Reaction pathways and textural aspects of the replacement of anhydrite by calcite at 25 °C

Reaction pathways in anhydrite carbonation

The carbonation reaction begins on anhydrite surfaces immediately after the crystals are immersed into the 0.5 M Na₂CO₃ aqueous solution. GXRD data (Fig. 1) confirm calcite as the main component forming on the outer layer of the reacted anhydrite samples from the very early stages of the carbonation process. This was irrespective the anhydrite surface considered. This reaction seems to progress via the dissolution of anhydrite and the simultaneous formation of calcite crystals. Evidences of both processes can be clearly detected on the images of anhydrite surfaces that even after 1 min are covered by small calcite crystals (Fig. 2a).

Figure 1

GXRD patterns obtained after the interaction of anhydrite (001) cleavage surfaces with 0.5 M Na₂CO₃ aqueous solution during (a) 1 h, (b) 3 h, and (c) 1 day. The main (hkl) reflections for (+) anhydrite (ANH) and (*) calcite (cc) are indicated. All other minor peaks correspond to either calcite or anhydrite.

Figure 2

SEM photomicrographs of an anhydrite (100) surface after reaction with a 0.5 M Na₂CO₃ aqueous solutions for (a) 1 min, (b) 30 min, (c) 5 h, and (d) 7 days. Note the progressive increase in both, the degree of coverage of the anhydrite surface and the size of the growing calcite crystals.

Although at the very beginning calcite nuclei are small and scarce (Fig. 2a), their density and sizes rapidly increase on all considered anhydrite surfaces during the first 30 min of interaction (Fig. 2b). The newly formed calcite crystals were not randomly distributed on the anhydrite surface but they were spatially related to etch pits or step edges (Fig. 2b) and their size increases with increasing interaction time: after 1 min the size of calcite crystals varied between ∼0.2–0.5 µm and it reached ∼12 µm after 1 week (Fig. 2d). The progress of the interaction leads to a complete coverage of the anhydrite surface after a few hours (∼5 h) (Fig. 2c). Calcite crystals show the typical rhombohedral habit, with flat (104) faces and sharp edges and no modification in the general characteristics of the rhombohedral habit of calcite were observed.

Significant differences in the extent of coverage by CaCO₃ phases and the pathway of carbonation were detected at the early stages of the reaction on different anhydrite surfaces. For example, after 1 h of reaction, the coverage was almost complete on the anhydrite (001) surface (Fig. 3a), while on the (100) surface it reached ∼33% (Fig. 3b) and on the (010) surface ∼47% only (Fig. 3c). In addition, both calcite and vaterite were present on the anhydrite surfaces. However, the amount of vaterite observed coexisting with calcite at the early stages of reaction also varies depending on the anhydrite surface considered. Numerous vaterite aggregates could be distinguished on anhydrite (010) and (001) surfaces, while these were scarce on the (100) surface (Figs. 3d, 3e, and 3f). GXRD patterns collected on reacted samples 2 h after the beginning of experiments evidence that irrespective the anhydrite surface studied calcite was the most abundant CaCO₃ polymorph. Weak reflections that could be assigned to vaterite were distinguishable in the GXRD pattern collected from (001) anhydrite surfaces (Fig. 4a). These reflections are extremely weak in the GXRD pattern collected on the (010) surface and absent on the patterns obtained from the (100) anhydrite surface (Figs. 4b and 4c). These results were also confirmed by the scarcity observed of vaterite aggregates in SEM images of each surface (Figs. 3a and 3d). Semi-quatitative estimates derived from GXDR patterns using the Reference Intensity Ratio (RIR) method (Snyder 1992) yielded vaterite percentages in the carbonate layer after 2 h reaction below 1% on the (010) and ∼1.5% on the (001) surface, respectively. After 15 h of reaction the GXRD patterns revealed the presence of only reflections that could be assigned to calcite and this was irrespective the anhydrite cleavage surface considered (e.g., Fig. 1c).

Figure 3

SEM photomicrographs showing anhydrite (001), (100), and (010) cleaved surfaces partially covered by calcite and vaterite crystals after 1 h of interaction with Na₂CO₃-aqueous solutions (a–f). White arrows in photomicrograph d are pointing out some vaterite aggregates.

