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The growth of calcite spherulites from solution
Journal of Crystal Growth 193 (1998) 382—388
The growth of calcite spherulites from solution II. Kinetics of formation S.L. Tracy!, D.A. Williams!, H.M. Jennings!,",* ! Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA " Department of Civil Engineering, Northwestern University, Evanston, IL 60208, USA Received 29 December 1997; accepted 28 May 1998
Abstract The reaction rate of calcite sphere formation was investigated. Samples of precipitates and solutions were taken at various times and characterized for morphology, phase, chemistry, and pH. It appears that the Mg2` and SO2~ 4 impurities have a controlling influence on both the phase and morphological development of precipitates. The calcite spheres initially precipitate as an amorphous phase, then go through a transition into a rough intermediate spherulitic morphology before the final development into near-perfect spheres. Slower crystallization and a diffusion-controlling boundary layer of rejected impurities apparently is part of the mechanism controlling spherical growth. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 81.10.Dn Keywords: Calcite; Morphology; Precipitation; Calcium carbonate; Impurities; Kinetics
1. Introduction Part I of this paper described the occurrence of calcite spherulites grown from solution [1]. It was determined that [Ca2`], [CO2~], [Ca2`]/ 3 [CO2~], [Mg2`], and [SO2~] all were necessary 4 3 for their formation. The present paper explores the kinetics of sphere formation and possible mechanisms for the effect that the combination of Mg2`
* Correspondence address: Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA. Fax: #1 847 491 5282; e-mail: [email protected].
and SO2~ ions have in promoting spherical calcite 4 growth. 2. Experimental procedure One composition was chosen for further study from the experimental design matrix of Part I [1] which yielded 100% calcite spheres (Trial 8). It consisted of an initial composition of [Ca2`]" 50 mM, [CO2~]"50 mM, [Mg2`]"50 mM, 3 and [SO2~]"50 mM, with or without K` as 4 the presence of this ion was found not to affect sphere formation. In addition, control experiments
0022-0248/98/$ — see front matter ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 5 2 1 - 1
S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
were performed at a composition of [Ca2`]" [CO2~]"50 mM (i.e. without the presence of ad3 ditive ions). Solutions were made with the NO~ 3 and NH` counterions only. 4 Identical procedures were used as described in Part I [1], except sampling techniques varied. For instance, early age crystals were not collected on glass coverslips because they had not yet settled to the bottoms of jars. Instead, crystals and solutions were removed from the reaction jar with pipettes, injected into aqueous ammonia to rinse and remove the supersaturated solution, vacuum filtered, and air dried. All solid samples were separated in this way for consistency. Solid samples were taken at 1, 5, and 10 min, and increments of 10 min up to 1 h. Samples were also taken at 2 h, 1 week, 18 days, and 1 month. 1 ml of the mother liquor was also removed, filtered through a membrane filter, and diluted 10 : 1 with deionized water in a volumetric flask for subsequent chemical analysis using an inductive coupled plasma atomic emission spectrometer (ICP). This technique measured the atomic concentrations of Ca, Mg, and S in the solution phase. The time increments as described above were used. The pH of the solution was also measured over time with a pH electrode immersed in the reaction jar. Initial pH measurements (time"0) were from the calcium solutions with and without additives before the carbonate component was added. Crystals were prepared for scanning electron microscopy (SEM) by placing the powders on to carbon tape on SEM stubs. The samples were coated with 160—240 A_ of gold. Some crystals were crushed between glass slides prior to analysis in order to further investigate the internal structures of the spheres. Energy dispersive spectroscopy (EDS) was used to chemically analyze the precipitates. X-ray diffraction (XRD) was used to characterize the phase of the precipitates sampled at different times.
383
allowed simply to air dry without washing, many different particles were formed, leaving it unclear which were the phases present in solution and which precipitated later from supersaturated, drying solutions. A rinse with basic solution (ammonia) was found to be the best way to preserve early age particles (younger than 30 min-old). One-minute samples were found to be made of nanosize particles and are shown in Fig. 1. Fig. 2 shows a cracked 10 min-old particle. Ten-minute-old particles were rough in nature, not quite spheres, but exhibited spherulitic growth from their centers. Frequently, a groove could be seen at the equator of the spheres, almost separating them into two halves. Also evident was the rough core at the center of the spheres akin to the cores mentioned previously in Part I [1]. Nicely formed spheres existed at all times longer than 30 min up to 1 month old. The size of the spheres did not change between 30 min and 1 month. The distribution of radii seemed to be bimodal, roughly at 5—10 lm and 20—30 lm. Image analysis studies did not indicate an increase in average radius over time. Fig. 3 compares the size distributions of 30 min to 1 month old spheres.
