α-Quartz Epitaxial Nucleation and Growth

Witherite (BaCO3)/r-Quartz Epitaxial Nucleation and Growth: Experimental Findings and Theoretical Implications on Biomineralization Erica Bittarello, Francesco Roberto Massaro, Marco Rubbo, Emanuele Costa, and Dino Aquilano*

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 971–977

Dipartimento di Scienze Mineralogiche e Petrologiche, UniVersita` degli Studi di Torino, Via Valperga Caluso 35, I - 10125 Torino, Italy ReceiVed July 18, 2008; ReVised Manuscript ReceiVed NoVember 3, 2008

ABSTRACT: Witherite (BaCO3) crystals have been synthesized by mixing BaCl2 · 2H2O and NaHCO3 aqueous solutions; their growth morphology is characterized by single and twinned individuals both [001] elongated and limited by {110} prism and {hhl} bipyramids. When quartz single crystals are used as substrates, heterogeneous nucleation of witherite occurs onto the {101j0}, {101j2}, and {011j2} forms of quartz and {001} witherite lamellae grow along the [100], [001], and [121j] directions of quartz. This witherite/ quartz three-dimensional epitaxy is the first evidence that two-dimensional epitaxial layers of quartz can form as well on witherite nucleating from aqueous solutions containing Na-metasilicate. As it is known, nano-polycrystalline structures, the “silica biomorphs”, are precipitated from such solutions: this implies a very high nucleation frequency of witherite. We propose that this is caused by the lowering of the specific surface energy of witherite for the two-dimensional localized adsorption of silica chains.

1. Introduction Since the pioneering and sound papers by Garcı´a Ruiz1 on “silica biomorphs”, a wide number of studies were aimed at deciphering the sequence of mechanisms responsible for the formation of such fascinating and complex aggregates. Indeed, in the last 30 years many scientists focused their attention on the dramatic changes of the morphology of some orthorhombic alkaline-earth carbonates when grown in aqueous solutions containing low amounts of silica or in silica gels. It was set, since the beginning, that these “induced morphology crystal aggregates (IMCA)” showed growth and morphological properties “... which are not governed by crystal to crystal relationships... but by an external substrate, usually a 2D membrane, which acts as the site for progressive 3D growth of the crystals forming the aggregate”.2 Further, it was also ascertained that “... the use of other gels (such as polyacrylamide, agar-agar, TMOS, or gelatine) was found unsuccessful and that the IMCA generation is closely related to the chemistry of silica gels”.3 It was also demonstrated that a co-orientation exists among the 3D crystallites forming the aggregate, and it was, consequently, suggested that epitaxial relations exist between the crystallites and the surrounding membrane which acts as a template.3 In the last decade, the research on “biomorphs” followed two main lines. The first, starting from the original milestone paper,1 aimed at unravelling the mechanism of self-assembling of these complex silica-carbonate structures.4-12 The second line concerns the carbonate (either orthorhombic or rhombohedral) biomorphs that form interacting with organic (polymeric) additives13-19 dissolved in aqueous solutions. The common opinion is that the various morphologies result from the sequential “bridged” growth of carbonate crystals in cooperation with inorganic polymeric anions (silicate), as well as organic polymers adsorbing on the carbonate crystals surfaces. The adsorption is generally viewed as a stereochemical effect, and no attention is paid to potential epitaxial relationships between the crystallizing carbonates and their inorganic/organic sub* To whom correspondence should be addressed. E-mail: [email protected].

strates. Furthermore, the adsorption is viewed as a hindrance to the further growth of each carbonate nanocrystal forming the biomorphs aggregate; correlations between the concentration of additive in solution, the amount adsorbed on the crystal faces, and the consequent variation of the size of the carbonate crystals are not investigated. Conversely, an interesting attempt was carried out to obtain the epitaxial crystallization of a pseudohexagonal carbonate (aragonite) on different trigonal substrates, without focusing only on the stereochemical compatibility, so giving new insights into biomineralization. Pokroy and co-workers20 tried to determine to which extent the lattice misfit affects the nucleation and growth of aragonite. To this end they used (0001) quartz slabs polished to optical quality, free of carbonate groups. Control experiments were performed on amorphous silica (SiO2). Aragonite crystallization at room temperature and pressure was accomplished by slow diffusion of CO2 into a CaCl2 solution placed on (0001) quartz substrate. Lattice misfits (∆%) at the interface between the (001) face of aragonite and the (0001) quartz are obtained comparing the repeat distances (in Å) along coincidence directions:

