Witherite (BaCO3)/r-Quartz Epitaxial Nucleation and Growth:Experimental Findings and Theoretical Implications onBiomineralization

Erica Bittarello, Francesco Roberto Massaro, Marco Rubbo, Emanuele Costa, andDino Aquilano*

Dipartimento di Scienze Mineralogiche e Petrologiche, UniVersita degli Studi di Torino, Via ValpergaCaluso 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; theirgrowth morphology is characterized by single and twinned individuals both [001] elongated and limited by 110 prism and hhlbipyramids. 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 witheritenucleating 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 bythe 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 Garcıa Ruiz1 on“silica biomorphs”, a wide number of studies were aimed atdeciphering the sequence of mechanisms responsible for theformation of such fascinating and complex aggregates. Indeed,in the last 30 years many scientists focused their attention onthe dramatic changes of the morphology of some orthorhombicalkaline-earth carbonates when grown in aqueous solutionscontaining low amounts of silica or in silica gels.

It was set, since the beginning, that these “induced morphol-ogy crystal aggregates (IMCA)” showed growth and morpho-logical properties “... which are not governed by crystal to crystalrelationships... but by an external substrate, usually a 2Dmembrane, which acts as the site for progressive 3D growth ofthe crystals forming the aggregate”.2 Further, it was alsoascertained that “... the use of other gels (such as polyacrylamide,agar-agar, TMOS, or gelatine) was found unsuccessful and thatthe IMCA generation is closely related to the chemistry of silicagels”.3 It was also demonstrated that a co-orientation existsamong the 3D crystallites forming the aggregate, and it was,consequently, suggested that epitaxial relations exist betweenthe crystallites and the surrounding membrane which acts as atemplate.3

In the last decade, the research on “biomorphs” followed twomain lines. The first, starting from the original milestone paper,1

aimed at unravelling the mechanism of self-assembling of thesecomplex silica-carbonate structures.4-12 The second line con-cerns the carbonate (either orthorhombic or rhombohedral)biomorphs that form interacting with organic (polymeric)additives13-19 dissolved in aqueous solutions. The commonopinion is that the various morphologies result from thesequential “bridged” growth of carbonate crystals in cooperationwith inorganic polymeric anions (silicate), as well as organicpolymers adsorbing on the carbonate crystals surfaces. Theadsorption is generally viewed as a stereochemical effect, andno attention is paid to potential epitaxial relationships betweenthe crystallizing carbonates and their inorganic/organic sub-

strates. Furthermore, the adsorption is viewed as a hindranceto the further growth of each carbonate nanocrystal formingthe biomorphs aggregate; correlations between the concentrationof additive in solution, the amount adsorbed on the crystal faces,and the consequent variation of the size of the carbonate crystalsare not investigated.

Conversely, an interesting attempt was carried out to obtainthe epitaxial crystallization of a pseudohexagonal carbonate(aragonite) on different trigonal substrates, without focusing onlyon the stereochemical compatibility, so giving new insights intobiomineralization. Pokroy and co-workers20 tried to determineto which extent the lattice misfit affects the nucleation andgrowth of aragonite. To this end they used (0001) quartz slabspolished to optical quality, free of carbonate groups. Controlexperiments were performed on amorphous silica (SiO2).Aragonite crystallization at room temperature and pressure wasaccomplished by slow diffusion of CO2 into a CaCl2 solutionplaced on (0001) quartz substrate. Lattice misfits (∆%) at theinterface between the (001) face of aragonite and the (0001)quartz are obtained comparing the repeat distances (in Å) alongcoincidence 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 substrateand the layer lattices plays an important role in the epitaxialgrowth of aragonite, a fact that was proven by a controlexperiment using amorphous silica (SiO2) as a substrate, whichshowed no traces of aragonite formation.”