3. Results 3.1. Precipitate analysis: morphology Early aged particles were hard to collect; they dissolved in water, or if the collected particles were
Fig. 1. Precipitate powders at 1 min old. Scale bar is 1 lm.
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S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
Fig. 2. Cracked, almost spherical precipitates at 10 min old. Core is already evident, as well as deep center groove. Scale bar is 5 lm.
Fig. 3. (a) Spheres at 30 min old. (b) Spheres at 1 month old. Scale bars are 150 and 120 lm.
An “equator” or groove evident on some spheres at 10 min (or possibly all, with some that could not be seen) was also present on older spheres, and an example is shown in Fig. 4. The occurrence of these equators could be explained if the spheres were formed via a sheaf-of-wheat mechanism as described in Ref. [2]. The equator would be due to the meeting of the opposite ends of the sheafs.
Fig. 4. Existence of an “equator” on some of the spheres.
Although at no time were sheafs-of-wheat collected as the predominant morphology, which would have proven the sheaf-of-wheat theory, one sheaf-ofwheat was found in a sample taken at 25 h old as shown in Fig. 5. The composition of the experiment was [Ca2`]"100 mM, [Ca2`]/[CO2~]"3, 3 [SO2~]"20 mM, and [Sr2`]"20 mM. It may 4 be surmised that sheafs-of-wheat, if they were present, either were modified when separated from their mother liquors, or that spheres form very rapidly once the sheaf-of-wheat is nucleated. It is interesting to compare 1 min and 30#-min old calcite sphere morphologies to similar morphologies reported by Reddy and Nancollas [3]. They added 10~3 M Mg2` to 5 mM [Ca2`]" [CO2~] solutions and found amorphous precipi3 tates at 3 min, and after 20 days, combinations of aragonite spherulites and irregularly formed calcite were observed. Control specimens as early as 1 min old were a combination of spherical spherulites and rhombohedra as shown in Fig. 6. But samples at 7 days consisted of 100% rhombohedra, so at some point between 1 min and 7 days the spherulites recrystallized as rhombohedra.
S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
385
Fig. 7. XRD spectra of precipitates formed in the presence of Mg2` and SO2~. At 1 min the particles are basically amorph4 ous, and at 10 min and older they are calcite. The broad peak between 20"15° and 20° is from the tape that supported the sample. At 1 min only a small amount of sample could be collected. Fig. 5. One particle collected in a sheaf-of-wheat morphology, possibly the precursor to sphere formation. System with Sr2` presence.
Fig. 8. XRD spectra of precipitates formed without additives at [Ca2`]"[CO2~]"50 mM at 120 min old compared to 3 JCPDS files for calcite and vaterite showing the sample contains both phases.
Fig. 6. 1 min old precipitates from 50 mM with no additives.
[Ca2`]"[CO2~]" 3
3.2. Precipitate analysis: phase XRD was performed on the solids precipitated at different times in the presence of Mg2` and SO2~ 4
additives. Because only a small amount of sample could be collected, the identity of the phase at 1 min could not be established (1 min old, Fig. 1). Indeed, it could be amorphous although one peak could be resolved. By 10 min and older they were calcitic as shown in Fig. 7. The 1 min old control combination of spherulites and rhombohedra were a combination of vaterite and calcite as shown in Fig. 8. Evidently, the spherulite morphologies were the vaterite phase, and with time transformed to the stable phase calcite of a rhombohedral morphology.
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S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
3.3. Precipitate analysis: chemistry EDS spectra were taken of the sphere surfaces on samples aged to various times. Fig. 9 shows the Mg/Ca atomic percent ratios found at the different ages. It appears that the younger particles are higher in Mg than the final calcite spheres. 3.4. Solution analysis Fig. 10 shows the [Ca], [S], and [Mg] concentrations for calcite sphere-forming solutions and the [Ca] concentration for control solutions. Immediate precipitation is evident as the concentrations fall sharply by 1 min (and visibly precipitate). [Mg] and [S] are relatively constant indicating they are not incorporated into the solid phase to a great extent during crystallization, nor do they fluctuate during morphological changes. There are, however, differences in the [Ca] between the calcite-sphere-forming solutions and the control solutions versus time. In the case of spheres, the presence of Mg and SO appear to keep a higher 4 concentration of Ca in solution for longer times than in the absence of these ions. The solubility of calcite is not altered, however, since 12-day-old
Fig. 10. Atomic solution compositions versus time. “Ca (with additives)”, S, and Mg concentrations are from the calcite sphere solutions, while the “Ca (without additives)” is from the control solutions.
