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CALCITE PASSIVATION BY GYPSUM: THE ROLE OF THE COOPERATIVE EFFECT Linda Pastero1,2,*, Roberto Giustetto1,2, Dino Aquilano1
1Department of Earth Sciences, University of Turin, via Valperga Caluso 35, 10125 Torino (Italy) 2NIS - Nanostructured Interfaces and Surfaces, via Quarello 15/A, 10135 Torino (Italy)
*Corresponding author: linda.pastero@unito.it
The overgrowth of gypsum (CaSO4·2H2O) on calcite (CaCO3), used as a substrate, has been
obtained in order to verify the hypothesis of a crystallographic fit between the two phases. Lattice coincidences show that many orientations of gypsum with respect to calcite can occur. Experimentally, only two among these are fulfilled during the early stages of growth, the selection criterion reasonably being the adhesion energy. Moreover, it has been shown that the presence of calcium carbonate as a specific surface impurity in solution modifies the growth habit of gypsum acting on the growth rate and surface morphology of some gypsum forms. At high concentration, the calcium carbonate in solution promotes the twinning of gypsum according to the Montmartre law.
INTRODUCTION
The passivation of calcium carbonate via sulfation raised deep interest during the years because of its implications in many fields, from conservation to medical, geochemical and environmental purposes 1–4.
Many efforts have been devoted to understand the sulfation mechanisms and rates in order to develop surface treatments for the prevention and slowdown of the decay of building materials and artworks.
Furthermore, special attention has been paid to the kinetics of the interfacial phenomena occurring during the dissolution of carbonate substrates and growth of gypsum. Compton and coworkers studied the dissolution kinetics of calcite in the presence of gypsum by the channel flow cell (CFC) method5–13. They experimentally showed that the direct surface reaction of hydrogen, in an acidic
environment (pH below 5-6) and under high convection regime, controls the dissolution kinetics of calcite. However, under slow mass transport, the passivation of the calcite substrate occurs. They adapted as well the CFC method to study the dissolution kinetics of a passivated calcite substrate, in order to demonstrate the effective reduction of the dissolution rate. Calcite cleaved substrates were passivated ex-situ, by using sulfuric acid at different concentrations and then the dissolution rates of
the substrate were measured in acidic conditions. These measurements confirmed the passivating effect of the gypsum overgrowth on calcite substrates.
Atanassova and coworkers 14 focused once again on the passivation kinetics of a carbonate substrate
coated by a layer of gypsum, discussing the role of the diffusion of calcium ions through the barrier represented by the gypsum layer overgrown on the substrate. They systematically showed the relationships occurring among the pH of the solution, the dissolution rate of the substrate and the quality of gypsum coverage and morphology, and distinguished between "needle-type" and "leaf-type" crystals, depending on the time of exposure and the concentration of the solution. We will discuss the occurrence of the two morphologies in details in the following paragraphs.
Only a few papers deal with the crystallographic relationships between the host (calcite) and guest (gypsum) phases. Starting from the alignment of gypsum along a defined direction of calcite, Booth and coworkers 8 suggested a “very specific interaction between the substrate and the
overgrowth”. In their work, they used freshly cleaved crystals or surfaces of calcite. They obtained a gypsum overgrowth by using a H2SO4/Li2SO4 solution and demonstrated that, during the
dissolution in the presence of sulfate species, the calcite substrate undergoes passivation due to gypsum overgrowth along a fixed direction on the host {10.4} calcite substrate. This preferred orientation was ascribed to an epitaxial constraint between gypsum and calcite due to the “fit of the cation-cation distance between the two phases”.
Recently, the overgrowth of gypsum, anhydrite, and bassanite on calcite has been carefully described15. In that work, isothermal reactions of calcite replacement by gypsum (pseudomorphs)
were carried out in PTFE lined reactors. The samples obtained were analyzed by 2D-XRD to confirm the strong structural control on the reaction of substitution. Moreover, pole figures describing the orientation of the calcite substrate and related calcium sulfates overgrowth were drawn and the calcite/gypsum epitaxial constraints were discussed in terms of Ca-Ca distances along the
⟨
´4 41⟩
and [001] PBCs belonging to calcite and gypsum, respectively.However, the cooperative effect between calcium carbonate and calcium sulfate during the dissolution of the former and the growth of the later has never been debated: our work can thus be placed in this lack of discussion.