Thus, for the first time, the epitaxial criterion initiallyinvoked by Garcıa Ruiz is applied quantitatively to explainthe growth of these carbonate aggregates. In our recent work21

on the crystallization of biomorphs, we put forward thehypothesis that Na-metasilicate should promote the 3Dnucleation of witherite, owing to the strong adhesion energybetween polymerized (SiO4)-4 groups and the witheritesurface. At that time, we could not produce evidence on theformation of a witherite/silica interface controlled by epitaxy;nevertheless, we argued that if an adhesion energy is gained* To whom correspondence should be addressed. E-mail: lxf@fjirsm.ac.cn.

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when silica deposits on witherite surfaces, the same occurswhen witherite grows on silica. Having in mind that theoptimal adhesion energy is obtained when epitaxial relation-ships are fulfilled between two ordered phases, we tried togrow from solution witherite crystals (guest) on single quartzcrystals (hosts) in order to use the information resulting froma direct and observable 3D epitaxy (witherite on quartz) tojustify the inverse and not observable 2D epitaxy (quartz onwitherite). Our final goal is to quantitatively explain thedramatic increase of the nucleation frequency of witheritewhen precipitated from aqueous solutions doped with in-creasing amounts of Na-metasilicate.

2. Experimental Procedures

The procedures we followed to obtain well-shaped witherite crystalseasily indexed are obviously different from those carried out forproducing industrial barium carbonate.22,23 BaCO3 is a sparingly solublesalt (Ks ) 2.58 × 10-9; solubility ) 0.0014 g/100 g water at 20 °C);then, high supersaturations are needed to obtain a reasonable nucleationfrequency, that implies a small size of critical nuclei. Witherite crystalwere synthesized at constant temperature (22 °C) and under environ-mental 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 solventevaporation.

Crystallization was carried out in two ways. In the first one, crystalsspontaneously formed in the solution bulk, while, in the second one,witherite nucleated and grew on quartz single crystals previously placedon the bottom of the crystallization cell.

Crystal morphology and chemical composition were determined byoptical microscopy (Diavert Leica and Zeiss Axiolab), scanning electronmicroscopy, and energy dispersive spectrometry (Cambridge StereoscanS-360 equipped with a Oxford Inca Energy 200). Crystals formed inthe solution bulk were structurally characterized with the aid of a X-raypowder diffractometer (XRPD-Siemens D5000 equipped with aBragg-Brentano geometry). Crystals grown on quartz substrates were

examined through the Raman spectrometry (micro/macro Jobin YvonMod. LabRam HRVIS) owing to their very small amount and size.The pH value in the mother solutions was controlled by a Thermo Orion4-star Benchtop pH/conductivity meter equipped with an Orion 91-09Triode 3-in-1 pH/ATC probe.

3. Results

Pure witherite was obtained in both crystallization ways, asit follows from XRPD and Raman spectra illustrated in Figure1a,b, respectively. The growth morphologies are very differentdepending on whether witherite was formed, or not, on quartzsubstrate.

Crystals Formed in the Solution Bulk. After two days theprecipitate consisted of pseudohexagonal rods, 0001 termi-nated and built by twinned lamellae piled-up along theircommon z-axis (Figure 2). Their length exceeds 100 µm, whiletheir size is generally less than 20 µm. The lamellar stackinggives rise to a rod consisting of repeated segments. Thesegmentation progressively vanishes as the supersaturationdecreases with time, owing to the closed crystallization system.When the mother solution approaches equilibrium the segmen-tation completely disappears and the rod-shaped twinned crystalsare laterally bounded by flat faces and top-terminated by smallrhombohedral forms. The mechanisms generating the multipletwins of witherite will be treated extensively elsewhere, ac-cording to the way previously followed for the aragonitecrystals.24,25 Here we will confine our attention to the dramaticmodification induced by the quartz substrate on the witheritehabit.

Crystals Formed on Quartz Surfaces Used As Sub-strates. Figure 3a shows the natural crystal of quartz used assubstrate; it is dominated by the 101j2 form. Figure 3b showsthe same crystal after the crystallization of witherite on it.

Figure 1. (a) Indexed XRPD diagram of witherite obtained from crystals formed in the solution bulk; (b) Raman diagram showing the shifts ofwitherite, 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.

