(Zabuyelite) Anomalous Mixed Crystals. Sector Zoning and Growth

CaCO3 (Calcite)/Li2CO3 (Zabuyelite) Anomalous Mixed Crystals. Sector Zoning and Growth Mechanisms Linda Pastero and Dino Aquilano* Dipartimento di Scienze Mineralogiche e Petrologiche, UniVersita` degli Studi di Torino, Via Valperga Caluso 35, I 10125 Torino, Italy

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 9 3451–3460

ReceiVed May 9, 2008; ReVised Manuscript ReceiVed May 27, 2008

ABSTRACT: A growth model of {0001} form bearing calcite crystals has been proposed in our previous papers. A detailed PBC (periodic bond chains) study demonstrated that calcite {0001} form could not be stable, owing to its K character. The stabilization involves a 2D epitaxial growth mechanism between calcium and lithium carbonate, the model being supported by parametric fit between calcium and lithium carbonates. In this paper, we experimentally evaluate the consistency of this model. Integrated spectroscopic and imaging characterization techniques, inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray powder diffraction (XRPD), cathodoluminescence (CL) and electron paramagnetic resonance (EPR) have been applied to synthetic calcite crystals grown in the presence of lithium in order to investigate whether and how lithium can be absorbed into the calcite lattice. Experiments and theoretical considerations prove that lithium enters the calcite crystals during their growth, according to strongly differentiated sector absorption mediated by 2D and 1D epitaxy on the corresponding {0001} and {101j4} crystal forms. Introduction The influence of lithium on the crystal habit of calcite has become a matter of some interest since Rajam and Mann1 found that the flat-shaped {0001} form was added to the classic cleavage {101j4} rhombohedron in crystals growing from Li+doped aqueous solutions. Later, Nefyodova et al.2 confirmed that pure {101j4} calcite seeds transform into final crystals always dominated by the {0001} form when growing from lithium-bearing hydrothermal solutions. In addition, IR spectroscopic analysis1 and evaluation of the segregation energy of lithium ions on the {0001} form of calcite3,4 suggested that lithium cannot be absorbed within the growing calcite bulk but can be randomly adsorbed in lattice and not interstitial sites on the {0001} surfaces, thus slowing their advancement rate. More recently, experimental and theoretical research on calcite crystallization from aqueous solutions in the presence of variable concentrations of lithium and on the growth morphology of single and twinned crystals of Li2CO3 (mineral zabuyelite) proved that lithium deeply affects the calcite morphology by changing the character of the {0001}-kinked and {011j8}-stepped forms of calcite into flat (F) ones.5–8 These and other morphological transformations are not due to the random adsorption of lithium ions onto the calcite surfaces but to epitaxial relationships setting up between {001}, {1j01}, and {100} forms of zabuyelite and {0001}, {011j8}, and {101j4} forms of calcite. Finally, it was shown9 that when the mother solution is slightly supersaturated with respect to Li2CO3, both single and 100 twinned zabuyelite crystals appear; further, if the supersaturation increases and the calcium amount in solution is such that calcite crystals become stable, then zabuyelite and calcite coprecipitate giving rise to (i) 3D epitaxy of single zabuyelite crystals on the {0001} form of calcite and (ii) 3D epitaxy of triple zabuyelite twins on the {101j4} cleavage rhombohedron of calcite. In addition, the growth morphology of zabuyelite is deeply modified by the presence of calcium in solution. All these findings surely prove that the crystal structures of zabuyelite and calcite can “interact” giving rise to complex bulk * Corresponding author. E-mail: [email protected]. Tel: +39116705125. Fax: +39116705128.

