The competition between {1 0 % 1 4} cleavage and {0 1 % 1 2} steep rhombohedra in gel grown calcite crystals

Journal of Crystal Growth 247 (2003) 472–482

The competition between {1 0 %1 4} cleavage and {0 1 %1 2}steep rhombohedra in gel grown calcite crystals

Linda Pastero*, Emanuele Costa, Bruno Alessandria, Marco Rubbo,Dino Aquilano

Dipartimento di Scienze Mineralogiche e Petrologiche, Universit "a di Torino, via Valperga Caluso 35, 10125 Torino, Italy

Received 13 September 2002; accepted 16 September 2002

Communicated by R. Kern

Abstract

Calcite crystals have been obtained from supersaturated solutions diffusing both in agarose and Na-metasilicate gels.

Crystal morphology in agarose is characterized mainly by spherulites grown around solid gel impurities and terminated

by the cleavage f1 0 %1 4g rhombohedron. Grown from Na-metasilicate gel, f1 0 %1 4g and f0 1 %1 2g rhombohedra compete

according to the influence of pH and of the acidifying agent, HCl or CH3COOH, the latter favouring the appearance of

the f0 1 %1 2g form.

r 2002 Elsevier Science B.V. All rights reserved.

PACS: 82.70.Gg; 81.10.Aj; 81.10.Dn; 68.37.Hk; 68.37.Ps

Keywords: A1. Crystal morphology; A1. Impurity adsorption; A2. Gel growth; B1. Calcite

1. Introduction

Calcite crystals have been obtained from crystal-lization in gels since the beginning of Thirties. Thesound paper by Mc Cauley and Roy [1], beside theexperimental part, deals with the historical re-searches on CaCO3 growth in silica gels during1931–1971. These studies demonstrated that fairlylarge crystals may be routinely prepared and thenseparated and studied independently by varioustechniques; moreover crystals may be observedduring their nucleation and growth either macro-

scopically or microscopically since the gel isoptically transparent. Further the pH and theconcentrations of the reactants and impurity ionscan also be independently adjusted. Extraneouseffects on the precipitates are also minimized sinceearly formed nuclei are held in the position of theirformation, separate from all other first formednuclei. This minimizes effects due to the precipi-tate–precipitate interaction and crystal impactonto the bottom of the container.

Various mechanisms have been suggested todescribe the function of the gel in the crystalgrowth process. In general, however, the gellimits the number of critical-sized nuclei that areformed and decreases the rate of growth bycontrolling diffusion of the reacting ions and,

*Corresponding author. Tel.: +39-011-6707125; fax: +39-

011-6707128.

E-mail address: [email protected] (L. Pastero).

0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 9 1 1 - 5

more importantly, by governing the speed ofreactions at the crystal’s growing interface. Final-ly, the gel is assumed to be chemically inert to anyreaction that takes place and may be thought of asanalogous to a glass sponge.

Various type of gel were used till now, frominorganic (water–glass, Na-metasilicate, Na-MTSthereinafter) to organic (gelatine); either HCl oracetic acid is generally added to improve gelifica-tion, the gel’s pH ranging between 7 and 10.5. Thesolution containing the Ca2+ ion was always moreor less concentrated CaCl2 aqueous solution,whereas the CO3

2� ion was obtained from aqueoussolutions either of (NH4)2CO3, or NaHCO3,Na2CO3 or, finally, as a by product of K2C2O4.Different growth morphologies were obtained,according to the type of gel, the acidifying agent,the reacting solutions and their absolute andrelative concentrations. In Table 1 a summary ofin gel experiments and related results is brieflyillustrated.

Notwithstanding the experiments are ratherheterogeneous. However some common character-istics in the growth morphology can be drawn out

* f1 0 %1 4g and f0 1 %1 2g are by far the mostrecurrent and flat forms, while the prismf1 0 %1 0g; which rarely occurs, shows striatedand curved faces,

* f0 1 %1 2g prevails on f1 0 %1 4g in those parts ofthe crystallization tubes where the Ca2+ con-centration exceeds that of the CO3

2� ion (i.e. onthe crystals grown in the gel just below theCaCl2 solution, where the Ca2+/CO3

2� is high).This is in good agreement with the observationsby Kirov et al. [3] for calcite growing, withoutgel, from counter-diffusing pure solutions.