972 Crystal Growth & Design, Vol. 9, No. 2, 2009 Bittarello et al.

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 rhombohedral101j2, 011j2, and prismatic 101j0 forms in Figures A, A′,and B, respectively.

(b) The habit of witherite is platy everywhere (40-50 µm insize and ≈2 µm in thickness), and the prevailing orientation ofthe growth twinned lamellae is perpendicular to the quartzsurfaces: this suggests that witherite crystals do not form in thesolution bulk and successively adhere by syneusis [two (more)crystals adhere by “syneusis” when they nucleate and growseparately and only successively coalesce forming an aggregategeometrically controlled either by twin or epitaxial rules] tothe quartz surfaces but that, on the contrary, they heteroge-neously nucleate and grow (guests) along privileged directionsof the pre-existing quartz faces (hosts).

From a closer examination of the geometrical relationshipsboth within the single lamella and between lamellae andsubstrate (Figure 5a-c) it ensues that three situations occur:

(i) one side of the pseudohexagonal lamella develops eitherparallel to the substrate (Figure 5a),

(ii) or intersects the substrate nearly perpendicular (Figure5b).

(iii) both these cases result in the same aggregate of 001witherite lamellae (Figure 5c).

It is practically impossible to identify, from SEM images,which side of a lamella corresponds to the three pseudoequiva-lent [100], [11j0], and [1j1j0] directions lying in the 001 plane of

witherite. As a matter of fact, the deviation from the perfecthexagonality easily appreciated in aragonite crystals (3.82°),reduces to 1.69° in witherite. Consequently, we were inducedto search for all possible coincidence lattices between the basic2D meshes of witherite, namely, [100] × [001], [010] × [001],[110] × [001], [310] × [001], and the corresponding 2D meshesof the quartz faces on which the lamellae grew.

4. Discussion

Geometrical and Structural Aspects of the Witherite/Quartz Epitaxies. The geometrical relationships, betweenquartz (host) and witherite (guest) lattices, that can be realizedat the three most frequently occurring epitaxial interfaces, arecollected in Table 1. The first one (Table 1a) refers to thewitherite lamellae which were observed developing perpen-dicular to both the 101j0 faces and to the [001] axis of quartz.

Hence, we look for lattice coincidences among the possiblewitherite 2D meshes and the basic or multiple [100] × [001]cell of quartz. The epitaxially grown witherite lamellae can havetwo 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 thequartz surface can be either 010 or 110 and the intersectionline will be, correspondingly, [100] or [110]. In both situationsthe linear misfit between quartz and witherite parallel latticerows is lower than 10%, its mean value reaching -6.78%.

Case 2: if, on the contrary, one side of the lamella isperpendicular to the substrate, the adhesion of the lamella willbe obtained through the 100 or 13j0 forms and then theintersection lines will result to be [010] and [310], respectively.The mean linear misfit (+9.06%) is higher compared to thepreceding one; further, lattice coincidences occur over repeatperiods which are twice the values of case 1. Finally, it shouldbe remembered that the forms 100 or 13j0 of witherite arenot flat (F) forms, in the sense of Hartman-Perdok.26 All thisindicates that the occurrence frequency of case 2 should be lowerthan that of case 1.

The same considerations apply when witherite lamellae growperpendicular to the 101j0 faces and parallel to the [001] axisof quartz (Table 1b). Lattice coincidences are strongly aniso-tropic in this case. As a matter of fact, the mean linear misfit isvery low (+3.02%) in case 3, while it reaches a very high mean

Figure 3. A quartz face belonging to the 101j2 form before (a) andafter (b) the crystallization of witherite lamellae on it. The SEM imagein (a) was obtained without any coating.

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

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 aggregatesoften develop according to both these orientations (c).

Witherite (BaCO3)/R-Quartz Epitaxial Nucleation Crystal Growth & Design, Vol. 9, No. 2, 2009 973

value (-11.14%) in case 4. It is easy to understand why case4 has never been observed considering that (i) higher latticemisfit determines lower adhesion energy between guest and hostphases in the epitaxial growth, and (ii) the nucleation frequencyof an epitaxial phase depends exponentially on (minus) theadhesion energy.

