opposite sides of a single (104) facet). We postulate that the different reactivity of Pb to these different step structures is a critical component leading to the observed dissolution structures, especially if defect structures (e.g., point defects and screw dislocations) are presented which are more favorable for adsorption of Pb ions compared with the defect free area. 4.2. Nucleation density influences the growth morphology of cerussite A striking feature of these results is that the nucleation of cerussite growth initiated preferentially at the confined environment at the calcite/Kapton interface. A homogeneous nucleation event can be energetically unfavorable until the nucleus reaches a critical size32. In heterogeneous nucleation, the energy barrier can be reduced due to the presence of surfaces and confinement33. This picture can be applied to understand the observed nucleation process of cerussite. Dissolution of calcite adjacent to the Kapton substrate likely created a confined pore space between calcite and substrate. The existence of such confined pore space was evident in the observation that cerussite crystals were found underneath the calcite (i.e., adjacent to the Kapton support), instead of next to the free calcite crystal (Figures. 5a and 6c). Carbonate species dissolved from calcite surfaces within the semi-confined volumes are therefore likely to be enriched, in contrast to other surfaces that were in direct contact with the bulk solution (Figure 7(II)). The Kapton substrate appears to have provided a stable nucleation site for cerussite. In contrast, nucleation of cerussite on the rapidly dissolving calcite surfaces was rarely observed on 18 ACS Paragon Plus Environment

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the samples measured by TXM. These factors could have favored the heterogeneous nucleation of cerussite at the calcite/substrate intersection regions. In an acid mine drainage site, the nucleation sites could be at pore spaces or grain boundaries between calcite and other minerals. In addition, previous work has shown consistent cerussite shell formation process initiated at the interface between calcite and glass substrate, indicating the properties of substrates does not play a major role in the nucleation event20. Two types of growth patterns of cerussite (i.e. lath-shaped vs. dendritic) can be attributed to the difference in the nucleation density that is influenced by the orientation of the dissolving calcite crystal (Figure S14). For a “lying down” calcite crystal, the intersection perimeters between calcite and substrate were longer than a “standing up” calcite of a similar size. Since nucleation occurred preferentially at the intersection area, a higher nucleation density was expected for a “lying down” calcite of longer intersection perimeter. In comparison, dendritic growth was observed for the “standing up” calcite crystals. Dendritic growth occurs in nonequilibrium, metastable conditions, in which crystal growth started from a single point and branched out into dendritic tree shape34. Figure 4b indicated less than five cerussite dendrites for each “standing up” calcite crystal, much fewer than the observed nucleation density around the “lying down” calcite crystal. 4.3. Origin of pores and templating growth on the nano fluid layer Generation

of

pores

requires

a

negative

volume

change

associated

with

a

dissolution/precipitation reaction, which can be influenced by factors such as solution chemistry, molar volume difference, relative solubility, and epitaxial growth between the host and secondary mineral phases35. Calcite (Trig, R-3C) and cerussite (Orth, Pmcn) have two different



crystal structures. Epitaxial growth of cerussite on calcite was not observed during cerussite replacement of calcite and the differences in the crystallization habit of these two minerals may favor the development of interstitial porosity20. The intrinsic volume increase of 10% for a oneto-one transformation from calcite to cerussite (with molar volumes of 36.93 and 40.63 cm3/mol, respectively), would be expected to hinder pore formation. However, the nearly five orders of magnitude difference in solubility counterbalanced the negative effect from increasing molar volume (Ksp(CaCO3) = 10-8.48 vs. Ksp(PbCO3) = 10-13.23)36-37. That is, the one-to-one transformation of dissolved carbonate species from calcite entirely to PbCO3 cannot be achieved. Transport of solution species through nanopores between calcite and cerussite enables the replacement reaction to proceed. Pore spaces smaller than 200 nm were resolved by TXM between calcite and cerussite regardless of the size and orientation of the calcite crystals (Figures. 5 and 6). Fluid-filled nanopores have been observed in numerous mineral replacement reactions in aqueous environments35, 38-42. This nano-fluid layers were located sufficiently close to the original calcite crystal that it apparently allowed the secondary growth phase to follow the morphology of the dissolving calcite surface. The fluid layer created a semi-confined environment and served as a mass transport medium, while dissolution of calcite and crystallization of cerussite occurred separately on each side of the layer, respectively. A thin fluid layer was necessary to achieve a pseudomorphic growth, and its critical role has been reported in various pseudomorphic replacement reactions39, 43-44. Based on the relative proximity to the reaction front, we infer the existence of three types of pores (Figure 7 (III)). Open pores likely form close to the reaction front at the triple junction between calcite, cerussite and bulk solution. In addition, trapped pores were observed between 20 ACS Paragon Plus Environment