According to the papers we already published on gypsum 16–21, we will adopt the de Jong-Bouman22
cell frame, as reported in Table 1. As concerns the calcite crystals, we used the well-known rhombohedral Space Group R ´3 c∧the reference frame: a0 = 4.9896 Å; c0 = 17.06 Å; ==90°,
=120°. Further, the shortened symbol {hk . l} will be used for the hexagonal system, hereinafter, instead of the extended one {hk ´i l}, where ´i = (h+k).
5.630 15.150 6.230 113.83 C2/c de Jong, Bouman (1938)22
5.670 15.201 6.274 113.91 A2/a Cole, Lancucki (1974)23
5.678 15.213 6.286 114.08 A2/a Hartman, Hejinen (1983)24
Table 1. Cell frames and Space Groups (S.G.) adopted in our previous papers for gypsum. MATERIALS AND METHODS
All reagents were analytical grade. Ultrapure water 18MΩ was obtained by using an Elga Purelab Flex3 water purification system. Calcite cleavage substrates were obtained from natural Iceland Spar single crystals. White Carrara and Prali silicate marbles were also used as substrates to check the effect of randomly oriented calcite crystals on the growth of gypsum crystals. In this paper, the attention will be focused on the growth of gypsum on cleaved Iceland Spar substrates, whereas the growth on marble substrates will be discussed in a forthcoming paper25.
Imaging was obtained by means of two scanning electron microscopes: i) Cambridge S360 (EHT 30 kV, working distance 5 mm, probe current 100 pA) and ii) JEOL JSM IT300LV (EHT 20 kV, working distance 10 mm).
Ex-situ atomic force microscopy was carried out in alternate contact mode, by using a DME SPM Microscope (DME Igloo) equipped with a DS95-50E scanner (scan volume 50x50x5µm). Data were acquired using MikroMasch Ultrasharp NSC16/Si3N4 Cr-Au back-coated cantilevers with a typical resonance frequency of 190 kHz, force constant 45 N/m, tip radius lower than 35nm and full tip cone angle of 40°.
The growth of gypsum on a cleaved calcite substrate
The {10.4 } freshly cleaved substrates were washed in ultrapure water 18MΩ and stored in a calcium carbonate saturated aqueous solution, kept at room temperature for at least one week to restore the structure of their original surfaces.
In sulfation experiments, some cleaved calcite crystals (5x5x1 mm) were passivated by dissolving calcite (cleaved Iceland spar and Carrara white marble or silicate marble) in diluted H2SO4
solutions, with concentration ranging from 0.01M to 1M. Gypsum crystals were generated at the interface between the substrate and the surrounding solution by the reaction between the calcite crystals and diluted sulfuric acid that produces gypsum and carbon dioxide:
CaCO3 + H2SO4 + H2O CaSO4 · 2H2O + CO2 (1)
The growth of gypsum crystals was obtained both in stagnant and stirred solutions to evaluate the effect of the concentration gradient of Ca2+ and CO
In stagnant solution, the first generation of gypsum crystals appears after a few seconds to a few hours of reaction, depending on the concentration of the acidic solution.
Irrespective of fluid-dynamic regime, a concentration gradient develops from the surface of the dissolving substrate toward the solution bulk. Actually, changing the stirring rate, the width of the gradient changes as well. For this reason, a couple of experiments was performed, with and without stirring, in order to establish the importance of the concentration gradient on the morphology and twinning frequency of gypsum. A rotating plate stirrer was used, operating at a constant rate of at 120 RPM for 5 minutes. The experiments were done using a 0,25 M solution of H2SO4 and crystals
of calcite prepared as described before.