and surface morphologies: in other words, from an aqueous solution containing Ca2+ and Li+ along with the different carbonate ions, a large variety of calcite and zabuyelite crystals (single, twinned, and 3D and 2D epitaxies) can generate according to the chemical equilibria ruling the mother phase and hence to different supersaturation levels with respect to the two crystal species. Up to now, the interaction between zabuyelite and calcite seemed to be confined to the interfaces forming between the two carbonates during growth, and the evidence found in literature3,4 seems to sustain this hypothesis as well. In this paper, we will demonstrate that a deeper investigation on the bulk structure of calcite crystals grown in aqueous solution and in the presence of lithium reveals a more intimate interaction between zabuyelite and calcite. As a matter of fact, it will be shown that lithium can be absorbed into the lattice of calcite during growth and that an anomalous CaCO3/Li2CO3 mixed crystal, in the sense of Johnsen,10 can form limited to the growth sectors of {0001} and {101j4} forms of calcite. A preliminary clue that directed our research consisted in the extraordinary parametric coincidence between host and guest lattices at the {0001} calcite/{001} zabuyelite interface (see Table 1). It is worth outlining that these lattice coincidences are not limited to the 2D lattice ruling the calcite/zabuyelite epitaxial interface but that also the thicknesses of the two interfacial layers show a negligible misfit. All these conditions, that is, the parametric coincidence in the three space directions between the adsorbing and the absorbing crystal phases, have to be fulfilled when an anomalous mixed crystal has to be formed.10–13 Materials and Methods Calcite crystals were grown from aqueous solutions and gel according to the methods described in our previous works.5,14,15 The crystalline population to be used for X-ray powder diffraction analysis (XRPD) and atomic force microscopy (AFM)/scanning electron microscopy (SEM) imaging was obtained from calcium carbonate solutions by bubbling CO2 into aqueous suspensions of CaCO3. Five millimolar solutions obtained were filtered using a 0.45 µm Millipore filter, and subsequently, LiCl solution was added, initial Li+/Ca2+ molar ratio

10.1021/cg800483g CCC: $40.75  2008 American Chemical Society Published on Web 08/09/2008

3452 Crystal Growth & Design, Vol. 8, No. 9, 2008

Pastero and Aquilano

Table 1. Lattice Coincidences at the {0001} Calcite/{001} Zabuyelite Interface

2D cell vectors and length (nm) layer thickness (nm)

host crystal, calcite form {0001}

guest crystal, zabuyelite form {001}

misfit %

obliquity

|[210]| ) 0.864 |[010]| ) 0.4989 d0006 ) 0.2843

|[100]| ) 0.8359 |[010]| ) 0.4972 d002 ) 0.2812

+3.3 +0.34 +1.1

0°

ranging from 0 to 25. Both CaCO3 and LiCl were Fluka analytical grade reagents. Crystals were filtered from mother solutions into qualitative filter papers, then washed flushing water throughout to eliminate chloride salts, and finally dried at 60 °C. Calcium and lithium amounts in residual solutions were analyzed using a Thermo Jarrel Ash Corp. IRIS Advantage II inductively coupled plasma optical emission spectrometer (ICP-OES; axial torch, flushing rate 2.22 mL/min, pump rate 2.22 mL/ min, nebulizer pressure 23 PSI, RF power 950 W). Survey XRPD patterns were obtained using a Huber-Guinier Camera G670 (Cu KR1 radiation, 2θ ranging from 7° to 100°, step size of 0.005°, and 2 h data acquisition). Owing to the instrumental peak asymmetry, which precludes shape study, the peak evaluation was carried out on XRPD patterns performed on a Siemens D5000 diffractometer (Bragg-Brentano geometry, Cu KR1,2 radiation, 2θ ranging from 21° to 96°, step size of 0.005°, and 2 s time step). More detailed information about the 0006 reflection of calcite was obtained from scanning a 2θ interval from 27.5° to 32.5° with a step size of 0.005°, and 16 s step time. NIST Silicon standard (640c) with 0.543 119 46 nm ( 0.000 000 92 nm certified cell parameter was used as XRPD internal standard. These data were refined using the Rietveld refinement software package GSAS.16 Since a unique crystal phase (calcite) was obtained from different crystal populations, a quasi-perfect overlapping of the diffraction lines all over the XRPD patterns was observed. Nevertheless, because cell parameter spreading of some hundredth of an angstrom resulted, the calculated values were compared with those obtained from a leastsquares cell refinement using UNITCELL software17 and starting from XRPD pattern decomposition. FITYK curve fitting software18 was used not only for XRPD pattern decompositions but also for cathodoluminescence (CL)spectra. For each experiment, we got some electron micrographs in order to evaluate the impurity effect on crystal morphology by means of a Cambridge S-360 scanning electron microscope (SE and BSE detectors) equipped with an Oxford Inca Energy 200 EDS system (EDS PentaFET detector) and an Oxford Instruments MonoCL cathodoluminescence system. In-situ AFM images (continuous contact mode performed using Mikromash silicon probes, Al-backside, k ) 40 N/m) were obtained with a DME Dualscope Rasterscope atomic force microscope equipped with a C-21 controller and a 50 µm scanner. Calcite crystals to be examined in cathodoluminescence and EPR were obtained by mixing of two solutions prepared to transfer Ca2+, Li+, and CO32- counterions into a 5% or 10% Na-metasilicate gel column polymerized with CH3COOH (2 M), as indicated in our previous works.6,14 Counterion-bearing solutions were 0.05 and 1 M. In this way, we obtained a few millimeter sized individuals showing well-defined sector zoning. EPR measurements were performed on a X band Bruker EMX spectrometer (center field 3530.000 G, sweep width 1500.000 G, resolution 1024 points, microwave frequency 9.492923 GHz).