* As concerns the influence of gel (its quality andtreatment) on the calcite crystal morphology,no attention was paid till now.

Aiming at understanding the gel’s effect on thecrystallization we will study in this paper thecalcite morphology we obtained, at room tem-perature and pressure, both in pure aqueoussolution and in gel (Na-MTS and agarose). Theinfluence of the acidifying agents (HCl and aceticacid) will be pointed out as well, along with that ofthe major impurities constituting the gels. T

able

1

Calcitecrystallization

from

counter-diffu

sion

oftw

oaqueo

usso

lutionsin

gel,asit

comes

outfrom

litera

ture

data

Exp.

Typeofgel

pH

Acidifying

agen

tSolution

(1)

Solution

(2)

Cry

stalmorp

hology

Commen

tsReferen

ce

#1

Water–

glass

7–8

None

CaCl 2

(1M

)(N

H4) 2CO

3(1

M)

f10% 14g

f10% 14grh

ombohed

ron

isth

eonly

form

Bartaet

al.[2]

#2

Gelatine

??

(a)CaCl 2

(0.5

g-ion/l)

(b)CaCl 2

(1.0

g-ion/l)

(a)Na2CO

3(0.5

g-ion/l)

(b)Na2CO

3(0.5

g-ion/l)

(a)f0

h% h

lg+

ara

gonitesp

herulites;

(b)f0

h% h

lg+f1

0% 14g+

ara

gonite

nee

dlesand

spherulites

f0h% h

lgprevailswhen

[Ca2+]>

[CO

32�]

wherea

sf1

0% 14gp

revailswhen

[Ca2+

]D[C

O32�]

Kirov[3]

#3

Na2SiO

3�9

H2O

7–10

CH

3COOH

CaCl 2

(0.45M

)Na2CO

3

(vary

ingmolarra

tio

CaCl 2/N

a2CO

3)

Single

crystals

(pro

bablyf0

1% 12g)+

hopper

-like

types,when

7o

pHo

9

When

pH

>9polycrystals

with

rhombohed

raloutline+

spicular

and

spherulitictypes

McC

auleyet

al.

[4]

#4

Na2SiO

3�9

H2O

8HCl(1

N)

CaCl 2

(1N)

K2C

2O4(1N)

f10% 14g+

nonsingular

curv

edface

sCurv

edface

sare

ascribed

toth

eregion

comprised

inbetwee

nth

ef1

0% 10gprism

andth

escalenohed

ron

Prieto

etal.

[5]

#5

Na2SiO

3�9

H2O

7–8

CH

3COOH

(2M

)CaCl 2

(0.16M

)(N

H4) 2CO

3(0.16M

)f1

0% 14g+

f01% 12g

rhombohed

ra+

f10% 10gstriated

and

curv

edprism

s

Small

crystals

with

f10% 14g+

f01% 12g+

f10% 10gform

swhereCa2+

>CO

32�

wherea

slarg

erf1

0% 14gc

rystals

prevail

when

[Ca2+]D

[CO

32�]

Heijnen

[6]

L. Pastero et al. / Journal of Crystal Growth 247 (2003) 472–482 473

2. Materials and methods

Growth experiments were carried out in solution(a), in gel (b) and in a peculiar solution obtainedby mixing two mother solutions diffusing throughtwo gel barriers (c). U-tubes and trident-tubeswere used for cases (b) and (c), respectively(Fig. 1).

Two types of gel were employed: agarose (CarloErba Agar–Agar, use for foods) and Na2SiO3

� 9H2O. Both acetic acid, CH3COOH and HClwere used as acidifying agents for Na-MTS gel (JTBaker Chemicals N.V.). The Na-MTS gel wasprepared from a 10% Na2SiO3 � 9H2O aqueoussolution (pHE12.8) and, successively, acidifiedeither with 2M acetic acid or 1M HCl until the pHreached the value of E7.5 which further increasedto 8–8.5 during the gelification.