To check the crystallographic constraints on the orientation of the gypsum crystals grown on calcite, an additional set of experiments was performed, following the procedure proposed by Booth and coworkers8. Host gypsum crystals were grown on the guest cleaved calcite substrates that have
been reacted for 60 s, without stirring, with a 0.1M HSO4-/1.98M Na2SO4 solution obtained by
mixing a 1.98M sodium sulfate solution with HCl (a 2M Li2SO4 solution was used, in the original
paper). We know, from our previous work26, that lithium is an effective modifier of the growth
morphology of calcite, since it works as a stabilizer of the {00.1} and the {01.8} forms, which are less stable with respect to the {10.4} cleavage rhombohedron. Hence, we modified the procedure by substituting Li2SO4 with Na2SO4. The HCl concentration ranged from 0.5 M to 9 M.
After 60 seconds of reaction under acidic conditions, the cleaved crystal was plunged into a calcium carbonate/calcium sulfate saturated solution to stop the reaction, so to avoid the sudden drying and the resulting formation of an amorphous layer on the sample (which could possibly hinder the imaging).
Some other overgrowth experiments on calcite substrate were performed by slowly evaporating a CaCO3/CaSO4·2H2O saturated solution, at room temperature. These trials were stopped after two
days before the complete drying of the solution was achieved.
The growth of gypsum crystals in the presence of calcium carbonate as a specific impurity in solution
The growth of gypsum in the presence of carbonate (and not used as a substrate) is mandatory to distinguish: i) the effect of the carbonate as a specific surface impurity on the growth morphology of gypsum from ii) the effect due to the high concentration of calcium and carbon dioxide at the interface with the spar. As a matter of fact, in this latter case, owing to the gradient of concentration at the interface, the supersaturation of the solution (with respect to gypsum) is high enough to allow both a sudden nucleation and very high growth rates.
In this set of experiments, a solution saturated with respect to both calcium carbonate and calcium sulfate was prepared by adding solid calcium sulfate di-hydrate (in amounts slightly exceeding the saturation in pure water) to a solution already saturated in calcium carbonate. Under these conditions, the calcium carbonate exceeding the solubility precipitated as calcite because of the common ion (Ca++) effect and the low solubility of calcite with respect to gypsum. This results in a
very low concentration of calcium carbonate (7.6 10-3 mM instead of 6.6 10-2 mM in a pure calcium
carbonate saturated solution) in a mixed calcium carbonate/calcium sulfate solution at the equilibrium. On the contrary, when working in dissolution, the system is completely out of equilibrium. The low pH value promotes the carbon dioxide as the most stable carbonate species and the concentration of calcium ions originated from the substrate is higher than in the solution at the equilibrium, depending on the sulfuric acid concentration. The surplus of solid calcium carbonate and calcium sulfate was kept at the bottom of the flask, so to ensure that saturation for both phases had been reached. Finally, the stored solution was equilibrated for some weeks at room temperature.
RESULTS AND DISCUSSION The growth of gypsum on a substrate.
In a stationary solution, the concentration of the sulfuric acid rules both the induction time of gypsum crystallization and the final coverage of the substrate. At high acidic concentration, a complete passivation of the substrate is reached in a very short time (e.g., a few minutes), even in a stagnant solution. Conversely, if the sulfuric acid concentration is low (≤ 0.01 M), passivation never reaches completion at the laboratory time scale, and the surface of the carbonate substrate undergoes to strong dissolution.
Stirring lowers the local supersaturation decreasing the gradient of concentration of calcium with respect to a pure diffusive regime. Accordingly, the number of nuclei of gypsum obtained but not the general gypsum crystal growth morphology.
As reported by many authors8,27, sulfation experiments do generate at least two different habits of
gypsum crystals (see Figure 1): i) large tabular and ii) needle-like ones.
Large tabular crystals, also described as “leaf-type” crystals14, are indeed gypsum penetration
(optically determined) twins (twinning plane ´1 01 and re-entrant twinning angle of 105.02°), known as “Montmartre twins”16,17,28. They grow near the interface with the substrate, though not
stuck to the substrate itself, where the highest supersaturation value with respect to gypsum is suddenly reached during the substrate dissolution (Figure1a-c).