Experimental Results and Interpretation A. Growth Morphology of Calcite from Scanning Electron and Atomic Force Microscopy. Initial and residual concentrations (after each crystallization run) of both Ca2+ and Li+ cations are shown in Figure 1. In depleted solutions, calcium concentration sensibly varied from one sample to another (Figure 1a), while there was no significant change in lithium concentrations (Figure 1b), meaning that bulky crystallization of lithium carbonate cannot occur, as further confirmed by crystal phase identification from XRPD patterns. SEM pictures showed well-formed calcite individuals with dominating {101j4} cleavage (Figure 2a) and narrow {011j8} rhombohedra (Figures 2b and 3a). Triangular- or pseudohex-

agonal-shaped {0001} pinacoids also appeared (Figures 2b-d and 3a), progressively enlarging with increasing lithium concentration1,2 and often characterized by regular sequences of macrosteps (see AFM topographic profile in Figure 3e). B. Sector Growth from Cathodoluminescence (CL) Measurements. Images obtained from backscattered electrons on two out of many single crystals (Figures 4a and 5a) do not evidence compositional zoning as concerns the main chemical elements, while the {0001} growth sectors show sharp zoning due to the presence of specific impurities, as revealed by cathodoluminescence analysis (Figures 4b and 5b). Sector zoning is due to the presence of Mn2+ in the gel matrix. This is further confirmed by EPR data (Figure 6), since Mn2+ replacing Ca2+ in calcite crystals leads to a complex EPR spectrum with more than three sharp lines, as reported in literature.19–23 A general view of an entire crystal confirms that these features occur in the remaining {101j4} sectors as well (Figure 7a,b). The {101j4} growth sectors show an oscillatory zoning due to the growth mechanism the crystal underwent (Figure 7d). As a matter of fact, once a critical supersaturation level is reached in the closed growth gel system, nucleation and growth occur. Then, supersaturation decreases, and the crystal stops growing until a new supersaturation pulse sets up. So, zoning reveals that pulse modulation is the origin of discontinuities in sector boundaries (Figure 5b). Special attention must be paid to the funnel-shaped (0001) sector (Figure 7d). A first supersaturation pulse leads to nucleation and growth of the {101j4} rhombohedron. At this stage, the (Li+/Ca2+) ratio is too small to generate the {0001} form; moreover, the three-dimensional Li2CO3 phase cannot nucleate because of its solubility. Hence, the (Li+/Ca2+) ratio can increase and the beginning of the {0001} form appears in the crystal morphology once a suitable value of the (Li+/Ca2+) ratio is reached. Cathodoluminescence spectra obtained from the growth sectors show three broad emission bands: the first one, at 375 nm, is related to the intrinsic luminescence of calcite, while the second (at 420 nm) is characteristic of calcite, and the last one (at 620 nm) is related to Mn2+ in calcite (Table 2).24–26 To characterize the emission lines associated with each growth sector, the emission spectra were decomposed (Figure 8a,b and Table 3). Assuming the 375 nm peak as a reference emission line, one can compare the 620 nm peak measured on both the darkest and brightest sectors (Table 3). In the darkest ones, the 375 nm intrinsic emission line broadens, while the Mn2+ line loses its intensity. Comparing the two emission line areas in both sectors, one can see that in the brightest ones Mn2+ vs intrinsic emission lines ratio is about 0.75, while in the darkest sectors, the same ratio decreases to 0.14. This means that in the brightest sectors, the Mn2+ emission line hides the intrinsic emission line. The correlation between the intensity of the emission line at ∼620 nm and the Mn2+ concentration in calcite being well-known,24,26,27 one can affirm that in the darkest sectors the Mn2+ amount is lower than in the brightest ones. Shortly, this surely proves that the concentration of Mn2+ in the {0001} growth sectors is lower than that in the {101j4} ones. However, it is well-known that Mn2+ can replace Ca2+ in calcite crystals