Lamellae developing on the surfaces of the 011j2 and101j2 rhombohedra represent a situation quite similar to thatdiscussed above. In fact, case 5 and case 6 illustrated in Table1c are related to very different linear misfits, that is, -4.08%and +11.9%, respectively. Hence, the same considerations applyas for cases 3 and 4.

A conclusive remark concerns the witherite/quartz misfitcalculated 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 thecommon feature of the witherite/quartz epitaxies is their

preferential one-dimensional character. In other words, thewitherite embryos forming on the quartz surfaces can easilydevelop along those directions where the linear misfit isminimum, while their advancement rate is hindered alongthe directions where the misfit largely exceeds the limitsfound by Royer.27

Highlighting evidence of the privileged conditions promotedby the rules of epitaxy is shown in Figure 6. As a matter offact, one can compare two situations differing only by thecrystallographic continuity of the quartz substrate. In Figure 6a,one witherite lamella nucleated onto one face of a 101j0 quartzprism extends on the adjacent face since there is no interruptionof the lattice order when going from one face to the other. Thisis no longer true if the surface of the adjacent face is stronglydamaged, as shown in Figure 6b: in this case the latticecontinuity is interrupted and the witherite crystal can continueto grow beyond the perfect prism face, but no contact can occurbetween the damaged face and the crystal hanging on it.

Table 1. (a) Witherite Lamellae Perpendicular to the 101j0 Faces and to [001] Axis of Quartz, (b) Witherite Lamellae Perpendicular to the101j0 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 <121j> Directions of Quartz

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

974 Crystal Growth & Design, Vol. 9, No. 2, 2009 Bittarello et al.

The Evaluation of the Witherite/Quartz Adhesion Ene-rgy. When systematically looking at the shape of the growthlamellae in adhesion on the quartz substrate, one can find thattheir pseudohexagonal profiles are truncated by the substrate ata level a little bit lower than their centers. For the center of alamella we mean the point of intersection of the perpendicularsdrawn through the midpoints of the lamella sides. This enablesus to draw the circumferences inscribing and circumscribingeach lamella profile and, consequently, determine the contactangle (R) between the epitaxially grown witherite and itssubstrate. From a preliminary rough estimation and havingconventionally assumed that Rf 0 when the wetting vanishes,R ranges between 80° and 85° indicating a good guest/hostwetting. It is worth recollecting here the thermodynamic andmechanic conditions ruling the guest/host equilibrium in termsof both the interfacial specific free energies and the adhesionenergy of the witherite/quartz coupling:

γwith/qtz ) γqtz + γwith- adhwith/qtz (1a)

adhwith/qtz

γwith) 1 - cos R (1b)

Relations 1a and 1b represent the system of Dupre’s andYoung’s equations.28 The specific free energies γqtz and γwith

are referred to vacuum, the excess specific energy γwith/qtz isrelated to the witherite/quartz interface, while adh

with/qtz is thespecific adhesion energy between the 3D phases of witheriteand quartz. Further, it should be mentioned that, following Table1, we use the γwith values related to either 010 or 110 formsof witherite.

Since nothing is known about these values, we madereference, as a first step, to the estimations we calculated, at 0K and without relaxation, on aragonite (CaCO3, isostructuralwith witherite) using a semiempirical potential:24 γ010 ) 1226and γ110 ) 1242 erg × cm-2. Considering that both relaxationand entropic surface contributions cannot be neglected, weroughly reduced by 30% the averaged value <γwith> ) 1234erg × cm-2, based on our recent results obtained on calcitesurfaces.29,30 Thus, having assumed γwith ) 864 erg × cm-2,and an averaged value of R ) 82.5°, from eq 1b it ensues thatadh

with/qtz ) 751 erg × cm-2.One might object that the observed lamellae represent growth

and not equilibrium forms, and then that relations 1a and 1bwere improperly applied. Nevertheless, it should be consideredthat growth and equilibrium shapes of the orthorhombiccarbonates, at least for the forms in zone with the [001] axis,are practically the same. As a matter of fact, the ratio betweenthe calculated values of the surface free energies (determining

the equilibrium shape) and the attachment energies (determin-ing the growth shape) for aragonite are (γ110)/(γ010) ) 1.013and (Eatt