Figure 4

(a) GXRD patterns taken on anhydrite (001), (100), and (010) cleaved surfaces after 1 h of interaction with Na₂CO₃ aqueous solutions. The main (hkl) reflections for (*) calcite and (o) vaterite are indicated. In plots in b, c, and d, the vertical scale has been enlarged to more clearly show vaterite reflections in the diagrams taken on (001), (100), and (010) surfaces, respectively. SEM photomicrographs of these surfaces are shown in Figure 3.

Textural characteristics of calcite pseudomorphs after anhydrite

SEM photomicrographs reveal that calcite crystals appear to share a mutual orientation with the initial anhydrite, suggesting epitactic growth on each anhydrite surface (Fig. 2). We focus from hereon on the observations on the anhydrite (100) surface. Similar features are also observed on (010) and (001). On (100) this epitactic relationship becomes apparent after 1 min of interaction (Fig. 2a) and, at least locally, remains well defined after prolonged interaction times (7 days; Fig. 2d).

The original external shape of the anhydrite crystals as well as fine details of their surfaces including their cleavage steps are preserved during the carbonation reaction (Fig. 5a). Calcite pseudomorphs after anhydrite are bounded by well-defined faces and sharp edges (Fig. 5a). The backscatter electron (BSE) micrograph in Figure 5b shows a section of an anhydrite crystal partially transformed into CaCO₃ after 3 days of reaction. Differences in contrast between the CaSO₄ and CaCO₃ phase evidence the extent of the transformation. It is apparent that the carbonation reaction progresses from outward to inward of the anhydrite crystal and a well-defined reaction front developed. Eventually, the reaction progresses and finishes with the formation of pseudomorphs exclusively consisting of calcite (Fig. 5a). The replaced layer consists of calcite crystals, which at least locally were oriented with respect to the original anhydrite surface (Figs. 5c and 5d). A gradual increase in the size of the formed calcite crystals is also apparent, with those closer to the interface with the retreating anhydrite surface being significantly larger (mean size 17 ± 2.5 µm) than those on the outer rim (mean size 2.2 ± 0.8 µm). The contact between the replaced layer and the retreating anhydrite surface is sharp and no significant gap was observed at the interface. The replaced layer armoring the original anhydrite shows a homogeneous aspect, yet the carbonate layer also shows an evenly distributed porosity consisting of small pores located at the contact between slightly misoriented calcite crystals or between calcite crystals that show different orientations with respect to anhydrite (Fig. 5d).

Figure 5

(a) SEM photomicrograph of an anhydrite surface fully replaced by calcite after 24 h interaction with a 0.5 M Na₂CO₃ aqueous solution. (b) BSE photomicrograph of a section of an anhydrite (ANH) crystal partially replaced by calcite (CC) after 3 days in contact with a 0.5 M Na₂CO₃ aqueous solution; the rectangle indicates the same area shown in c. (c) BSE photomicrograph of calcite (cc) at the carbonate-anhydrite (ANH) interface. The edge of several calcite crystals have been outlined to highlight their common orientation. (d) SEM photomicrograph of the carbonate-anhydrite interface revealing the porosity associated with the replacement process. The image was acquired from a polished section of a partially replaced anhydrite crystal reacted for three days with a 0.5 M Na₂CO₃ aqueous solution

Reaction kinetics of anhydrite carbonation

The change in the width of the transformed region over time as obtained from evaluating SEM images is show in Figure 6. The thickness of the carbonate layer increase linearly with time, indicating no change in the transformation rate as the thickness of the carbonate layer (coating the anhydrite core) increases. It takes approximately 14 days for the whole replacement of an anhydrite single crystal to occur (crystal volume ≈ 6 mm³).

Figure 6

Linear increase in thickness of the transformed layer as a function of time. Thickness values correspond to the average of a minimum of 20 measurements on 3 sections of partially replaced anhydrite crystals at each time step. The error bars corresponds to the error of measurement.