Growth of CaCO3/Li2CO3 Anomalous Mixed Crystals

Crystal Growth & Design, Vol. 8, No. 9, 2008 3453

Figure 1. Initial and residual concentrations for calcium (a) and lithium (b) ions, measured in growth solutions.

Figure 2. Changes in calcite morphology (from {101j4} rhombohedron to rhombohedron plus {0001} pinacoid) when Li+/Ca2+ ratio increases in the mother solution.

(while it cannot replace Li+ in the Li2CO3 structure). Consequently, on the grounds of these results and on the final

considerations proposed in the Introduction (see Table 1), the darkest funnel shaped (0001) growth sector has to be associated



Figure 3. SEM (a,b) and AFM (c,d) images of {0001} cobble-shaped calcite surfaces. No monolayered steps are detectable, as it can be seen in the surface profile (e).5

with calcite deficiency and, correspondingly, with zabuyelite abundance. Now, the question is how calcite and zabuyelite structures could reciprocally arrange within the growing {0001} sectors. The XRPD patterns are the only technical

support to give a reasonable answer, especially when the small value of the ratio between the volume of the {0001} growth sectors and the total volume of the crystallized calcite is taken into account.



Figure 4. (a) Backscattered electron and (b) CL panchromatic image of the (0001) growth sector showing its layered structure evidenced by the oscillatory pattern of the CL signal.

Figure 5. (a) Backscattered electron and (b) CL panchromatic image of the (0001) growth sector. Discontinuities in sector boundary (dotted line) are due to the pulsed growth mechanism in gel.

Figure 6. EPR diagram (77 K) of gel-grown calcite showing sector zoning due to Mn2+ contained as paramagnetic impurity in the growth gel.

C. XRPD Patterns. From X-ray powder diffraction patterns two behaviors appear depending on whether each single pattern

is considered as a whole or some characteristic diffraction peaks are analyzed in their shape evolution as a function of the Li+/ Ca2+ ratio in growth solutions. 1. Considering Each XRPD Pattern As a Whole and Deriving a Unique Calcite Unit Cell from It. Cell parameters were calculated from GSAS Rietveld refinement and UNITCELL software. From pure solution, two slightly different cells were obtained, while three cells were identified with increase of the relative lithium content in solution. The dispersion of cell parameters and volumes is illustrated in Figure 9. The dispersion of a0 is rather symmetric around its mean value (0.49927 nm) and regularly increases with the Li+/Ca2+ ratio in growth solutions, while the deviation of the mean a0 value measured in the presence of lithium with respect to that coming from pure Ca solution does not exceed 0.045%. However, the dispersion of c0, which is more or less symmetric around its mean value (1.70741 nm), varies less regularly than a0, and the corresponding just mentioned deviation is even lower (0.00615%). Correspondingly, the dispersion of the cell volume follows the trend of a0, and its deviation does not reach 0.1%. In summary,



Figure 7. (a) CL panchromatic image of the whole gel-grown calcite crystal, (b) backscattered electron image of the same crystal shown in panel a, (c) crystal orientation, and (d) sector zoning (A ) (101j4) sector; B ) funnel-shaped (0001) sector). Table 2. Peak Position of Mn2+-Activated Cathodoluminescence in Calcitea

a

peak position (nm)

ref

615-633 580 578 615 645 (low temperature) ∼600 ∼ 610 556-605 590 605 625-633 640 610 615

Mukherjee (1948)* Coy-Yll (1969)* Amieux et al. (1983)* El Ali (1989)* Walker et al. (1989)* Mason and Mariano (1990)25 Diaz et al. (1991)* Mukherjee (1948) * Sommer (1972)* Mariano in Marshall (1988)* Walker et al. (1989)* Hemming et al. (1989)* El Ali et al. (1991)* El Ali et al. (1993)*

Data from various authors (* references in El Ali et al.24).