Moreover, due to their flat morphology and extended coverage, the “leaf-type” crystals play a shielding role on the supersaturation, preventing the extent of the concentration gradient of calcium and carbon dioxide toward the volume of the solution. Because of this shielding effect, the habits of the successive generation of crystals can be compared with the needle-shaped ones obtained from pure calcium sulfate solution, with the well-developed {010 } form encompassed by small {120 } and
{
10 ´1}
forms, as shown in Figure 1d. This second generation of needle-like crystals frequently shows rounded apexes, which have to be related to the morphological instability of the{
10 ´1}
and vicinal forms due to the high growth rate along the c axis and – as we will discuss later – to the effect of calcium carbonate as a specific surface impurity. Sometimes, penetration twins with composition plane 100 and twinning angle of 105.02° 17,29 are also observed (theso-called “Swallow tail” twins). Generally, those crystals grown in the presence of a carbonate substrate at high supersaturation values, show the greatest finding frequency of Montmartre twins, if compared with samples of gypsum crystals grown from pure calcium sulfate solution.
Figure 1. a) A ´1 01 twin generated on the cleavage surface of a calcite crystal. A second generation of acicular prismatic crystals grows from the twin toward the solution. b) A ´1 01 twin lying on the calcite substrate. c) Gypsum twins usually grow apart from the substrate. In this case, gypsum grows on the edge of a calcite crystal, proving that the stabilization of this twin is not strictly ascribable to the effect of the substrate. d) Gypsum single crystals grown in pure solution showing the {010} , {120} and {´1 11} forms. e) Comparison between the ´1 01 and the
Ex-situ AFM measurements were performed on “Montmartre twins” to extensively investigate the domains formed near the twin plane. The contact plane of the twin is evidenced by a broken line, as shown in Figures 2a and S1c. The reentrant angle between the two individuals of the twin should be sealed during growth, but if the process is suddenly stopped, at the far end of the crystal such an angle persists, as shown in Figure 2a and c.
The height of the step ranges from 10 (7.6 nm thick) up to 70 monolayers, as shown in Figure 2b. Along the contact plane, the growth steps run in a quite regular succession. On the terraces, some elongated 2D-islands with rounded edges are found (Figure 2d). As reported in other papers30, no
evidence of growth spirals was found on the {010} form.
Gypsum twins grow by the juxtaposition of individuals developed along the c axis (Figure S1a, c). These crystals may twin in different positions, as shown in Figure S1a and b, thus generating the broken line described before; a very complex morphology may then arise from the repeated twinning. The “leaf-type” morphology is due to an incomplete sealing at the far ends of the crystals (Figure S1d).
F igure 2. Crystals of gypsum twinned following the ´1 01 “Montmartre law”. In (a) the twin plan follows a polygonal path and the reentrant angle at the far end of the crystal may not be sealed [see magnification in (c)]; the height of steps ranges from 7.6 nm (matching 10 d020 spacing) to 53 nm
(70 monolayers). Somewhere on the twinned (010) face, 2D islands are found, spreading along the surfaces of the single crystals composing the twin (d).
The increasing twinning percentage with the supersaturation is not surprising: in fact, during the dissolution of the substrate surrounded by a steady solution, a concentration gradient of the ionic species (Ca++ in particular) originates from the substrate and then decreases from the interface
toward the solution bulk. The highest concentration of ions will be placed near the surface where, in the presence of sulfate ions, the supersaturation with respect to gypsum reaches its maximum value. Accordingly, if the supersaturation increases, the corresponding value of activation energy lowers, both for 2D-nucleation and 3D formation of twins31–34.
For this reason, near the surface of the substrate the frequency of twinning reaches its maximum, generating the observed Montmartre twins. Moreover, the presence of large tabular crystals close to
the substrate decreases the diffusion rate of calcium from the surface toward the solution bulk and, consequently, the supersaturation value with respect to gypsum decreases. Under these new conditions, gypsum can grow slowly, so reducing the twinning frequency and showing its typical elongated prismatic habit.