Reaction pathways in anhydrite carbonation vs. gypsum carbonation

The interaction between anhydrite (100) surface and a 0.5 M Na₂CO₃ aqueous solution involves the formation of the CaCO₃ polymorph calcite since the very early stages of the reaction, with vaterite appearing only as a very minor phase that readily transforms into calcite after a few hours. The vaterite to calcite transformation is well known to proceed via dissolution and reprecipitation specially in the presence of sulfate (Fernández-Díaz et al. 2010; Rodriguez-Blanco et al. 2011; Bots et al. 2012). This relatively simple reaction pathway contrast with the more complex sequence of dissolution-crystallization reactions observed during the carbonation of gypsum under identical experimental conditions (Fernández-Díaz et al. 2009). In the case of gypsum carbonation the reaction started with the formation of a thin layer of amorphous calcium carbonate (ACC) within the first minutes of the replacement process. This early ACC layer readily transformed to a mixture of vaterite and calcite. Furthermore, both crystalline carbonate polymorphs were still present after the carbonation reaction had finished, although the proportion of vaterite continued to decrease as a result of its solvent mediated transformation into calcite and after 72 h of reaction only calcite was detected.

Two main factors can explain the different reaction pathways observed in the carbonation of both calcium sulfate phases. The first factor and, probably, the most important one is the existence or absence of epitactic relationships between the calcite overgrowth and the calcium sulfate substrate. Whereas the overgrowth clearly is epitactic on anhydrite (100) surface (Figs. 2 and 3), no evident epitactic relationship were observed when calcite crystals initially form on gypsum (010) surfaces (Fernandez-Diaz et al. 2009). Indeed, during the carbonation of gypsum calcite crystals first nucleate randomly oriented within the thin ACC layer that carpeted the initial gypsum (010) surface after the first minutes of reaction. Only later did the calcite crystals become oriented with their 𝒸 axis perpendicular to the retreating (010) gypsum surface as their growth progressed. The formation of metastable phases like ACC and vaterite at the early stages of the interaction in the case of gypsum carbonation was the consequence of the predominance of kinetic factors over thermodynamics. This predominance is associated with the energy barrier that needs to be overcome for the heterogeneous nucleation of calcite on the surface of a calcium sulfate mineral to occur. This energy barrier significantly decreases when heterogeneous nucleation has an epitactic character, as it is the case for calcite forming on anhydrite (100) surface (Chernov 1984). The difference in complexity of the reaction pathways operating during the interaction between a carbonate-bearing aqueous solution and anhydrite or gypsum highlights the importance of crystallographic relationships in defining mineralogical evolutions during mineral replacement processes. A significant influence of crystallographic relationships in the development of mineral replacement processes has been found in several systems. For example, Xia et al. (2009a) showed that the similarity of the crystal structures of pentlandite, (Fe, Ni)₉S₈, and violarite, (NiFe)₃S₄, which facilitates the epitactic nucleation, promotes the replacement of petlandite by violarite under conditions at which other sulfides ([e.g., vaesite (NiS₂) and polydymite (Ni₃S₄)] are thermodynamically more stable than violarite. Similarly, Qian et al. (2011) concluded that the replacement of pyrrhotite by marcasite instead of pyrite under low-temperature hydrothermal conditions is facilitated by the higher crystallographic similarity between the S lattices in pyrrhotite and marcasite structures.

The second factor that might influence the different reaction pathways observed during the carbonation reaction is the differences in dissolution kinetics of gypsum vs. anhydrite. Bulk experiments revealed that anhydrite dissolves around two orders of magnitude slower than gypsum (Jeschke et al. 2001; Jeschke and Draybrodt 2002). Similarly, macroscopic dissolution measurements conducted on anhydrite (010) and gypsum (010) surfaces yielded dissolution rate constants (k) of 2.6 and 46 ± 6 µmol m⁻²s⁻¹, respectively (Pachon-Rodriguez and Colombani 2013). Based on atomic force microscopic (AFM) studies much faster dissolution kinetics for gypsum than for anhydrite were also confirmed (Bosbach et al. 1995; Shindo et al. 1996). Consequently, supersaturation of the aqueous solution at the interface with respect to CaCO₃ phases increases much more rapidly during interaction with gypsum (010) surfaces than with anhydrite (100) surfaces, thereby explaining the formation of metastable ACC as well as higher amounts of vaterite during the early stages of the carbonation of gypsum compared to the total lack of ACC and only minor vaterite for anhydrite carbonation.