[010]quartz ) 4.9136 [100]aragonite ) 4.9620 ∆% ) -0.98 [210]quartz ) 8.5106 [010]aragonite ) 7.9696 ∆% ) +6.79 According to these authors “... the proximity of the substrate and the layer lattices plays an important role in the epitaxial growth of aragonite, a fact that was proven by a control experiment using amorphous silica (SiO2) as a substrate, which showed no traces of aragonite formation.” Thus, for the first time, the epitaxial criterion initially invoked by Garcı´a Ruiz is applied quantitatively to explain the growth of these carbonate aggregates. In our recent work21 on the crystallization of biomorphs, we put forward the hypothesis that Na-metasilicate should promote the 3D nucleation of witherite, owing to the strong adhesion energy between polymerized (SiO4)-4 groups and the witherite surface. At that time, we could not produce evidence on the formation of a witherite/silica interface controlled by epitaxy; nevertheless, we argued that if an adhesion energy is gained

10.1021/cg800771q CCC: $40.75  2009 American Chemical Society Published on Web 01/07/2009

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Figure 1. (a) Indexed XRPD diagram of witherite obtained from crystals formed in the solution bulk; (b) Raman diagram showing the shifts of witherite, as obtained on microcrystals nucleated and grown on quartz faces used as substrates.

Figure 2. (a-c) Evolution with time of witherite twinned rods nucleated and grown in the mother solution bulk.

when silica deposits on witherite surfaces, the same occurs when witherite grows on silica. Having in mind that the optimal adhesion energy is obtained when epitaxial relationships are fulfilled between two ordered phases, we tried to grow from solution witherite crystals (guest) on single quartz crystals (hosts) in order to use the information resulting from a direct and observable 3D epitaxy (witherite on quartz) to justify the inverse and not observable 2D epitaxy (quartz on witherite). Our final goal is to quantitatively explain the dramatic increase of the nucleation frequency of witherite when precipitated from aqueous solutions doped with increasing amounts of Na-metasilicate.

2. Experimental Procedures The procedures we followed to obtain well-shaped witherite crystals easily indexed are obviously different from those carried out for producing industrial barium carbonate.22,23 BaCO3 is a sparingly soluble salt (Ks ) 2.58 × 10-9; solubility ) 0.0014 g/100 g water at 20 °C); then, high supersaturations are needed to obtain a reasonable nucleation frequency, that implies a small size of critical nuclei. Witherite crystal were synthesized at constant temperature (22 °C) and under environmental pressure, by mixing two solutions of BaCl2 · 2H2O (0.02 M; pH ) 6.4) and NaHCO3 (0.002 M; pH ) 8.4) with a 1:2 volume ratio. After mixing, the crystallization vessel was closed to avoid solvent evaporation. Crystallization was carried out in two ways. In the first one, crystals spontaneously formed in the solution bulk, while, in the second one, witherite nucleated and grew on quartz single crystals previously placed on the bottom of the crystallization cell. Crystal morphology and chemical composition were determined by optical microscopy (Diavert Leica and Zeiss Axiolab), scanning electron microscopy, and energy dispersive spectrometry (Cambridge Stereoscan S-360 equipped with a Oxford Inca Energy 200). Crystals formed in the solution bulk were structurally characterized with the aid of a X-ray powder diffractometer (XRPD-Siemens D5000 equipped with a Bragg-Brentano geometry). Crystals grown on quartz substrates were

examined through the Raman spectrometry (micro/macro Jobin Yvon Mod. LabRam HRVIS) owing to their very small amount and size. The pH value in the mother solutions was controlled by a Thermo Orion 4-star Benchtop pH/conductivity meter equipped with an Orion 91-09 Triode 3-in-1 pH/ATC probe.