It is worth to stress that in a pure environment and within the broad range of supersaturation values covered during the evaporation of a pure calcium sulfate solution, the most common twins of gypsum obey to the 100 law, whereas the ´1 01 twinning does not occur. Only when carbonate is present, the ´1 01 law dominates.
Finally, the flattening of gypsum habit is strongly enhanced in the presence of carbonate in solution. Growth of gypsum in the presence of low concentration of calcium carbonate as a specific impurity in solution
Reducing the concentration of the calcium carbonate in solution allows evaluating the effect of the impurity as a specific surface modifier of the growth habit of gypsum. In this way, one can distinguish the role of the impurity from the effect of the supersaturation.
Two sets of experiments were carried out:
i) growth of gypsum on a cleaved calcite crystal as a stable substrate (dissolution being avoided)
ii) growth of gypsum without a substrate.
The first set of experiments allows evaluating both the gypsum habit and orientation with respect to the substrate, whereas the second is devoted to the evaluation of the sole habit. In both cases, a solution saturated in both calcium carbonate and calcium sulfate was used, as described in the experimental section.
As confirmed by the experiments of evaporation on a stable carbonate substrate, during the early growth stages and at low impurity concentration, the “Swallow-tail twin” law dominates (Figure 3a, Figure 7c). Later, when supersaturation increases by evaporation, the “Montmartre twin” law appears, crossing the previous one (Figure 3b).
Figure 3. The first generation of “Swallow-tail” twins (a) is followed by a further generation of “Montmartre twins” (b). In the inset figure, the growth shape of the double twin is outlined.
In the absence of a substrate, the presence of calcium carbonate deeply modifies the growth morphology of gypsum. Crystals become squatter than in a pure calcium sulfate solution, suggesting that a lower growth rate along the c axis occurs, owing to the impurity effect on the
{
11 ´1}
prismatic form (Figure 4a). The effect of the calcium carbonate solution is even more unusual on the {010 } form (Figure 4), which starts to grow macroscopically, layer by layer (Figure 4b). Each layer appears as separated from the successive one by a discontinuity, running along the 100 plane. The whole morphology appears as to be built of piled-up sheets, and the crystals take on a “book-like” habit.This unstable behavior, however, is fixed during growth, as one can observe in Figure 4b (white arrow): the facet, probably the (120) , encompassing the “book-like” structure progressively grows, healing up the grooves generated by the far ends of the piled-up macro-layers.
Figure 4. Gypsum crystals grown in the presence of calcium carbonate in solution show incomplete, rounded ends (a), instead of the
{
1 ´1 1}
form (which typically complete the growth morphology of gypsum grown in a pure environment; see Figure 1d). Besides, some crystals show a peculiar look, with sort of “bearded-edges” replacing the {120} -flat F form (b, c and d)In order to prove the effect of the presence of calcium carbonate on the {010 } form of gypsum, a natural centimetric crystal of selenite was cleaved. The two corresponding specular surfaces were plunged into a gypsum saturated solution (Figure 5a and c) and a gypsum + calcite saturated solution (Figure 5b and d), respectively. After a few days, the two surfaces show different behaviors: the one regenerated in pure gypsum solution is macroscopically striated with a nacreous look, whereas the one regenerated in gypsum + calcite saturated solution is macroscopically flat and polished. This behavior reflects the shape of the nuclei on the {010 } form. In the “pure” case, in fact, the nuclei have an extremely elongated shape, usually reported in the literature30, whereas in
the presence of calcium carbonate as an impurity, the nuclei on the surface become shorter – as already stressed for the entire morphology.
Moreover, the
[
´10 ´1]
edges between the{
´111}
and {010 } forms bent, proving the loss of stability of the{
´111}
form already observed in Figure 4a.At the AFM scale, the change of surface morphology and roughness is marked, as shown in Figure 5c and d. In a pure calcium sulfate solution, the surface is made up of units surrounded by perfect and straight edges, elongated along the c axis of the crystal. Changing the composition of the solution by adding calcium carbonate leads to shorter and broadened islands, with rounded and blurry edges.