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calcite and the pseudomorphic cerussite shell. These pores were connected to the bulk solution through open pores at the reaction front and by grain boundary diffusion between cerussite crystals. Fluid phase within the trapped pore is likely to be less acidic and the Pb concentration could be relatively low due to the complex diffusion pathways. As a result, the dissolution rate of calcite and growth rate of cerussite near the trapped pores may be much slower compared with that at the reaction front. This explains the observed difference in the thickness (~ 2 µm) vs. length (~ 19 µm) of grains within the cerussite shell shown in Figure 5a, where the rapid growth along the calcite (104) surface likely resulted from open pores formed at the reaction front, while the sluggish growth perpendicular to the calcite surface is limited by diffusion among the pores trapped underneath the cerussite shell. Eventually, growth of cerussite crystals may block the diffusion pathways and create isolated pores, as observed in Figure 5e, where trapped fluid layers reached equilibrium with respective to both solid phases.



Figure 7. Schematic diagram shows three observed features during Pb-calcite interactions. I. Formation of micro pyramids. II. Nucleation of cerussite at the micro-environment located at the calcite/substrate interface. III. Mass transport in open, trapped and isolated pore spaces as moving away from the reaction front where growth of cerussite and dissolution of calcite occurred. 5. Conclusion A suite of 2D/3D imaging techniques were used to study the interactions of aqueous Pb2+ with micron-sized calcite crystals under far-from-equilibrium conditions. During the early reaction stages, micro pyramids formed on calcite (104) surfaces. These features were absent in control experiments conducted in acidic Pb-free solutions, indicating the Pb2+ ions can modify calcite 22 ACS Paragon Plus Environment

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surface dissolution features likely through adsorption on the retreating steps. Further dissolution of calcite led to supersaturation with respect to cerussite (PbCO3) and the heterogeneous nucleation of cerussite was initiated on calcite surfaces adjacent to the Kapton substrate. The orientation of calcite crystals attached to the substrate influenced the nucleation density of cerussite, which then directed the growth patterns. Transmission X-ray microscopy resolved the presence of a nano-scale fluid layer separating the calcite and cerussite phases, which was sufficiently close to the dissolving calcite surface that it effectively templated the pseudomorphic growth of cerussite. The results shown that metal ions can significantly change the dissolution patterns commonly observed before on calcite. It indicates that regular surface features and hierarchical structures observed during abiotic and biomineralization of carbonate minerals may not be uniquely caused by crystal growth, and that dissolution reactions in the presence of metal ions may also play a role in generating micro structures. In addition, the sensitivity of the growth of a secondary phase to the shape and orientation of the parent crystal, as well as the location of nucleation sites, demonstrate the importance of heterogeneous micro-environments in controlling the macroscopic crystal growth. Supporting Information SEM and EDS analysis of micro pyramids on calcite surfaces; Statistical distribution of the angular variation of lateral planes of micro pyramids based on TXM cross sectional images; Thickness distribution of the pores; Numerical modelling results of the coupled calcite dissolution/cerussite precipitation reaction; Example of nucleation site at the calcite/substrate interface; A fully replaced calcite pseudomorph made of polycrystalline cerussite; Formation mechanism of micro pyramids; Growth patterns of cerusite on calcite of different orientations; Kinetics modelling methods; Movie showing the 3D structure of a cerussite shell (AVI). 23 ACS Paragon Plus Environment


Author Information Corresponding Author Email: [email protected] ORCID Ke Yuan: 0000-0003-0565-0929 Sang Soo Lee: 0000-0001-8585-474X Paul Fenter: 0000-0002-6672-9748 Notes The authors declare no competing financial interest. Acknowledgement This material is based on work supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences through Argonne National Laboratory. Argonne is a U.S. Department of Energy laboratory managed by UChicago Argonne, LLC, under contract DE-AC02-06CH11357. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. References 1. Orme, C. A.; Noy, A.; Wierzbicki, A.; McBride, M. T.; Grantham, M.; Teng, H. H.; Dove, P. M.; DeYoreo, J. J. Formation of Chiral Morphologies through Selective Binding of Amino Acids to Calcite Surface Steps. Nature 2001, 411, 775-779. 2. Davis, K. J.; Dove, P. M.; De Yoreo, J. J. The Role of Mg2+ as an Impurity in Calcite Growth. Science 2000, 290, 1134-1137. 3. Teng, H. H.; Dove, P. M.; Orme, C. A.; De Yoreo, J. J. Thermodynamics of Calcite Growth: Baseline for Understanding Biomineral Formation. Science 1998, 282, 724-727. 4. Zachara, J. M.; Cowan, C. E.; Resch, C. T. Sorption of Divalent Metals on Calcite. Geochim. Cosmochim. Acta 1991, 55, 1549-1562. 5. Teng, H. H.; Dove, P. M.; De Yoreo, J. J. Kinetics of Calcite Growth: Surface Processes and Relationships to Macroscopic Rate Laws. Geochim. Cosmochim. Acta 2000, 64, 2255-2266. 24 ACS Paragon Plus Environment