3. Results Pure witherite was obtained in both crystallization ways, as it follows from XRPD and Raman spectra illustrated in Figure 1a,b, respectively. The growth morphologies are very different depending on whether witherite was formed, or not, on quartz substrate. Crystals Formed in the Solution Bulk. After two days the precipitate consisted of pseudohexagonal rods, {0001} terminated and built by twinned lamellae piled-up along their common z-axis (Figure 2). Their length exceeds 100 µm, while their size is generally less than 20 µm. The lamellar stacking gives rise to a rod consisting of repeated segments. The segmentation progressively vanishes as the supersaturation decreases with time, owing to the closed crystallization system. When the mother solution approaches equilibrium the segmentation completely disappears and the rod-shaped twinned crystals are laterally bounded by flat faces and top-terminated by small rhombohedral forms. The mechanisms generating the multiple twins of witherite will be treated extensively elsewhere, according to the way previously followed for the aragonite crystals.24,25 Here we will confine our attention to the dramatic modification induced by the quartz substrate on the witherite habit. Crystals Formed on Quartz Surfaces Used As Substrates. Figure 3a shows the natural crystal of quartz used as substrate; it is dominated by the {101j2} form. Figure 3b shows the same crystal after the crystallization of witherite on it.

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Figure 3. A quartz face belonging to the {101j2} form before (a) and after (b) the crystallization of witherite lamellae on it. The SEM image in (a) was obtained without any coating.

Figure 5. Witherite lamellae grow perpendicularly to the quartz faces, the intersection between a lamella and the substrate being either parallel (a) or perpendicular (b) to one side of the lamella. Lamellar aggregates often develop according to both these orientations (c).

witherite. As a matter of fact, the deviation from the perfect hexagonality easily appreciated in aragonite crystals (3.82°), reduces to 1.69° in witherite. Consequently, we were induced to search for all possible coincidence lattices between the basic 2D meshes of witherite, namely, [100] × [001], [010] × [001], [110] × [001], [310] × [001], and the corresponding 2D meshes of the quartz faces on which the lamellae grew.

4. Discussion Figure 4. Witherite lamellae grown on quartz rhombohedra {101j2} (A), {011j2} (A′), and on the prismatic {101j0} quartz surfaces (B) show that the surface structure of quartz favors the epitaxy with witherite. Overall image of the epitaxy between witherite and quartz single crystal (C).

Further, Figure 4 illustrates the following: (a) Quartz crystal as a whole behaves as an epitaxial host for witherite (Figure 4C), as shown in detail for the rhombohedral {101j2}, {011j2}, and prismatic {101j0} forms in Figures A, A′, and B, respectively. (b) The habit of witherite is platy everywhere (40-50 µm in size and ≈2 µm in thickness), and the prevailing orientation of the growth twinned lamellae is perpendicular to the quartz surfaces: this suggests that witherite crystals do not form in the solution bulk and successively adhere by syneusis [two (more) crystals adhere by “syneusis” when they nucleate and grow separately and only successively coalesce forming an aggregate geometrically controlled either by twin or epitaxial rules] to the quartz surfaces but that, on the contrary, they heterogeneously nucleate and grow (guests) along privileged directions of the pre-existing quartz faces (hosts). From a closer examination of the geometrical relationships both within the single lamella and between lamellae and substrate (Figure 5a-c) it ensues that three situations occur: (i) one side of the pseudohexagonal lamella develops either parallel to the substrate (Figure 5a), (ii) or intersects the substrate nearly perpendicular (Figure 5b). (iii) both these cases result in the same aggregate of {001} witherite lamellae (Figure 5c). It is practically impossible to identify, from SEM images, which side of a lamella corresponds to the three pseudoequivalent [100], [11j0], and [1j1j0] directions lying in the 001 plane of

Geometrical and Structural Aspects of the Witherite/ Quartz Epitaxies. The geometrical relationships, between quartz (host) and witherite (guest) lattices, that can be realized at the three most frequently occurring epitaxial interfaces, are collected in Table 1. The first one (Table 1a) refers to the witherite lamellae which were observed developing perpendicular to both the {101j0} faces and to the [001] axis of quartz. Hence, we look for lattice coincidences among the possible witherite 2D meshes and the basic or multiple [100] × [001] cell of quartz. The epitaxially grown witherite lamellae can have two possible orientations with respect to the substrate: Case 1: if one side of the lamella is parallel to the substrate (see last column in Table 1a), its microform adhering onto the quartz surface can be either {010} or {110} and the intersection line will be, correspondingly, [100] or [110]. In both situations the linear misfit between quartz and witherite parallel lattice rows is lower than 10%, its mean value reaching -6.78%. Case 2: if, on the contrary, one side of the lamella is perpendicular to the substrate, the adhesion of the lamella will be obtained through the {100} or {13j0} forms and then the intersection lines will result to be [010] and [310], respectively. The mean linear misfit (+9.06%) is higher compared to the preceding one; further, lattice coincidences occur over repeat periods which are twice the values of case 1. Finally, it should be remembered that the forms {100} or {13j0} of witherite are not flat (F) forms, in the sense of Hartman-Perdok.26 All this indicates that the occurrence frequency of case 2 should be lower than that of case 1. The same considerations apply when witherite lamellae grow perpendicular to the {101j0} faces and parallel to the [001] axis of quartz (Table 1b). Lattice coincidences are strongly anisotropic in this case. As a matter of fact, the mean linear misfit is very low (+3.02%) in case 3, while it reaches a very high mean