Mother solutions were prepared from analyticalgrade CaCl2 � 2H2O and NaHCO3 or Na2CO3.Types of gel, initial concentration of the mothersolutions, acidifying agents, along with the ob-served morphologies are described in Table 2.

Cation concentrations were determined by ICP-AES (IRIS II Advantage/1000, Thermo-Jarrel AshCorp.) and the analytical microprobe associated tothe scanning electron microscope (Cambridge S360equipped with an EDS Link QX2000, detectorPentafet) was employed to determine qualitativelythe chemical elements present in the core of calcitespherulites. X-ray data on crystallization productswere obtained by means of powder diffractometerSiemens D5000 (Bragg–Brentano geometry). Op-tical, electronic and atomic force microscopy(Danish Micro Engineering DME-Dualscopet)were employed to study the crystal morphology.

Fig. 1. Two types of tubes used in gel growth experiments: (left side) the three-arms device, where the central arm receives the

counterdiffusing solutions and holds the seed crystals, and (right side) the classic U tube.

L. Pastero et al. / Journal of Crystal Growth 247 (2003) 472–482474

3. The morphology of calcite crystal

3.1. The observed morphology

From Table 2 it is easy to find that the calcitecrystal’s morphology highly varies according towhether the diffusing medium is the agarose orNa-MTS. When the gel is agarose, the mostrecurrent morphologies are:

* Simple (or ‘‘twinned’’) spherulites in which theindividuals are radially distributed around acrystallization centre, the shape of their termi-nating tips being the cleavage rhombohedronf1 0 %1 4g (Fig. 2a).

* Single crystals in which: (i) the macroscopicallyflat f1 0 %1 4g rhombohedron prevails on theremaining completely rounded forms (Fig. 2b),(ii) the f1 0 %1 4g form competes with striated,rough and often curved acute f4 %4 0 1g rhom-bohedron (Fig. 2c) and the f1 0 %1 0g prism(Fig. 2d).

When the gel is Na-MTS the morphology getsrich:

* The steep f0 1 %1 2g rhombohedron becomes thedominating form. It appears with flat faces,growing by lateral spreading of macrostepsgenerated by hollow core dislocations, as wewill show in detail elsewhere. Moreover, it is

Table 2

Types of gel, initial concentration of the mother solutions, acidifying agents, and the observed crystal morphologies

Exp.

code

Gel Reacting solutions Observed crystal morphology

U1a Agarose U.A 1% 10ml Na2CO3 1M; 10ml CaCl2 � 2H2O 1M (a) Simple and ‘‘twinned’’ spherulites terminated

byf1 0 %1 4g; (b)f1 0 %1 4g+rough f4 %4 0 1grhombohedron+curvedf1 0 %1 0gprism

U2a Agarose U.A 1% 10ml Na2CO3 1M; 10ml CaCl2 � 2H2O 1M f1 0 %1 4g+striatedf1 0 %1 0gprism

U3a Agarose U.A 1% 10ml Na2CO3 1M; 10ml CaCl2 � 2H2O 1M f1 0 %1 4g+striatedf1 0 %1 0gprism

U4a Agarose U.A 1% 10ml NaHCO3 0.1M; 10ml CaCl2 � 2H2O 0.1M (a) Simple and ‘‘twinned’’ spherulites terminated

byf1 0 %1 4g; (b)f1 0 %1 4g+curvedf1 0 %1 0gprism

U1 Na-MTS

10%+Acetic acid

10ml Na2CO3 1M; 10ml CaCl2 � 2H2O 1M f0 1 %1 2g rhombohedron+dendrites generated

by the morphological instability off1 0 %1 4g

U2 Na-MTS

10%+Acetic acid

10ml Na2CO3 1M; 10ml CaCl2 � 2H2O 1M (a) Morphologically unstable f0 1 %1 2g;(b) f0 1 %1 2g+striatedf1 0 %1 0g+unstablef1 0 %1 4g;(c) dendriticf1 0 %1 4g