Figure 5. SEM and AFM images of a gypsum crystal cleaved and regenerated in pure calcium sulfate solution (a and c) and in the presence of calcium carbonate (b and d). The surface behavior is clearly different: narrow 3D nuclei along the crystal occur in the case of the pure gypsum solution and short, whereas broad islands of a few monolayers of height appear when the doped solution is used.
The epitaxial constraint between Gypsum (010) and Calcite (10.4)
As mentioned above, great attention has been paid to the oriented growth of gypsum on a substrate of cleaved calcite crystal. In Table S1, we reported the coincidence cells between the two forms, having taken into account the coupling of the A2 rotation axis perpendicular to the {010}
The 2D-rectangular cell describing the 10. 4 calcite lattice is limited by the vectors: [010] = 4.989 and 13 [42 ´1] = 8.103 Å. Thus, the repeat period along the edge of the (10.4) face is 1
3 [ ´4 41] = 12.85 Å. The 2D-cell describing the 010 gypsum lattice is limited by the vectors: [100 ] = 5.63 and [001] = 6.23Å, the angle comprised within them being 113. 91°. Then, the 2D coincidence lattices needed for the (10.4) calcite / (010) gypsum epitaxy, should be
found by mutually rotating the two just quoted lattices around a common lattice point (assumed as a fixed origin).
In Figure 6 and in Table 2 an example is given of a lattice coincidence. The most sensible 2D-coincidence cells are listed in Table S1, following the order of the increasing value of their common area. When the percent misfit between the areas of the proposed coincidence cells exceeds the value of 10%, the quality of the lattice coincidence is bad, and the corresponding probability of epitaxy becomes very low. Accordingly, when the areas of the 2D coincidence cell are very large (i.e. more than 500 Å2), the epitaxy probability should be low as well. Hence, such coincidences will not be
considered in Table S1.
Figure 6. The smallest 2D-coincidence cell (red straight lines) between the 010 lattice plane of gypsum (grey dots) and the 10.4 lattice plane of calcite (black dots).
Host crystal (calcite) form {10.4} Guest crystal (gypsum) form {010} Misfit (%) Notes
Cell vectors Cell vectors ¿
1
3¿ 4 ´1 ´1 ] =
9.516
3[010] = 14.969 [301] =15.4615 +3.29 Area (Å2) and 2D-cell
multiplicity 121.292 (3) 128.339 (4) +5.81
Linear misfits in the same sense Table 2. Lattice vectors limiting the smallest 2D-coincidence cell between 010 gypsum and calcite 10. 4 planes, as drawn in Figure 5. The percent misfit between gypsum and calcite comparable vectors is indicated along with the misfit between the corresponding areas of the respective 2D-cells.
All the possible orientations of a crystal of gypsum adhering with its (010) face to a (10.4) face of calcite (as follows from Table S1), are drawn in Figure 7a. If the calcium sulfate growth does not stop at the early stages, the combination of all possible orientations of a gypsum crystal on a cleavage rhombohedron of calcite, added up to the twinning of gypsum, may generate a very complex morphology, similar to that of a spherulite. It is rather evident that, if the process is not interrupted just at its beginning, the final aspect of such an overgrowth can be quite confusing, and not easily ascribable to any ordered reciprocal arrangement.
As shown in Figure 7b and c, following the procedure proposed by Booth in 19978 and modified as
described in a previous section, we obtained two lineups of the seeds of gypsum on calcite. These lineups are oriented along the two equivalent directions,
[
´4 41]
and[
48 ´1]
, which represent the edges of the cleavage rhombohedron of calcite. These directions are experimentally revealed by the edges of the etch pits (arrows on AFM images in Figure 7b). This result complies with the results previously obtained by other authors8,15.Figure 7. a) Graphical representation of all possible orientations – between 0 and 180° – of a gypsum crystal, leaning on its (010) face, on the (10.4) face of calcite (derived from Table S1). In grey, the gypsum crystals aligned along the equivalent directions
[
´4 41]
and[
48 ´1]
of the calcite cleavage rhombohedron are represented. From b) to d): AFM and SEM images of gypsum nuclei aligned along the edges of the cleavage rhombohedron of calcite (white background). In c), the nuclei are twinned according to the 100 law.The seeds of gypsum shown in Figure 7 are one monolayer (7.6 Å) thick and 100 to 300 nm long. Later, new crystals grow on the same gypsum seeds, oriented following some of the directions represented in Figure 5. The growth structure, however, becomes quite complicated.