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Table 1. (a) Witherite Lamellae Perpendicular to the {101j0} Faces and to [001] Axis of Quartz, (b) Witherite Lamellae Perpendicular to the {101j0} Faces and Parallel to the [001] Axis of Quartz,a and (c) Witherite Lamellae Perpendicular to the {101j2} and {111j2} Faces, and Parallel to the Directions of Quartz

a

The dotted crystal shape drawn in the last column indicates a very low occurrence probability.

value (-11.14%) in case 4. It is easy to understand why case 4 has never been observed considering that (i) higher lattice misfit determines lower adhesion energy between guest and host phases in the epitaxial growth, and (ii) the nucleation frequency of an epitaxial phase depends exponentially on (minus) the adhesion energy. Lamellae developing on the surfaces of the {011j2} and {101j2} rhombohedra represent a situation quite similar to that discussed above. In fact, case 5 and case 6 illustrated in Table 1c are related to very different linear misfits, that is, -4.08% and +11.9%, respectively. Hence, the same considerations apply as for cases 3 and 4. A conclusive remark concerns the witherite/quartz misfit calculated along the direction perpendicular to each lamella. As it can be inferred from the last row of the Tables 1a-c, the parametric misfit ranges from the minimum value of -18.9% to a maximum of -30.83%. This means that the common feature of the witherite/quartz epitaxies is their

preferential one-dimensional character. In other words, the witherite embryos forming on the quartz surfaces can easily develop along those directions where the linear misfit is minimum, while their advancement rate is hindered along the directions where the misfit largely exceeds the limits found by Royer.27 Highlighting evidence of the privileged conditions promoted by the rules of epitaxy is shown in Figure 6. As a matter of fact, one can compare two situations differing only by the crystallographic continuity of the quartz substrate. In Figure 6a, one witherite lamella nucleated onto one face of a {101j0} quartz prism extends on the adjacent face since there is no interruption of the lattice order when going from one face to the other. This is no longer true if the surface of the adjacent face is strongly damaged, as shown in Figure 6b: in this case the lattice continuity is interrupted and the witherite crystal can continue to grow beyond the perfect prism face, but no contact can occur between the damaged face and the crystal hanging on it.

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Figure 6. (a) Witherite lamella epitaxially grown astride two adjacent prism faces of a quartz substrate. (b) When the epitaxial continuity is interrupted, owing to a concoidal fracture damaging one of the two contiguous faces, then the witherite crystal continues growing, its shape being no longer conditioned by the witherite/quartz contact.

The Evaluation of the Witherite/Quartz Adhesion Energy. When systematically looking at the shape of the growth lamellae in adhesion on the quartz substrate, one can find that their pseudohexagonal profiles are truncated by the substrate at a level a little bit lower than their centers. For the center of a lamella we mean the point of intersection of the perpendiculars drawn through the midpoints of the lamella sides. This enables us to draw the circumferences inscribing and circumscribing each lamella profile and, consequently, determine the contact angle (R) between the epitaxially grown witherite and its substrate. From a preliminary rough estimation and having conventionally assumed that R f 0 when the wetting vanishes, R ranges between 80° and 85° indicating a good guest/host wetting. It is worth recollecting here the thermodynamic and mechanic conditions ruling the guest/host equilibrium in terms of both the interfacial specific free energies and the adhesion energy of the witherite/quartz coupling:

γwith/qtz ) γqtz + γwith- βwith/qtz adh

(1a)

βwith/qtz adh ) 1 - cos R γwith

(1b)