samples have equivalent [Ca], within experimental error. The solution pH versus time during precipitation is shown in Fig. 11, with and without the presence of Mg2` and SO2~ additives. The initial pH differ4 ence between the two solutions measures the effect that Mg2` and SO2~ ions (and extra NO~ and 3 4 NH`) have on pH. When carbonate is added, the 4 two solutions increase to the same point, but during solid-phase precipitation they decrease at different rates, with the additive-containing solutions decreasing more slowly. The pH of the solution with additives has a higher final pH than the one without additives.
4. Discussion
Fig. 9. Plot of the EDS data for the Mg/Ca atomic%’s of the developing particles formed in the presence of Mg2` and SO2~. 4
Both Keith and Padden [4] and Goldenfeld [5] have discussed the general crystallization phenomenon of spherulitic growth. While Keith and Padden relied on 2-D nucleation as controlling crystallization, they still appealed to diffusion as
S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
Fig. 11. The pH versus time of the two solutions with and without additives.
key in segregating impurities at a boundary layer causing fibrillation of spherulites. Goldenfeld shows how a linear growth law can still be expected even if crystallization is diffusion controlled. It is interesting to note, however, that both references, when discussing the formation of spherulitic minerals and inorganic salts, speak of their formation in viscous magmas, devitrified glasses, and in gels. A common feature to all spherulitic systems seems to be growth in highly viscous environments where crystallization is slow. In the case of the growth of spherulitic vaterite, spherulitic crystallization appears to be quite rapid, occurring at least at 1 min after the reactants were mixed. Vaterite is unstable with respect to calcite, and as it recrystallizes (in the control experiments when no additives were present), the calcite was of rhombohedral morphology. In the case of spherical spherulitic calcite formation, the crystallization process is slower. Initially amorphous precipitates appear, followed by roughly spherical spherulites and finally almost perfectly spherical spherulites. The solution phase analysis shows the elevated calcium level over time. It appears the impurities interfere with vaterite and cal-
387
cite crystallization initially. As crystallization proceeds, these impurities probably form a boundary layer of impurities being rejected from the growing solid as described in Refs. [4] and [5]. This delayed, slower crystallization also allows the more stable calcite phase then to develop, even though vaterite appears kinetically to be favored early on. We cannot provide a mechanism for the formation of the spherulites, but a couple of observations presented here may be relevant. The size of the cation impurity seems to be more important than the charge. The young spheres have more Mg than the older spheres. Finally, the concentration of calcium in solution is maintained at a higher level for a long period of time when spherulites form, i.e. in the presence of both additives. Since the early spheres contain impurities, it is possible that they alter the solubility of calcite and that the increased solubility changes the morphology. In other words, higher energy points on the surfaces, such as edges and corners, become unstable as a result of increased solubility and dissolve, resulting in a sphere.
5. Conclusions A description of the morphological development of calcite spheres is the following: Initially, a nanophase amorphous precipitate forms of slightly higher magnesium content than the final spheres. At 10 min the nanophase has disappeared and roughly spherical particles have appeared. Between 10—30 min the spheres perfect their shape and become almost perfectly round, except for residual grooves which sometimes remain and show evidence of possible sheaf-of-wheat origins. At times between 30 min and 1 month, the spheres remain essentially the same. The Mg2` and SO2~ impurities in solution seem 4 to interfere with initial precipitate formation and cause an initially amorphous phase to precipitate, which later transforms to calcite spheres through a few intermediate steps. The impurities do not change the final solubility of calcite, but they do change the precipitation pathways and pH of the system. These impurities, by interfering with precipitation, force the precipitates to crystallize more
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slowly than their control counterparts. Slower crystallization is a hallmark of spherulitic growth in several systems. The impurities could set up a rejected boundary layer and cause fibrillation of the crystal. This pathway also allows calcite to have a spherulitic morphology instead of being vaterite, as expected. Acknowledgements S.L.T. would like to thank the Department of Defense for awarding her a graduate fellowship,
and the USG Corporation for sponsoring this research.