U3 Na-MTS

10% Acetic acid

10ml Na2CO3 1M; 10ml CaCl2 � 2H2O 1M (a) Dendriticf1 0 %1 4g;(b)f0 1 %1 2g+f1 0 %1 4g+striatedf1 0 %1 0g

U4 Na-MTS

10%+Acetic acid

10ml Na2CO3 0.1M; 10ml CaCl2 � 2H2O 0.1M f1 0 %1 4g + rough and curved f1 0 %1 0gprism+sometimes occurring small f0 1 %1 2gfaces

U5 Na-MTS

10%+HCl

10ml NaHCO3 1M; 10ml CaCl2 � 2H2O 1M f1 0 %1 4g+sometimes occurringf1 0 %1 0g prism

T1 Na-MTS

10%+Acetic acid

10ml Na2CO3 0.1M; 10ml CaCl2 � 2H2O 0.1M Flatf0 1 %1 2g rhombohedra

T1a Agarose U.A 1% 10ml Na2CO3 0.1M; 10ml CaCl2 � 2H2O 0.1M Simple spherulites formed by single crystals

terminated by f1 0 %1 4grhombohedra


often accompanied by small vicinal forms as thef1 0 %1 5g and f1 0 %1 7g rhombohedra (Fig. 3a).Otherwise the acute vertices of the f0 1 %1 2gform become the privileged sites for repeated2D-nucleation (probably owing to higher con-centration gradient which sets up around theacute vertices with respect to that of the obtuseones), as shown in Fig. 3b.

* The morphological instability of the f0 1 %1 2gform sets up when the situation illustrated inFig. 3b degenerates to that depicted in Fig. 3c,especially in those regions of the gel tube werethe supersaturation reaches its highest values.

* Sometimes the combination of f0 1 %1 2g andf1 0 %1 4g forms occurs, while the f1 0 %1 4g formalone is found particularly far from the inter-face between the CaCl2 solution and the gel,when the pH reaches its highest value, asmentioned by Kirov et al. [3] and Heijnen [6].

Also for the growth of the f1 0 %1 4g form themorphological instability is pointed out bythe preferential surface nucleation around thecorners of the rhombohedron (Fig. 3d).

3.2. The influence of gel composition on the calcite

morphology

The gel composition not only influences themorphology of crystals grown inside the very gel,but also that of crystals which grew (onto glasscapillaries) inside the solution of the central arm oftrident-tubes, where the gel barriers have only thetask to slow down the diffusion rate of reactingions. In fact also in this case we obtained the samecalcite morphologies we just described (Figs. 4aand b). This implies that the counter-diffusingsolutions leached the gel barriers and hencetransferred to the growth solution, contained in

Fig. 2. Calcite crystals grown in gel (agarose). (a) Simple and ‘‘twinned’’ spherulites terminated by cleavage rhombohedron faces; the

hollow centre of the spherulites has revealed the presence of calcium sulphates and phosphates. (b) Flat cleavage rhombohedron prevail

on the completely rounded crystal shape. The cleavage rhombohedron competes either with the (c) acute f4 %4 0 1g rhombohedron or (d)

not well defined f1 0 %1 0g prism.


Fig. 3. Calcite crystals grown in gel (Na-MTS). (a) The sequence shows the surface of a steep f0 1 %1 2g rhombohedron face, with the

macrosteps starting from the hollow core dislocations wall and the vicinal f1 0 %1 5g rhombohedron. (b) When 2D-nucleation occurs on

the acute angle sites of the f0 1 %1 2g form, the faces look no longer flat; the morphological instability of the form sets up when the local

supersaturation futher increases (c). (d) The cleavage f1 0 %1 4g rhombohedron loses its morphological stability as well when 2D-

nucleation starts around both acute and obtuse vertices of the form.


the central arm, the impurities conditioning thenucleation and growth of calcite crystals.

To control the quality of these impurities andtheir concentration in the growth solutions wetested by leaching the gels we employed.