Relations 1a and 1b represent the system of Dupre´’s and Young’s equations.28 The specific free energies γqtz and γwith are referred to vacuum, the excess specific energy γwith/qtz is related to the witherite/quartz interface, while βadhwith/qtz is the specific adhesion energy between the 3D phases of witherite and quartz. Further, it should be mentioned that, following Table 1, we use the γwith values related to either {010} or {110} forms of witherite. Since nothing is known about these values, we made reference, as a first step, to the estimations we calculated, at 0 K and without relaxation, on aragonite (CaCO3, isostructural with witherite) using a semiempirical potential:24 γ010 ) 1226 and γ110 ) 1242 erg × cm-2. Considering that both relaxation and entropic surface contributions cannot be neglected, we roughly reduced by 30% the averaged value ) 1234 erg × cm-2, based on our recent results obtained on calcite surfaces.29,30 Thus, having assumed γwith ) 864 erg × cm-2, and an averaged value of R ) 82.5°, from eq 1b it ensues that βadhwith/qtz ) 751 erg × cm-2. One might object that the observed lamellae represent growth and not equilibrium forms, and then that relations 1a and 1b were improperly applied. Nevertheless, it should be considered that growth and equilibrium shapes of the orthorhombic carbonates, at least for the forms in zone with the [001] axis, are practically the same. As a matter of fact, the ratio between the calculated values of the surface free energies (determining

the equilibrium shape) and the attachment energies (determining the growth shape) for aragonite are (γ110)/(γ010) ) 1.013 and (Eatt110)/(Eatt010) ) 0.957, respectively.24 The Adhesion Energy of the 2D Polymerized Silica Network Adsorbed on the Nucleating Witherite, and the Nucleation Frequency of Its Nanosized Individuals. The witherite/quartz epitaxy allows one to understand the dramatic differences observed in the frequency of nucleation of the alkaline-earth carbonates, according to whether their precipitation occurs from pure aqueous solution or in the presence of increasing concentrations of silica, all other conditions (temperature, supersaturation, fluid-dynamics) being equal. Let us consider the expression of the excess surface energy γwith/sol defining the interface between a nucleating witherite 3D embryo and the surrounding supersaturated and pure aqueous solution. The Dupre´’s relation reads in this case:

γwith/sol ) γwith + γsol - βwith/sol adh

(2a)

sol

We measured γ by the capillary method; its value, 72 erg cm-2, is that expected for aqueous solutions of sparingly soluble salts. Determining γwith is not necessary for our purpose, as we will see very soon. It is well-known that the βadhwith/sol value cannot exceed the cohesion energy of the solution, that is, βadhwith/sol e 2 × γsol. Then, the minimum value for γwith/sol should be, in erg × cm-2: with/sol γmin ) γwith - 72

(2b)

A different situation is set up when, for instance, Na2SiO3 · 9H2O is poured into the mother solution in concentrations varying from 500 to 5000 ppm, as we did to obtain silica biomorphs. Under these new conditions, the solution is surely unsaturated with respect to any compound ranging from SiO2 · nH2O to SiO2 or other hydrated phases that, consequently, cannot precipitate, neither as amorphous nor as crystals. Nevertheless, either (SiO3)-2 or (Si4O11)-6 chains can be adsorbed onto the facets of the nucleating witherite through the strong bonds among the exposed Ba2+ ions and the oxygen atoms of the silica chains. Inspired by the just found witherite/ quartz epitaxy, we suggest that this adsorption can be organized in 2D epitaxial islands assuming the quartz structure. For the 2D heterogeneous nucleation to occur, the specific adhesion energy between the two phases in contact must be higher than the cohesion energy of the guest phase (i.e., 3D quartz in this case).28 It is sufficient to compare the melting points of quartz and witherite to forecast that the cohesion energy of quartz is surely higher than the adhesion energy between witherite and quartz. Thus, it comes out that βadhwith/qtz ) 751 erg × cm-2 can be reasonably assumed as the lower limit of the adhesion energy between a witherite embryo and its surrounding silicadoped solution. In other words we can write down the Dupre´’s relation when 2D silica adsorption occurs, remembering that the just mentioned lower limit corresponds to the maximum value of the excess interfacial energy witherite/doped solution: with/qtz (γwith/sol )max ) γwith + γsol ads ads - βadh

(3a)

From our measurements it results that the surface energy of the solution slightly varies when silica is added to the growth medium, γadssolreaching a maximum of 80 erg × cm-2. Consequently, relation 3a can be written, in analogy with relation 2b:

(γwith/sol )max ) γwith - 671 ads

(3b)

Finally, subtracting eq 3b from 3a, we obtain the lower limit of the difference

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(4)