References [1] S.L. Tracy, C.J.P. Franc7 ois, H.M. Jennings, J. Crystal Growth 193 (1998) 374. [2] R.B. Williamson, J. Crystal Growth 3—4 (1968) 787. [3] M.M. Reddy, G.H. Nancollas, J. Crystal Growth 35 (1976) 33. [4] H.D. Keith, F.J. Padden Jr., J. Appl. Phys. 34 (1963) 2409. [5] N. Goldenfeld, J. Crystal Growth 84 (1987) 601.
The growth of calcite spherulites from solution II. Kinetics of formation S.L. Tracy!, D.A. Williams!, H.M. Jennings!,",* ! Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA " Department of Civil Engineering, Northwestern University, Evanston, IL 60208, USA Received 29 December 1997; accepted 28 May 1998
Abstract The reaction rate of calcite sphere formation was investigated. Samples of precipitates and solutions were taken at various times and characterized for morphology, phase, chemistry, and pH. It appears that the Mg2` and SO2~ 4 impurities have a controlling influence on both the phase and morphological development of precipitates. The calcite spheres initially precipitate as an amorphous phase, then go through a transition into a rough intermediate spherulitic morphology before the final development into near-perfect spheres. Slower crystallization and a diffusion-controlling boundary layer of rejected impurities apparently is part of the mechanism controlling spherical growth. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 81.10.Dn Keywords: Calcite; Morphology; Precipitation; Calcium carbonate; Impurities; Kinetics
1. Introduction Part I of this paper described the occurrence of calcite spherulites grown from solution [1]. It was determined that [Ca2`], [CO2~], [Ca2`]/ 3 [CO2~], [Mg2`], and [SO2~] all were necessary 4 3 for their formation. The present paper explores the kinetics of sphere formation and possible mechanisms for the effect that the combination of Mg2`
* Correspondence address: Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, USA. Fax: #1 847 491 5282; e-mail: [email protected].
and SO2~ ions have in promoting spherical calcite 4 growth. 2. Experimental procedure One composition was chosen for further study from the experimental design matrix of Part I [1] which yielded 100% calcite spheres (Trial 8). It consisted of an initial composition of [Ca2`]" 50 mM, [CO2~]"50 mM, [Mg2`]"50 mM, 3 and [SO2~]"50 mM, with or without K` as 4 the presence of this ion was found not to affect sphere formation. In addition, control experiments
0022-0248/98/$ — see front matter ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 5 2 1 - 1
S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
were performed at a composition of [Ca2`]" [CO2~]"50 mM (i.e. without the presence of ad3 ditive ions). Solutions were made with the NO~ 3 and NH` counterions only. 4 Identical procedures were used as described in Part I [1], except sampling techniques varied. For instance, early age crystals were not collected on glass coverslips because they had not yet settled to the bottoms of jars. Instead, crystals and solutions were removed from the reaction jar with pipettes, injected into aqueous ammonia to rinse and remove the supersaturated solution, vacuum filtered, and air dried. All solid samples were separated in this way for consistency. Solid samples were taken at 1, 5, and 10 min, and increments of 10 min up to 1 h. Samples were also taken at 2 h, 1 week, 18 days, and 1 month. 1 ml of the mother liquor was also removed, filtered through a membrane filter, and diluted 10 : 1 with deionized water in a volumetric flask for subsequent chemical analysis using an inductive coupled plasma atomic emission spectrometer (ICP). This technique measured the atomic concentrations of Ca, Mg, and S in the solution phase. The time increments as described above were used. The pH of the solution was also measured over time with a pH electrode immersed in the reaction jar. Initial pH measurements (time"0) were from the calcium solutions with and without additives before the carbonate component was added. Crystals were prepared for scanning electron microscopy (SEM) by placing the powders on to carbon tape on SEM stubs. The samples were coated with 160—240 A_ of gold. Some crystals were crushed between glass slides prior to analysis in order to further investigate the internal structures of the spheres. Energy dispersive spectroscopy (EDS) was used to chemically analyze the precipitates. X-ray diffraction (XRD) was used to characterize the phase of the precipitates sampled at different times.