Figure 9 proves that lithium does enter the calcite structure, and the resulting lattice modifications increase with the Li+/ Ca2+ ratio in growth solutions. Further, the lithium effect on the calcite lattice should be higher than that evidenced by XRPD spectra since, as mentioned before, the crystalline volume, which is strongly affected by lithium incorporation, represents only a small fraction (roughly less than 5%) of the entire diffracting calcite mass.

2. Considering the Evolution of Some Characteristic Diffraction Peaks As a Function of Lithium Amount in Growth Solutions. The 101j4 is by far the main diffraction peak of calcite. In pure CaCO3 samples, the highest intensity elementary curve among the ones resulting from the decomposition (Figure 11a) is located at d101j4 ) 0.3043 nm. With increasing lithium amount, the maximum intensity progressively shifts toward lower interplanar spacing, that is, at d101j4 ) 0.3031 and 0.3028 nm when Li+/Ca2+ ) 5 and 25, respectively (Figure 11b,c). Hence the dispersion of the maximum is not symmetric around the averaged value of the peak 〈d101j4〉 ) 0.3035 nm. It is worth remembering here that the d111 reflection of pure Li2CO3 crystal occurs at 0.30311 nm and that, consequently, the presence of mixed CaCO3-Li2CO3 layers in the {101j4} growth sectors of calcite should affect the position of the maximum corresponding to the 101j4 peak. As a matter of fact, a new epitaxial relationship can be realized at the {101j 4}calcite / {111}zabuyelite interface, as illustrated in Table 4. Comparing Tables 1 and 4, one can argue that the {101j4}calcite/ {111}zabuyelite epitaxy is geometrically well grounded, even if the lattice coincidences at the {0001}calcite/{001}zabuyelite interface show an overall lower misfit, both for 2D cell parameters and angular discordance (obliquity). Furthermore, it is worth considering that the carbonate groups within the d111 slice of zabuyelite lie nearly parallel to the corresponding groups


Crystal Growth & Design, Vol. 8, No. 9, 2008 3457 Table 3. Comparison between Brightest and Darkest Sectors Emission Lines at 375 nm (Calcite Intrinsic Emission Line) and 620 nm (Mn2+ as Calcite Impurity Emission Line)

Figure 8. (a) CL emission spectra obtained from the brightest and darkest growth sectors along with their decomposition and (b) emission lines obtained from the decomposition (Voigt functions).

characterizing the d101j4 layers of calcite, even better than it occurs at the {0001}calcite/{001}zabuyelite interface (Figure 10).

emission line

height ratio (brightest vs darkest)

area ratio (brightest vs darkest)

fwhm ratio (brightest vs darkest)

intrinsic emission Mn2+ emission

1.5 3.3

0.8 4.3

0.6 2.9

However, Figure 10 illustrates a deep difference between these two interfaces: at the{0001}calcite/{001}zabuyelite interface. the epitaxy forms between two faces having F character, while at the {101j4}calcite/{111}zabuyelite interface, the {111} form is a stepped one because any bond can be found among adjacent [1j10] PBCs of zabuyelite within a d111 slice. This implies that the growth of the elementary d111 slice can occur only through 1D nucleation of noncorrelated [1j10] chains, and hence their adsorption on the {101j4} rhombohedron of calcite is less favored with respect to that of zabuyelite nuclei (of thickness d002) on the {0001} pinacoid. Nevertheless, if the adsorption occurs, 1D zabuyelite chains can be easily buried within the {101j4} growth sectors by new freshly created calcite layers, owing to the quasi-perfect coincidence between the layer thicknesses of the two crystals (Table 4). This seems to be the most reasonable way of explaining the progressive shift of the highest intensity component of the 101j4 peak (and hence of the lowering of the d101j4 value) of our crystals, with the increasing amount of lithium in the growth solution. The 0006 diffraction peak of calcite is the fourth in order of importance in calcite (its intensity ratio being 73/100 of the 101j4 one). The decomposition in Figure 11 d reveals that, from the

Figure 9. (a–c) Dispersion of parameters and volume of the calcite unit cell from Rietveld refinement. Gray dotted line refers to the volume mean value.



crystals not showing the {0001} growth sector). (ii) Another growth sector affected by lithium absorption, even if to a lesser extent, is that of the {101j4} rhombohedron. This is not surprising, since we already observed that pure calcite seeds limited only by {101j4} are encompassed by thick growth layers starting from the new generated {0001} form, when lithium is added in the growth solution.5 Discussion