Changing the growth conditions and working in the presence of a stable substrate, we obtained the “Swallow-tail” twinned crystals previously described. These twins are undeniably iso-oriented along one or both of the equivalent edges of the calcite cleavage rhombohedron (Figure 7c and d). In summary, the growth morphology of gypsum obtained from a solution containing calcium carbonate as a specific impurity, changes according to the calcium carbonate concentration.
i) During a reaction of sulfation of a calcium carbonate substrate under strongly acidic conditions, the concentration of calcium carbonate in solution may reach very high values, following a gradient spreading from the surface of the crystal through the solution bulk. In this case, in the early stages of gypsum growth, the primeval seeds nucleate on the substrate and grow along the equivalent edges of the cleavage calcite rhombohedron. We confirmed and extended the findings of other authors about the epitaxial constraints exerted by calcite (the host phase) on the growth of gypsum (the guest phase). When considering all feasible geometrical coincidences (listed above), only the ones aligned with the directions
[
´4 41]
and[
48 ´1]
are verified in highly perturbed systems, suggesting a strong variance in the values of the adhesion energy among all possible orientations. After a while, depending on the concentration of the sulfuric acid, a coverage of gypsum crystals will form on the surface of the substrate. These gypsum crystals are flattened, with respect to those grown from pure calcium sulfate solution, and tend to grow layer by layer, frequently forming twins that follow the ´1 01 law (“Montmartre twins”).ii) At low concentration of calcium carbonate in solution, in the presence of calcite as a substrate, the early seeds are formed by large {010 } forms and subordinated {120 } and {11 ´1 } forms, laying down the {10.4 } rhombohedron of calcite. These crystals regularly twin following the 100 law (“Swallow-tail twins”). Later during growth, new twins may form, intersecting the previous ones.
iii) At low concentration, but without a carbonate substrate, the action of the impurity focuses on the { 010 } form of gypsum. This effect may be ascribed to the temporary formation of an epitaxial layer of calcite adsorbed on the cleavage surfaces of gypsum, thus generating the “book-like” morphology described above. Such a layer will not be absorbed because of the large misfit between the d10.4 spacing of calcite and d020 of
gypsum; consequently, no anomalous mixed crystals (as described by literature26,35,36)
will form. The temporary adsorption is efficient enough to modify the growth habit of gypsum.
These results confirmed that the surface of calcite exerts a strong crystallographic control over the orientation of gypsum crystals. The epitaxial agreement between the two phases is also confirmed by the changes in the growth morphology of gypsum in the presence of carbonate containing solutions.
If the concentration of calcium carbonate in solution increases, the twin law switches from 100 to ´1 01 . The 100 twinning mainly occurs in gypsum crystals grown in a pure environment. On a stable calcite substrate, this twinning becomes ubiquitous, with the {010 } form of gypsum
sticking on the {10.4 } form of calcite (Figure 3 and Figure 7). In the presence of high concentration of calcium carbonate in an unsettled solution, the twinning law turns to the ´1 01 . The two twinning laws show the same re-entrant twin angle,17 but differ in the mutual orientation of
the individuals. From the results of our experiments, we may hypothesize a double effect of the calcium carbonate on the twinning of gypsum.