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26. Schindelin, J., et al. Fiji: An Open-Source Platform for Biological-Image Analysis. Nat. Methods 2012, 9, 676-682. 27. Liang, Y.; Baer, D. R.; McCoy, J. M.; Amonette, J. E.; LaFemina, J. P. Dissolution Kinetics at the Calcite-Water Interface. Geochim. Cosmochim. Acta 1996, 60, 4883-4887. 28. Arvidson, R. S.; Ertan, I. E.; Amonette, J. E.; Luttge, A. Variation in Calcite Dissolution Rates: A Fundamental Problem? Geochim. Cosmochim. Acta 2003, 67, 1623-1634. 29. Schott, J.; Brantley, S.; Crerar, D.; Guy, C.; Borcsik, M.; Willaime, C. Dissolution Kinetics of Strained Calcite. Geochim. Cosmochim. Acta 1989, 53, 373-382. 30. Stack, A. G.; Grantham, M. C. Growth Rate of Calcite Steps as a Function of Aqueous Calciumto-Carbonate Ratio: Independent Attachment and Detachment of Calcium and Carbonate Ions. Cryst. Growth Des. 2010, 10, 1409-1413. 31. Sturchio, N. C., et al. Lead Adsorption at the Calcite-Water Interface: Synchrotron X-Ray Standing Wave and X-Ray Reflectivity Studies. Geochim. Cosmochim. Acta 1997, 61, 251-263. 32. De Yoreo, J. J.; Vekilov, P. G. Principles of Crystal Nucleation and Growth. In Biomineralization; Dove, P. M., DeYoreo, J. J., Weiner, S., Eds.; Mineralogical Society of America: Washington, DC, 2003. 33. Stephens, C. J.; Ladden, S. F.; Meldrum, F. C.; Christenson, H. K. Amorphous Calcium Carbonate Is Stabilized in Confinement. Adv. Funct. Mater. 2010, 20, 2108-2115. 34. Gibbs, J. W.; Mohan, K. A.; Gulsoy, E. B.; Shahani, A. J.; Xiao, X.; Bouman, C. A.; De Graef, M.; Voorhees, P. W. The Three-Dimensional Morphology of Growing Dendrites. Scientific Reports 2015, 5. 35. Pollok, K.; Putnis, C. V.; Putnis, A. Mineral Replacement Reactions in Solid Solution-Aqueous Solution Systems: Volume Changes, Reactions Paths and End-Points Using the Example of Model Salt Systems. Am. J. Sci. 2011, 311, 211-236. 36. Godelitsas, A.; Astilleros, J. M.; Hallam, K.; Harissopoulos, S.; Putnis, A. Interaction of Calcium Carbonates with Lead in Aqueous Solutions. Environ. Sci. Technol. 2003, 37, 3351-3360. 37. Mucci, A. The Solubility of Calcite and Aragonite in Seawater at Various Salinities, Temperatures, and One Atmosphere Total Pressure. Am. J. Sci. 1983, 283, 780-799. 38. Putnis, A. Transient Porosity Resulting from Fluid-Mineral Interaction and Its Consequences. In Pore-Scale Geochemical Processes; Steefel, C.I., Emmanuel, S., Anovitz, L.M., Eds.; Mineralogical Society of America: Washington, DC, 2015. 39. Putnis, A. Mineral Replacement Reactions. In Thermodynamics and Kinetics of Water-Rock Interaction; Oelkers, E. H., Schott, J., Eds.; Mineralogical Society of America: Washington, DC, 2009. 40. Perdikouri, C.; Piazolo, S.; Kasioptas, A.; Schmidt, B. C.; Putnis, A. Hydrothermal Replacement of Aragonite by Calcite: Interplay between Replacement, Fracturing and Growth. Eur. J. Mineral. 2013, 25, 123-136. 41. Qian, G.; Xia, F.; Brugger, J.; Skinner, W. M.; Bei, J.; Cren, G.; Pring, A. Replacement of Pyrrhotite by Pyrite and Marcasite under Hydrothermal Conditions up to 220 °C: An Experimental Study of Reaction Textures and Mechanisms. Am. Mineral. 2011, 96, 1878-1893. 42. Xia, F.; Brugger, J.; Chen, G. R.; Ngothai, Y.; O'Neill, B.; Putnis, A.; Pring, A. Mechanism and Kinetics of Pseudomorphic Mineral Replacement Reactions: A Case Study of the Replacement of Pentlandite by Violarite. Geochim. Cosmochim. Acta 2009, 73, 1945-1969. 43. González-Illanes, T.; Borrero, M. T.; Herráez, M. M.; Pimentel, C.; Pina, C. M. Pseudomorphic Replacement of Mg-Ca Carbonates after Gypsum and Anhydrite. ACS Earth Space Chem. 2017, 1, 168178. 44. Fernández-Díaz, L.; Pina, C. M.; Astilleros, J. M.; Sánchez-Pastor, N. The Carbonatation of Gypsum: Pathways and Pseudomorph Formation. Am. Mineral. 2009, 94, 1223-1234.