Figure 10. The unrelaxed {101j4}calcite/{111}zabuyelite (a) and {0001}calcite/ {001}zabuyelite (b) interfaces showing the stepped (S) and flat (F) character of the d111 and d101j4 slices, respectively. The underlying calcite d101j4 slice is seen along the 〈010〉 direction, while the d111 and d002 slices of zabuyelite are projected along the [1j10] and [010] directions, respectively.

two diffraction components (KR1 and KR2) of this peak, a unique value d0006 ) 0.284 69 nm comes out corresponding to c0 ) 1.7080 nm, which is very close to the value (1.7073 nm) calculated from the overall XRPD spectra in non-lithium-bearing calcite crystals. But, when the amount of lithium in growth solution reaches the values indicated in Figure 11e,f, the peak shape dramatically changes and the spacing coming out from the decomposition spreads over a ∆d0006 interval of 0.0169 nm, resulting in a fairly symmetrical peak dispersion around the averaged value of the peak 〈d0006〉 ) 0.284 69 nm. The highest peak dispersion is reached when Li+/Ca2+ ) 5, and correspondingly, the maximum of the peak intensity shifts toward higher spacing, being located at d0006 ) 0.285 89 nm, while the minimum intensity peak locates at d0006 ) 0.283 08 nm. When Li+/Ca2+ ) 25, the peak dispersion slightly narrows, and the highest intensity shifts toward lower interplanar distances (d0006 ) 0.283 97 nm). All this evidence further proves the following: (i) Lithium absorbed in calcite crystals mainly locates in the {0001} growth sectors perturbing the ordered stacking of the elementary CaCO3 layers (d0006 ) 0.284 69 nm). As a matter of fact, the formation of Li2CO3 monolayers (d002 ) 0.2812 nm) should induce zones of small compression and distension that are reflected in the dispersion of the cell parameter c0 from 1.7154 to 1.6985 nm (around the value of 1.7073 nm, measured for the pure calcite

Integrated characterization techniques (ICP, SEM, AFM, XRPD, cathodoluminescence (CL), and EPR) have been applied to calcite crystals grown both from solution and from gel in the presence of well-defined amounts of lithium, to investigate whether and how lithium can be absorbed into the calcite lattice. Cathodoluminescence measurements take advantage of the incompatibility between the structure of lithium carbonate and the manganese captured in it. Moreover, it is well-known that manganese capture in carbonates shows marked CL effect, since manganese-depleted sectors are frankly darker than the richer ones. From our measurements and from these considerations, we can argue the existence of irregular stacking sequences of Mn2+-rich and Mn2+-depleted layers within the (0001) growth sector of calcite. This proves, indirectly, than lithium has been buried during growth in the Mn2+-depleted layers. In addition, X-ray powder diffraction diagrams clearly showed that when lithium concentration is continuously increased in the mother phase, lithium is not only adsorbed through 1D or 2D epitaxies on {0001} and {101j4} forms of calcite5,6 but is absorbed in {0001} and {101j4} growth sectors as well. Diffraction diagrams evaluated through Rietveld refining confirm the coexistence of several calcite cells in the same crystal population, while the detailed analysis on 0001 and 101j4 calcite peaks reveals that the peak shapes are strongly related to the lithium presence in the growth medium. In summary, the incorporation of lithium into calcite does not randomly occur but needs an epitaxy mediation. This means that 2D coincidence lattices between lithium and calcium carbonates (Tables 1 and 4) are not confined to a geometrical meaning but represent the necessary condition for lithium to be incorporated through the epitaxial adsorption, followed by the absorption, of periodic bond chains (1D) or islands (2D) into growing calcite crystals. This epitaxial mechanism is selective, since the probability of lithium entering the calcite lattice varies from {0001} to {101j4} growth sectors, the first ones being largely favored, as it comes out from the contrast shown by cathodoluminescence imaging. On this ground, the formed anomalous calcite-zabuyelite mixed crystal (in the sense of Johnsen10 and Neuhaus11) is not homogeneous, as proved by both the cathodoluminescence and the different spreading of the 0001 and 101j4 XRPD peaks. The epitaxial way to lithium incorporation represents also the ultimate answer to the question open by the following: (a) Rajam and Mann1 who found that calcite crystals containing up to 0.06 mol % lithium showed, from high-resolution XRPD spectra, that the cell parameters slightly varied (∆a0 ) +0.14% and ∆c0 ) +0.16%). Moreover, IR spectra evidenced the absence of bulk structural effects, the absorption bands being characteristic of well-ordered calcite. Finally, no morphological