i) At low concentration of calcium carbonate in the presence of calcite as a substrate, the number of 100 twins rises surprisingly, possibly induced by the good lattice coincidences between the two phases along the equivalent directions of the calcite (Figure 8a). In fact the
[
´4 41]
and[
48 ´1]
directions, which coincide with the edges of the cleavage calcite rhombohedron, form an angle of 101.9° – very close to that of the twin (105.02°). Moreover, the outmost double layer of water on the {010 } surfaces of calcite can easily adapt to compensate this small angular misfit.Figure 8. a) 100 twin of gypsum (gray) on a calcite cleavage rhombohedron as a substrate (white background). The dotted arrows represent the two equivalent directions
[
´4 41]
and[
48 ´1]
of calcite. b) a double twinned crystal. The ´1 01 twin doesn’t match with the two equivalent directions of calcite.ii) At high concentration of calcium carbonate, the occurrence frequency of ´1 01 twins increases with respect to the 100 law. In some cases, this may be related to the same induction effect of the symmetry on the twinning, as previously described for the 100 twin and reported in other papers15. Is worth noting that the ´1 01 twins rarely adhere to the
Moreover, in the case of the double twinning, the reciprocal orientation of the individuals is not compatible with a constraint effect of the same substrate for both twins, as shown in Figure 8b. This commits us to evaluate other possibilities than the symmetry constraint. Comparing the surface lattices of the most stable form of calcite (the cleavage rhombohedron) with the surface lattices of the 100 and ´1 01 planes (the “Swallow-tail” and “Montmartre” twinning plane, respectively), we found many excellent coincidence lattices in both cases. Furthermore, in the case of the Montmartre twin, the misfit between the surface cells is very low. The calcium carbonate in solution may act as a stabilizer of the twin composition faces, in particular of the ( ´1 01 ) twin face, and this may be the reason for the increase in the occurrence frequency of this twin in the presence of calcium carbonate in solution. This hypothesis, along with the new coincidence lattices, will be discussed in depth in a forthcoming paper.
CONCLUSIONS
In our experiments, a double role of calcium carbonate in a gypsum/calcite system can be observed: i) on one hand, calcite acts as a substrate for the nucleation and growth of gypsum and ii) on the other hand, calcium carbonate as a specific impurity in solution alters the growth habit of gypsum. From the overgrowth experiments, we confirmed the crystallographic control exerted by calcite as a substrate on the orientation of the overgrowing crystals of gypsum. Experimentally, we found two preferred orientations of gypsum, running along the
[
´4 41]
and the[
48 ´1]
directions of the cleaved surfaces of a calcite crystal. Theoretically, the possible orientations are much more than two, as demonstrated in the previous paragraphs. To explain this selection, the host/guest adhesion energy can be invoked.The calcium carbonate as a specific impurity in solution destabilizes the
{
´111}
form. That form is replaced by a rounded surface representing the convolution of all the vicinal forms, meaning that the surface energies and the relative normal growth rates are changed by the effect of the impurity. The crystals grown in the presence of carbonate become squat. Moreover, the presence of calcium carbonate in solution changes the surface behavior of the {010} cleavage form of gypsum. Both the preferred orientation of growth and the surface modification can be ascribed to what we described in our previous works as a “cooperative effect” between phases during growth ordissolution. This effect is not peculiar of the system calcite/gypsum but recurs each time a suitable surface parametric agreement between phases sets up, whatever the saturation of the guest phase. This synergy between phases may induce as well the formation of an “anomalous mixed crystal” as in the case of lithium carbonate and calcite for example 26,35. If the conditions for the formation of an
“anomalous mixed crystal” are not fulfilled, the effect of the host phase (calcite) on the guest phase (gypsum) are limited to kinetic effects on the stability and relative growth rates of crystal forms, as in the case of lead chloride and potassium chloride (sylvite)37 and, of course, of calcium carbonate
and gypsum .
As expected from the classical nucleation theory, the twinning frequency of gypsum depends on the supersaturation of the solution with respect to gypsum. We found a correlation between the concentration of calcium carbonate in solution and the prevailing twin. As the concentration of calcium carbonate in solution increases, the twinning law switches from the 100 to the ´1 01 . This result may be explained in terms of stabilization of the the original composition face of the twin by the presence of calcium carbonate as a specific impurity. This mechanism may promote the appearance of the Montmartre twins in the presence of high concentrations of carbonate and will be discussed in detail in a forthcoming paper.
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