110)/(Eatt010) ) 0.957, respectively.24

The Adhesion Energy of the 2D Polymerized SilicaNetwork Adsorbed on the Nucleating Witherite, and theNucleation Frequency of Its Nanosized Individuals. Thewitherite/quartz epitaxy allows one to understand the dramaticdifferences observed in the frequency of nucleation of thealkaline-earth carbonates, according to whether their precipita-tion occurs from pure aqueous solution or in the presence ofincreasing concentrations of silica, all other conditions (tem-perature, supersaturation, fluid-dynamics) being equal. Let usconsider the expression of the excess surface energy γwith/sol

defining the interface between a nucleating witherite 3D embryoand the surrounding supersaturated and pure aqueous solution.The Dupre’s relation reads in this case:

γwith/sol ) γwith + γsol - adhwith/sol (2a)

We measured γsol by the capillary method; its value, 72 ergcm-2, is that expected for aqueous solutions of sparingly solublesalts. Determining γwith is not necessary for our purpose, as wewill see very soon. It is well-known that the adh

with/sol valuecannot exceed the cohesion energy of the solution, that is,adh

with/sole 2 × γsol. Then, the minimum value for γwith/sol shouldbe, in erg × cm-2:

γminwith/sol ) γwith - 72 (2b)

A different situation is set up when, for instance, Na2SiO3 ·9H2Ois poured into the mother solution in concentrations varyingfrom 500 to 5000 ppm, as we did to obtain silica biomorphs.Under these new conditions, the solution is surely unsaturatedwith 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 beadsorbed onto the facets of the nucleating witherite through thestrong bonds among the exposed Ba2+ ions and the oxygenatoms of the silica chains. Inspired by the just found witherite/quartz epitaxy, we suggest that this adsorption can be organizedin 2D epitaxial islands assuming the quartz structure. For the2D heterogeneous nucleation to occur, the specific adhesionenergy between the two phases in contact must be higher thanthe cohesion energy of the guest phase (i.e., 3D quartz in thiscase).28 It is sufficient to compare the melting points of quartzand witherite to forecast that the cohesion energy of quartz issurely higher than the adhesion energy between witherite andquartz. Thus, it comes out that adh

with/qtz ) 751 erg × cm-2

can be reasonably assumed as the lower limit of the adhesionenergy between a witherite embryo and its surrounding silica-doped solution. In other words we can write down the Dupre’srelation when 2D silica adsorption occurs, remembering thatthe just mentioned lower limit corresponds to the maximumvalue of the excess interfacial energy witherite/doped solution:

(γadswith/sol)max ) γwith + γads

sol - adhwith/qtz (3a)

From our measurements it results that the surface energy of thesolution slightly varies when silica is added to the growthmedium, γads

solreaching a maximum of 80 erg × cm-2.Consequently, relation 3a can be written, in analogy with relation2b:

(γadswith/sol)max ) γwith - 671 (3b)

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

Figure 6. (a) Witherite lamella epitaxially grown astride two adjacentprism faces of a quartz substrate. (b) When the epitaxial continuity isinterrupted, owing to a concoidal fracture damaging one of the twocontiguous faces, then the witherite crystal continues growing, its shapebeing no longer conditioned by the witherite/quartz contact.