The influence of the different dissolution kinetics for each face could also explain the slightly different mineralogical pathways observed on anhydrite (010) and (001) surfaces in comparison to observed on anhydrite (100) surface. While only calcite is detected on (100) anhydrite surface by GXRD at any reaction step, SEM observations evidence the formation of small amounts of vaterite together to calcite at early stages of the interaction on both (010) and (001) surfaces, being these amounts larger in the later. AFM studies by Shindo et al. (2010) showed that anhydrite (001) dissolves at much faster rate than (100) and (010), which are similarly reactive. This means that Ca²⁺ ions are released to the solution at a faster rate from (001), leading to larger departures from equilibrium in the layer of aqueous solution close to this surface. This can explain both the formation of higher percentages of vaterite and its preservation during longer times as a main component of the carbonate layer on (001) anhydrite cleavage surfaces.

Textural characteristics of calcite pseudomorphs after anhydrite vs. calcite pseudomorphs after gypsum

Calcite pseudomorphs after gypsum are characterized by a thin, fine-grained rim (15–20 µm) of randomly oriented calcite crystals and a core of parallel columns of calcite crystals in a stockade-like arrangement (Fernández-Díaz et al. 2009). This textural feature is the consequence of an initial nucleation of randomly oriented calcite crystals on gypsum (010). Due to the faster calcite growth along the [001] direction, differently oriented crystals compete for space as they grow. Only those calcite crystals whose c-axis is perpendicular to the original gypsum surface are able to grow without interfering with other crystals. As a result, the progress of the replacement leads to the development of parallel column-like calcite crystals elongated along [001], which grow perpendicular to the retreating gypsum surface.

Stockade-like textures are absent in calcite pseudomorphs after anhydrite. As explained above, calcite nucleation on anhydrite cleavage surfaces is epitactic. In all the cases, the first calcite crystals to form have one of their (104) surfaces laying on the corresponding anhydrite cleavage surface. As all these calcite crystals are equally oriented on anhydrite surface, competitive growth between them does not occur. On the contrary, the existence of epitactic relationships promotes their growth parallel to the anhydrite surface (Sunagawa 1987), which leads to their coalescence and the formation of a compact calcite layer that shields the anhydrite.

Further textural differences between gypsum and anhydrite result from the very different change in molar volume involved in the carbonation of these two calcium sulfate phases. Both gypsum and anhydrite have larger molar volumes than calcite. This means that in both cases carbonation is accompanied by a negative volume change. Consequently for a pseudomorphic process this volume change needs to be balanced by the generation of porosity. This is indeed the case in the replacement of anhydrite by calcite, which involves a volume change of ∼−9.2 cm³/mol; ∼20% (Smyth and McCormick 1995). The porosity generated to compensate this volume change is confined within the transformed layer and shows a basically homogeneous distribution during the early stages of the carbonation process. The difference between the molar volumes of gypsum and calcite is significantly larger (∼−37.6 cm³/mol; >50%) (Smyth and McCormick 1995), which determines that the volume change associated to the carbonation reaction can only be partially compensated by the generation of porosity. Part of this volume change is compensated by the formation of a gap between the replaced layer and the retreating surface of gypsum. The size of this gap increases as the transformation advances. The formation of similar gaps at the interface between primary and secondary phases has been observed during replacement processes, which either involve large volume changes, i.e., replacement of pyrochlore as a result of hydrothermal alteration (Pöml et al. 2007), or are kinetically limited by the precipitation of the secondary phase, i.e., pentlandite, (Fe, Ni)₉S₈, by violarite (NiFe)₃S₄ (Xia et al. 2009b). The development of such a gap during the carbonation of gypsum determines the loss of adhesion between the retreating gypsum surface and the replaced layer and eventually leads to the formation of a hollow core within calcite pseudomorphs after gypsum. This, together to the larger amount of porosity formed, confers these pseudomorphs with a much more fragile nature than calcite pseudomorphs formed after anhydrite. In nature the transformation of anhydrite to calcite generates reduction of volume that may cause the development of porosity or deformation in the sediment layers. For example: the nodules of sulfur of the Messinian of Cyprus (Rouchy et al. 1994), and the sulfate evaporates in the Middle Miocene (Pierre and Rouchy 1988) in the Gulf of Suez are associated with abnormally deformed secondary carbonates formation associated to sulfate evaporates in the Middle Miocene (Pierre and Rouchy 1988).