Three Na-MTS columns and three of agarosewere prepared. Three different leaching werecarried out: the first one using 10ml of ultra-purewater (18MO), the second one with 20ml of ultra-pure water and the third one with 10ml of 1Msolution of CaCl2 � 2H2O. The results are drawn inFigs. 5a and b. From Fig. 5a it comes out that theleaching of the agarose gel transferred in solutionmainly sulphur, sodium, calcium and magnesium;potassium and phosphorus are present in minoramounts. Calcium, when introduced with theleaching solution, is not considered. Crystals thatspontaneously nucleated in solution after leachingwere studied by XRPD and EDS: they resulted tobe a mixture of Ca–Na–K sulphates and phos-phates. It is worth to mention here that the solidinclusions found at the centre of calcite spherulites(Fig. 2a) mainly contain calcium, sodium andsulphur, as it was shown by microprobe analyses.From Fig. 5b it ensues that, apart the Ca2+ ion,higher amounts of Na+ and CH3COO� and loweramounts of K+ and Mg2+ ions are leached fromthe gel of Na-MTS. Thus, it was easily shown that

Na+ ion cannot intervene in the appearance of thef0 1 %1 2g form. As a matter of fact it is worth toremember that f1 0 %1 4g was the only form incrystals precipitated from two aqueous solutionsequally supersaturated with respect to calcite (oneout of them containing only calcium carbonateand NaOH species [7] and the other one alsoNa-MTS to simulate the gel composition).

This proves that the acetate ion plays anexclusive role in the competition between the twomost important calcite rhombohedra.

Four further simple experiments confirm thisstatement.

In the first one, two crystallization tests werecarried out in two separate U-tubes, both filledwith Na-MTS gel, the sole difference betweenthem being the acidifying agent: f1 0 %1 4g is thesole form in the presence of HCl, while f0 1 %1 2gdominates on the f1 0 %1 4g; in the presence of theacetate ion.

In the second experiment f0 1 %1 2gshaped crys-tals were used as seeds in a pure aqueous solutionsupersaturated with respect to calcite. It was notsurprising to observe (after one month) that thenew nucleated crystals were perfect f1 0 %1 4grhombohedra, as generally occurs in pure solu-tions, but it was quite unexpected to point out thatthe seeds grew changing their shape: as it may be

Fig. 4. Calcite crystals, grown around glass capillaries, from solutions diffused trough gel barriers, in the central arm of a trident tube:

spherulites when the gel is agarose (a) and single crystal showing the sole f0 1 %1 2g form (b), when the gel is Na-MTS.


seen in Fig. 6, at each corner of the originalsteep rhombohedron a small f1 0 %1 4g cleavagerhombohedron develops, giving rise to a verypeculiar hopper crystal in which the last borncrystallographic forms tend to envelop the ancientone.

A third experiment was carried out to confirmthe influence of the acetate ion on the stability ofthe f0 1 %1 2g form. A CaCl2 aqueous solution was

mixed with a Na-acetate solution buffered at pH 5;then droplets of NaHCO3 were added and the pHwas maintained lower than 7. f0 1 %1 2g was by farthe prevailing form shown by the early nucleatedcrystals. When the experiment was repeated, at thesame pH value, but in the absence of Na-acetate,only cleavage rhombohedra were observed.

In the last experiment a f1 0 %1 4g cleavage facewas etched in slightly undersaturated CaCO3

Fig. 5. Ionic concentrations (expressed in 10�3mol) in solution, as it comes out from the leaching of agarose (a) and Na-MTS (b) gels.


aqueous solutions. When the solution was pure theetch pits were rhombic shaped (Fig. 7a), the edgesof the rhomb being the /%4 4 1S directions and thefacets of the hollow pyramids limiting the pitsbeing those of the rhombohedron, as it was shownby several authors [8–10]. But when acetic acid orNa-acetate was added, at pHB5, the morphologyof the pits became richer (Fig. 7b): new edges

became stable and the pit developed along thesymmetry plane of the face. The shortest side ofthe pit is parallel to the /0 1 0S direction, which isrelated to an important PBC of the face, whilst thelongest ones run parallel to the other importantPBC ½4 2 %1�: Finally, it is worth remembering thatthe obtuse angle (101.901) of the etch pit obtainedin pure solution reduces to the acute one (B831) in

Fig. 6. Growth of f0 1 %1 2g shaped seeds of calcite in pure supersaturated aqueous solution supersaturated with respect to calcite. (a)

The original seeds were dominated by the f0 1 %1 2g form and show narrow f1 0 %1 4gfaces. (b) Small f1 0 %1 4g shaped crystal nucleated in

solution, while at each corner of the seed cleavage rhombohedra developed and encompassed the seed, so changing its morphology.