383
allowed simply to air dry without washing, many different particles were formed, leaving it unclear which were the phases present in solution and which precipitated later from supersaturated, drying solutions. A rinse with basic solution (ammonia) was found to be the best way to preserve early age particles (younger than 30 min-old). One-minute samples were found to be made of nanosize particles and are shown in Fig. 1. Fig. 2 shows a cracked 10 min-old particle. Ten-minute-old particles were rough in nature, not quite spheres, but exhibited spherulitic growth from their centers. Frequently, a groove could be seen at the equator of the spheres, almost separating them into two halves. Also evident was the rough core at the center of the spheres akin to the cores mentioned previously in Part I [1]. Nicely formed spheres existed at all times longer than 30 min up to 1 month old. The size of the spheres did not change between 30 min and 1 month. The distribution of radii seemed to be bimodal, roughly at 5—10 lm and 20—30 lm. Image analysis studies did not indicate an increase in average radius over time. Fig. 3 compares the size distributions of 30 min to 1 month old spheres.
3. Results 3.1. Precipitate analysis: morphology Early aged particles were hard to collect; they dissolved in water, or if the collected particles were
Fig. 1. Precipitate powders at 1 min old. Scale bar is 1 lm.
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S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
Fig. 2. Cracked, almost spherical precipitates at 10 min old. Core is already evident, as well as deep center groove. Scale bar is 5 lm.
Fig. 3. (a) Spheres at 30 min old. (b) Spheres at 1 month old. Scale bars are 150 and 120 lm.
An “equator” or groove evident on some spheres at 10 min (or possibly all, with some that could not be seen) was also present on older spheres, and an example is shown in Fig. 4. The occurrence of these equators could be explained if the spheres were formed via a sheaf-of-wheat mechanism as described in Ref. [2]. The equator would be due to the meeting of the opposite ends of the sheafs.
Fig. 4. Existence of an “equator” on some of the spheres.
Although at no time were sheafs-of-wheat collected as the predominant morphology, which would have proven the sheaf-of-wheat theory, one sheaf-ofwheat was found in a sample taken at 25 h old as shown in Fig. 5. The composition of the experiment was [Ca2`]"100 mM, [Ca2`]/[CO2~]"3, 3 [SO2~]"20 mM, and [Sr2`]"20 mM. It may 4 be surmised that sheafs-of-wheat, if they were present, either were modified when separated from their mother liquors, or that spheres form very rapidly once the sheaf-of-wheat is nucleated. It is interesting to compare 1 min and 30#-min old calcite sphere morphologies to similar morphologies reported by Reddy and Nancollas [3]. They added 10~3 M Mg2` to 5 mM [Ca2`]" [CO2~] solutions and found amorphous precipi3 tates at 3 min, and after 20 days, combinations of aragonite spherulites and irregularly formed calcite were observed. Control specimens as early as 1 min old were a combination of spherical spherulites and rhombohedra as shown in Fig. 6. But samples at 7 days consisted of 100% rhombohedra, so at some point between 1 min and 7 days the spherulites recrystallized as rhombohedra.
S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
385
Fig. 7. XRD spectra of precipitates formed in the presence of Mg2` and SO2~. At 1 min the particles are basically amorph4 ous, and at 10 min and older they are calcite. The broad peak between 20"15° and 20° is from the tape that supported the sample. At 1 min only a small amount of sample could be collected. Fig. 5. One particle collected in a sheaf-of-wheat morphology, possibly the precursor to sphere formation. System with Sr2` presence.
Fig. 8. XRD spectra of precipitates formed without additives at [Ca2`]"[CO2~]"50 mM at 120 min old compared to 3 JCPDS files for calcite and vaterite showing the sample contains both phases.
Fig. 6. 1 min old precipitates from 50 mM with no additives.
[Ca2`]"[CO2~]" 3
3.2. Precipitate analysis: phase XRD was performed on the solids precipitated at different times in the presence of Mg2` and SO2~ 4
additives. Because only a small amount of sample could be collected, the identity of the phase at 1 min could not be established (1 min old, Fig. 1). Indeed, it could be amorphous although one peak could be resolved. By 10 min and older they were calcitic as shown in Fig. 7. The 1 min old control combination of spherulites and rhombohedra were a combination of vaterite and calcite as shown in Fig. 8. Evidently, the spherulite morphologies were the vaterite phase, and with time transformed to the stable phase calcite of a rhombohedral morphology.