Table 4. Lattice Coincidences at the {101j4} Calcite/{111} Zabuyelite Interface

2D cell vectors and length (nm) layer thickness (nm)

calcite{101j4} form 1/3|[421j]| ) 0.811 |[020]| ) 0.99792 d101j4 ) 0.3043

zabuyelite {111} form |[01j1]| ) 0.79436 |[1j10]| ) 0.97266 d111 ) 0.30311

misfit %

obliquity

+2.09 +2.6 +0.39

2.34°



Figure 11. (a–f) Evolution of the 101j4 and 0006 calcite reflections with the Li+/Ca2+ ratio. Voigt function simulates the instrumental line broadening. When Li+/Ca2+ ) 5, different degrees of crystallinity of the two couples of peaks are well-evidenced (both KR1 and KR2 have been instrumentally resolved).

effects were observed for calcite crystals containing up to 0.04 mol % sodium, this being probably due to the size of Na+ ion (0.099 nm radius, 4-fold coordination) which is too large for specific occupancy of the in-plane (0001) sites of calcite (0.06 nm radius). (b) Titiloye et al.3,4,28,29 who evaluated, from defect surface energy calculation, that the polar {0001} form of calcite

becomes the most stable form (on substitution of Ca2+ with Li+), while the other nonpolar forms were destabilized. Further, the segregation energy of lithium (-776 kJ/mol with respect to the crystal bulk and -407.9 kJ/mol in solution) for the {0001} form indicates that lithium “prefers” to remain at the {0001} calcite surface, while all other faces considered allow the


dissolution of lithium in the crystal bulk. By simulation of the effect of sodium on the equilibrium morphology of calcite, by replacing Ca2+ ions with Na+, it was found that also sodium ions stabilize the {0001} form of calcite, but to a lesser extent with respect to lithium. In our opinion, the steric effect evidenced by these calculations is not the ultimate reason for the lower efficiency of sodium, the sensible misfits between calcite and Na2CO3 available structures being probably the true items responsible. 2CO3 3 In other words, the relative parametric misfit (aNa - aCaCO )/ o o 3 (aCaCO ) in the common 0001 plane between calcite and the o hexagonal polymorph of Na2CO3 (space group P63/mmc) is +4.42%. Furthermore, if the 001 plane of the monoclinic (C2/ m) polymorph of Na2CO3 is allowed to make coincidences with 2CO3 the 0001 plane of calcite, the relative parametric misfits (bNa o CaCO3 CaCO3 Na2CO3 CaCO3 CaCO3 - ao )/(ao ) and (ao - 2ao cos γ)/(2ao cos γ) turn out to be +4.54% and +4.48%, respectively. Then, 2D epitaxy between both polymorphs of Na2CO3 and 0001 plane of calcite should be much more difficult to obtain in comparison with that occurring with Li2CO3. Moreover, the thicknesses of the slices chosen to built epitaxies do not fulfill the anomalous mixed crystal conditions, their misfits being not lower than 13%. Finally, from survey experiments, we can say that sodium added in growth solution does not substantially modify the growth morphology of calcite. All these findings that we shall describe and interpret in a forthcoming paper emphasize once more that the “ordered adsorption”, that is, the 1D and the 2D epitaxies, should be the ultimate way to understand both the morphological changes and the bulk poisoning of crystals due to the presence of impurities in the growth medium. Acknowledgment. We wish to thank Dr. Roberto Cossio for many helpful discussions about luminescence, Dr. Francesco Roberto Massaro for fruitful scientific discussions and technical assistance during work, and Prof. Elio Giamello (Inorganic Chemistry Department, Torino University) for his support in EPR experiments, along with the IGG-National Research Council for the ICP-OES facility.

References (1) Rajam, S.; Mann, S. J. Chem. Soc., Chem. Commun. 1990, 1789– 1791.

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CG800483G

(Zabuyelite) Anomalous Mixed Crystals. Sector Zoning and Growth

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