Relation 4 simply means that the interfacial energy witherite/ solution, that is, the main barrier hindering the nucleation, dramatically falls from pure to silica-doped mother solution. Since the behavior of 3D nucleation frequency (J3D) essentially depends on the exponential term containing the activation energy for nucleation31 one can write, for pure and doped solutions:

ln

ads J3D pure J3D

≈

{

}

f × Ω2 (γ3 - γ3ads) 3 2 pure (kBT) (ln β)

(5a)

where f is the shape factor of the crystal embryo, Ω the molecular volume of the witherite crystal, kB the Boltzmann’s constant, T the absolute temperature, β the supersaturation of the solution. γpure and γads coincide with γminwith/sol ) 792 erg × cm-2 and (γadswith/sol) max ) 193 erg × cm-2, respectively. Adopting, for the sake of simplicity, f ) (16/3) for a spherical embryo, Ωwith ) 2.4 × 10-22 cm3, T ) 300 K, one finds that relation 5a gives

ln

ads J3D pure J3D

= 4 × 105 and 1 × 105

(5b)

for two sensible values of the supersaturation of sparingly soluble salt solutions, that is, β ) 10 and 100, respectively. Accordingly, the size of the critical nuclei, r* ) (2Ωγ)/(kBT(ln β)), assumes the values of rads* ) 9 and 4.5 nm and rpure* ) 37 and 18.5 nm, for β ) 10 and 100, respectively. Finally, it is worth comparing the values we estimated for the critical witherite nuclei with the observed sizes of the crystals. The calculated minimum size of witherite crystals coincides with the size of the {001} rod section, 10 µm (Figure 2), grown from pure solutions after a crystallization run of two days. This means that the advancement rate of both {010} and {110} forms of witherite reaches, in pure solution, a minimum mean value of 0.11 nm s-1, having assumed that the witherite nucleation occurred since the mixing of the mother solutions. On the contrary, the size of the {001} rod section measured on crystals populating the witherite-silica biomorphs ranges from 5 nm21 to 40 nm.7 This proves that, on one side, the epitaxial silica adsorption promotes a dramatic nucleation frequency of witherite while, on the other side, strongly reduces the advancement rate of the {010} and {110} facets limiting the just nucleated crystals.

5. Conclusions Summing up, we showed that the huge variation in the number of nucleated witherite crystals, occurring when going from pure mother solution to a silica-doped one, is essentially attributable to the large difference of the interfacial specific energy between the crystal phase and the solution. This equilibrium quantity, that intrinsically hinders the birth of a new phase, strongly decreases when silica polymers are adsorbed on the growing facets of the critical witherite nuclei (2D quartz/witherite epitaxy) forming ordered R-quartz like 2D islands. It is worth outlining that we are proposing here a model, as we did not observe, till now, such epitaxy. On the other hand it was proved, in this paper and for the first time, that the inverse 3D witherite /quartz epitaxy occurs at least on three important forms of R-quartz crystal. From these observations and by applying both the Young’s and Dupre´’s relationships along with the rules of the epitaxy, the order of magnitude of the specific adhesion energy witherite /quartz was estimated.

Hence, describing the very early stage of formation of “silica biomorphs” in terms of a model based on the 2D quartz/witherite epitaxy, we could evaluate the relevant interfacial energies and explain how the silica adsorption promotes the nucleation of 3D witherite. This gives a reasonable answer to the fundamental question put forward by Garcı´a Ruiz on the relationships between the ultrastructure of the silica membrane and the carbonate crystals of its biomorphs “... no quantitative epitaxial relations can be established between the carbonate crystals and the silicate membrane. Nevertheless, the existence of such epitaxial relations can be indirectly inferred”. Our considerations on the role exerted by the epitaxy are not confined to the witherite/silica interface but can be usefully transferred to the interpretation of the “vaterite biomorphs” mediated by charged polyelectrolites (such as polyaspartate),13 to sheets and helical forms of strontium carbonate grown in silica gel6 and to the formation of planar crystals of orthorhombic carbonates on an insoluble polyalcohol substrate.8 Moreover, the selective adsorption of racemic DHBC polymers on the {110} surfaces of witherite nanocrystals, inducing helical alignment17 should be fruitfully reviewed in the light of our hypothesis. In the second step of our research on this topic, we will deal with both the equilibrium and growth shapes of witherite crystallizing in a silica environment along with a model of witherite nanocrystal aggregation through intergranular silica membrane formation.

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