Reaction kinetics of anhydrite carbonation vs. gypsum carbonation

The carbonation of anhydrite occurs at much lower rate than the carbonation of gypsum. For example, a gypsum single crystal with volume ≈ 6 mm³ is completely replaced by CaCO₃ after being in contact with a 0.5 M Na₂CO₃ aqueous solution during 1 day (Fernández-Díaz et al. 2009). In contrast, the whole replacement of an anhydrite single crystal of similar volume takes approximately 14 days. This much slower kinetics of anhydrite carbonation is a consequence of two factors. The first factor is the different dissolution kinetics of gypsum and anhydrite. Since the solution layer closer to the calcium sulfate mineral surface becomes rapidly supersaturated with respect to any CaCO₃ polymorphs, CaCO₃ precipitation occurs immediately. Therefore, dissolution of the calcium sulfate mineral is the rate determining step of the carbonation process. Consequently, the much slower dissolution of anhydrite in comparison to gypsum can explain the much slower anhydrite carbonation. The second factor that can contribute to make anhydrite carbonation a slower process is the significantly smaller volume of porosity that is generated during the former reaction. For the carbonation process to proceed, a permanent communication between the interface where the reaction occurs and the bulk solution is required. This communication through the formed porosity guarantees continuous supply of CO32− ions toward the calcium sulfate surface and those can react with the Ca²⁺ released to the fluid as a result of mineral dissolution. A calcium carbonate layer with a significantly smaller amount of porosity will decrease the diffusion of CO32− toward the surface of the calcium sulfate due to partial armoring of the primary phase, anhydrite in this case. This leads then to a slower carbonation process.

It is interesting that, although anhydrite carbonation occurs at a much slower rate than that of gypsum, its rate remains constant until the process is complete and anhydrite single crystals are fully transformed into aggregates of small calcite crystals. On the contrary, gypsum carbonation rate is very high during the early stages of the process but it significantly slows down at later stages. Fernández-Díaz et al. 2009 explained this change in the gypsum carbonation kinetics as due to three main causes: (1) the formation of a thick layer coating the gypsum surface that prevent percolation of the fluid to the interface, (2) the depletion of CO32− in the aqueous solution due to the formation of CaCO₃ phases, and (3) the progressive increase of the SO42−/CO32− ratio in the aqueous solution at the interface. However, this explanation is not in good agreement with the observation that anhydrite carbonation rate remains constant during the whole process. Indeed, the formation of a thick layer of CaCO₃, which is significantly less porous than that formed during the carbonation of gypsum, should lead to a clearer slow down of the reaction in the case of anhydrite. The chemical composition of the sulfate phases can also play a decisive role. During gypsum dissolution two molecules of water for each Ca²⁺ and SO42− ions are being released to the aqueous solution confined into the gap developed between the replaced layer and the retreating surface of gypsum. This has a dilution effect that leads to a reduction of the supersaturation with respect to any CaCO₃ phase at interface. As a result, the carbonation reaction will temporally slow down or even stop until a high enough amount of CO32− ions diffuse to the aqueous solution filling the gap between the carbonate layer and the gypsum surface. Since the communication between the carbonate-bearing bulk fluid and the gypsum surfaces becomes progressively more difficult as the carbonate layer becomes thicker, the slow down of the carbonation reaction becomes apparent as the process progresses. In contrast, the dissolution of anhydrite is not accompanied by any dilution effect, which can, at least partially, explain that anhydrite carbonation proceeds at approximately constant rate, even though the volume of porosity generated during anhydrite carbonation is much smaller than during gypsum carbonation. Qualifying the influence of factors like the textural characteristics of the porosity distribution on the kinetics of the pseudomorphic carbonation of calcium sulfate phases will be the object of further investigation.