Fig. 7. Etch pits on a freshly cleaved f1 0 %1 4g face of calcite. (a) When the etching is obtained in pure CaCO3 aqueous solution the pit

shape is rhombic shaped (optical microscopy). (b) When the acetate ion is present in the etchant solution the pit morphology becomes

richer; its microfacets running along the symmetry (m) plane belong to the f0 1 %1 2g form, as it ensues from the image profile (atomic

force microscopy).


the presence of acetic acid: this new angle isdetermined by the intersection of the /%8 5 2Sdirections. As we will show in detail in a forth-coming paper [11], the reconstruction of the 3Dgeometry of the pit (obtained by AFM measure-ments) allows to say that:

* The microfacets subparallel to ½4 2 %1� directionbelong to the f0 1 %1 2g form, as it ensues fromthe dihedral angle of nearly 1201 formed withthe cleavage plane and measured through theAFM profile of the cross section depicted inFig. 7c;

* Those related to the /%8 5 2S directions corre-spond to the bipyramid f2 2 %4 3g which is theonly form of the type fh h 2h lg and shows anoccurrence frequency of near 5% in naturalcrystals [11];

* The microfacet related to the [0 1 0] directionforms an angle of 1351 with the f1 0 %1 4g faceand then is parallel to the hexagonal prismf1 0 %1 0g which represents the most recurrentform in natural crystals.

4. Conclusions

The growth of calcite crystals from agarose andNa-MTS gels showed that the crystal morphologyis strongly affected by the impurities contained inthe gel. We have pointed out that:

* Calcium sulphates and phosphates contained inthe agarose constitute the central nucleus of thecalcite spherulites; we have certainly to do, inthis case, with a heterogeneous nucleation, evenif we are not able, for the time being, toascertain the amorphous or crystalline nature ofthese agents favouring the radial nucleation andgrowth;

* The only form which showed a flat profile, ingrowth from agarose, is the cleavage f1 0 %1 4grhombohedron, while all other crystallographicforms tend to assume a rounded shape. Fromthis point of view it seems that the agarose doesnot modify the results of the theoreticalresearches [6,12–14] which foresee that thef1 0 %1 4g form can be the only one present atthe equilibrium (both in the crystal–vapour

system and with water adsorption) and theslowest one, having by far the lowest attach-ment energy [6,12,13];

* The acetate ion strongly affects the calcitemorphology, both in gel and aqueous solution.Its presence seems to reverse the ratio betweenthe specific surface energies of the two compet-ing rhombohedra, since the f0 1 %1 2g formwidely dominates at the early stages of thenucleation; furthermore it modifies the relativekinetics of these rhombohedra, as it comes outfrom the f0 1 %1 2g-f1 0 %1 4g transition in solu-tion growth, when the presence of the acetateion leaves off.

Within this paper we did not consider the role ofthe supersaturation on the competition betweenthe two most important F faces of the calcitecrystal, even if from our observations and fromliterature data [3,6] it ensues that the morpholo-gical importance of the f0 1 %1 2g form increaseswith supersaturation. This effect being qualitative,we are carrying out isothermal growth experimentson calcite spheres, at varying supersaturationvalues, both in pure solution and in the presenceof acetate ion.

Acknowledgements

This work was supported by MIUR (Ministerodell’Istruzione, Universit"a e Ricerca). Linda Pas-tero and Dino Aquilano are grateful to Drs.St!ephane Veesler and Jean Pierre Astier (CRMC2-CNRS Luminy-Marseille) for the experiencegained in their laboratory (in AFM techniques).

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The competition between {1 0 % 1 4} cleavage and {0 1 % 1 2} steep rhombohedra in gel grown calcite crystals

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