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Figure 1. “Spiky” vs. smooth calcite crystals. SEM images of calcite crystals of two different orientations (a, b “lying down” vs. c, d “standing up”) reacted in (a, c) 5 mM Pb2+ solution having initial pH = 2.8 and in (b, d) Pb-free solution having initial pH = 2.8 for 1 h. Scale bars are both 1 μm on the inserted images in (a) and (c).

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Figure 2. TXM 3D reconstructed calcite crystal with micro pyramids. (a) 3D view of a reacted calcite rhomb in 5 mM Pb2+ solution having initial pH = 2.8 for 1 h. (b) A magnified view of the aligned micro pyramids highlighted by the yellow box in (a). (c) Cross-sectional view of the micro pyramids cut along the plane indicated by the dashed black line in (a). Dark boundary at the calciteair interface in (c) was due to a reconstruction artifact (See SI, Figure S4a), and not due to adsorption of Pb. (d) A calcite micro pyramid model based on TXM measurement composed of four lateral surfaces of calcite (-1 2 0), (1 0 -8), (0 1 8), and (-2 1 0) planes.



Figure 3. Formation of early stage micro pyramids on the (104) surface of a calcite micro-crystal. Images were recorded after 1 min of reaction in 5 mM Pb2+ solution of pH = 2.8. AFM amplitude images (up) and corresponding height profiles (down) measured along the horizontal dashed lines. The inserted optical image of the calcite in (a) marked the two spots where images (a) and (b) were scanned. Rhombic black boxes highlighted the shape of the etch pits and the red box highlighted the base plane of one micro pyramid.


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Figure 4. Lath-shaped and dendritic cerussite crystals. SEM images of (a) “lying down” and (c) “standing up” calcite crystals reacted with 5 mM Pb2+ solutions of initial pH = 2.8 for 5 h.



Figure 5. TXM data of a calcite crystal covered by a cerussite dendrite. Crystal was reacted in 5 mM Pb2+ solution of initial pH = 2.8 for 3 h. (a) Cross-sectional view shows the distribution of calcite (grey), cerussite (black) and interstitial pore space (white). (b, c) 3D views of the reconstructed calcite crystal (grey) covered partially by a growing cerussite dendrite (cyan). (d) 3D pore distribution and (e) its spatial relation with the cerussite dendrite.


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Figure 6. The formation of cerussite shells on calcite. Two different perspectives viewed from the side (a, c) and top (b, d). (a, b) 3D reconstructed structure of the cerussite shell (calcite core was not shown). (c, d) Cross-sectional views of the reacted crystal (cerussite in black and calcite in grey with pore space in-between). Scale bars represent 5 μm.



Figure 7. Schematic diagram shows three observed features during Pb-calcite interactions. I. Formation of micro pyramids. II. Nucleation of cerussite at the micro-environment located at the calcite/substrate interface. III. Mass transport in open, trapped and isolated pore spaces as moving away from the reaction front where growth of cerussite and dissolution of calcite occurred.


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Pb2+âCalcite Interactions under Far-from-Equilibrium Conditions

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Pb2+âCalcite Interactions under Far-from-Equilibrium Conditions