Witherite (BaCO3)/R-Quartz Epitaxial Nucleation Crystal Growth & Design, Vol. 9, No. 2, 2009 975

∆γ) γminwith/sol - (γads

with/sol)max ) 599 erg × cm-2 (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) essentiallydepends on the exponential term containing the activation energyfor nucleation31 one can write, for pure and doped solutions:

lnJ3D

ads

J3Dpure

≈ f × Ω2

(kBT)3(ln )2(γpure

3 - γads3 ) (5a)

where f is the shape factor of the crystal embryo, Ω themolecular volume of the witherite crystal, kB the Boltzmann’sconstant, T the absolute temperature, the supersaturation ofthe solution. γpure and γads coincide with γmin

with/sol ) 792 erg ×cm-2 and (γads

with/sol) max ) 193 erg × cm-2, respectively.Adopting, for the sake of simplicity, f ) (16/3) for a sphericalembryo, Ωwith ) 2.4 × 10-22 cm3, T ) 300 K, one finds thatrelation 5a gives

lnJ3D

ads

J3Dpure

= 4 × 105 and 1 × 105 (5b)

for two sensible values of the supersaturation of sparinglysoluble salt solutions, that is, ) 10 and 100, respectively.Accordingly, the size of the critical nuclei, r* ) (2Ωγ)/(kBT(ln)), assumes the values of rads* ) 9 and 4.5 nm and rpure* ) 37and 18.5 nm, for ) 10 and 100, respectively.

Finally, it is worth comparing the values we estimated forthe critical witherite nuclei with the observed sizes of thecrystals. The calculated minimum size of witherite crystalscoincides with the size of the 001 rod section, 10 µm (Figure2), grown from pure solutions after a crystallization run of twodays. This means that the advancement rate of both 010 and110 forms of witherite reaches, in pure solution, a minimummean value of 0.11 nm s-1, having assumed that the witheritenucleation occurred since the mixing of the mother solutions.On the contrary, the size of the 001 rod section measured oncrystals populating the witherite-silica biomorphs ranges from5 nm21 to 40 nm.7 This proves that, on one side, the epitaxialsilica adsorption promotes a dramatic nucleation frequency ofwitherite while, on the other side, strongly reduces the advance-ment rate of the 010 and 110 facets limiting the justnucleated crystals.

5. Conclusions

Summing up, we showed that the huge variation in thenumber of nucleated witherite crystals, occurring when goingfrom pure mother solution to a silica-doped one, is essentiallyattributable to the large difference of the interfacial specificenergy between the crystal phase and the solution.

This equilibrium quantity, that intrinsically hinders the birthof a new phase, strongly decreases when silica polymers areadsorbed on the growing facets of the critical witherite nuclei(2D quartz/witherite epitaxy) forming ordered R-quartz like 2Dislands. 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 thefirst time, that the inverse 3D witherite /quartz epitaxy occursat least on three important forms of R-quartz crystal. From theseobservations and by applying both the Young’s and Dupre’srelationships along with the rules of the epitaxy, the order ofmagnitude of the specific adhesion energy witherite /quartz wasestimated.

Hence, describing the very early stage of formation of“silica biomorphs” in terms of a model based on the 2Dquartz/witherite epitaxy, we could evaluate the relevantinterfacial energies and explain how the silica adsorptionpromotes the nucleation of 3D witherite. This gives areasonable answer to the fundamental question put forwardby Garcıa Ruiz on the relationships between the ultrastructureof the silica membrane and the carbonate crystals of itsbiomorphs “... no quantitative epitaxial relations can beestablished between the carbonate crystals and the silicatemembrane. Nevertheless, the existence of such epitaxialrelations can be indirectly inferred”.

Our considerations on the role exerted by the epitaxy are notconfined to the witherite/silica interface but can be usefullytransferred to the interpretation of the “vaterite biomorphs”mediated by charged polyelectrolites (such as polyaspartate),13

to sheets and helical forms of strontium carbonate grown insilica gel6 and to the formation of planar crystals of orthor-hombic carbonates on an insoluble polyalcohol substrate.8

Moreover, the selective adsorption of racemic DHBC polymerson the 110 surfaces of witherite nanocrystals, inducing helicalalignment17 should be fruitfully reviewed in the light of ourhypothesis.

In the second step of our research on this topic, we will dealwith both the equilibrium and growth shapes of witheritecrystallizing in a silica environment along with a model ofwitherite nanocrystal aggregation through intergranular silicamembrane formation.

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