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S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
3.3. Precipitate analysis: chemistry EDS spectra were taken of the sphere surfaces on samples aged to various times. Fig. 9 shows the Mg/Ca atomic percent ratios found at the different ages. It appears that the younger particles are higher in Mg than the final calcite spheres. 3.4. Solution analysis Fig. 10 shows the [Ca], [S], and [Mg] concentrations for calcite sphere-forming solutions and the [Ca] concentration for control solutions. Immediate precipitation is evident as the concentrations fall sharply by 1 min (and visibly precipitate). [Mg] and [S] are relatively constant indicating they are not incorporated into the solid phase to a great extent during crystallization, nor do they fluctuate during morphological changes. There are, however, differences in the [Ca] between the calcite-sphere-forming solutions and the control solutions versus time. In the case of spheres, the presence of Mg and SO appear to keep a higher 4 concentration of Ca in solution for longer times than in the absence of these ions. The solubility of calcite is not altered, however, since 12-day-old
Fig. 10. Atomic solution compositions versus time. “Ca (with additives)”, S, and Mg concentrations are from the calcite sphere solutions, while the “Ca (without additives)” is from the control solutions.
samples have equivalent [Ca], within experimental error. The solution pH versus time during precipitation is shown in Fig. 11, with and without the presence of Mg2` and SO2~ additives. The initial pH differ4 ence between the two solutions measures the effect that Mg2` and SO2~ ions (and extra NO~ and 3 4 NH`) have on pH. When carbonate is added, the 4 two solutions increase to the same point, but during solid-phase precipitation they decrease at different rates, with the additive-containing solutions decreasing more slowly. The pH of the solution with additives has a higher final pH than the one without additives.
4. Discussion
Fig. 9. Plot of the EDS data for the Mg/Ca atomic%’s of the developing particles formed in the presence of Mg2` and SO2~. 4
Both Keith and Padden [4] and Goldenfeld [5] have discussed the general crystallization phenomenon of spherulitic growth. While Keith and Padden relied on 2-D nucleation as controlling crystallization, they still appealed to diffusion as
S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
Fig. 11. The pH versus time of the two solutions with and without additives.
key in segregating impurities at a boundary layer causing fibrillation of spherulites. Goldenfeld shows how a linear growth law can still be expected even if crystallization is diffusion controlled. It is interesting to note, however, that both references, when discussing the formation of spherulitic minerals and inorganic salts, speak of their formation in viscous magmas, devitrified glasses, and in gels. A common feature to all spherulitic systems seems to be growth in highly viscous environments where crystallization is slow. In the case of the growth of spherulitic vaterite, spherulitic crystallization appears to be quite rapid, occurring at least at 1 min after the reactants were mixed. Vaterite is unstable with respect to calcite, and as it recrystallizes (in the control experiments when no additives were present), the calcite was of rhombohedral morphology. In the case of spherical spherulitic calcite formation, the crystallization process is slower. Initially amorphous precipitates appear, followed by roughly spherical spherulites and finally almost perfectly spherical spherulites. The solution phase analysis shows the elevated calcium level over time. It appears the impurities interfere with vaterite and cal-
387
cite crystallization initially. As crystallization proceeds, these impurities probably form a boundary layer of impurities being rejected from the growing solid as described in Refs. [4] and [5]. This delayed, slower crystallization also allows the more stable calcite phase then to develop, even though vaterite appears kinetically to be favored early on. We cannot provide a mechanism for the formation of the spherulites, but a couple of observations presented here may be relevant. The size of the cation impurity seems to be more important than the charge. The young spheres have more Mg than the older spheres. Finally, the concentration of calcium in solution is maintained at a higher level for a long period of time when spherulites form, i.e. in the presence of both additives. Since the early spheres contain impurities, it is possible that they alter the solubility of calcite and that the increased solubility changes the morphology. In other words, higher energy points on the surfaces, such as edges and corners, become unstable as a result of increased solubility and dissolve, resulting in a sphere.
5. Conclusions A description of the morphological development of calcite spheres is the following: Initially, a nanophase amorphous precipitate forms of slightly higher magnesium content than the final spheres. At 10 min the nanophase has disappeared and roughly spherical particles have appeared. Between 10—30 min the spheres perfect their shape and become almost perfectly round, except for residual grooves which sometimes remain and show evidence of possible sheaf-of-wheat origins. At times between 30 min and 1 month, the spheres remain essentially the same. The Mg2` and SO2~ impurities in solution seem 4 to interfere with initial precipitate formation and cause an initially amorphous phase to precipitate, which later transforms to calcite spheres through a few intermediate steps. The impurities do not change the final solubility of calcite, but they do change the precipitation pathways and pH of the system. These impurities, by interfering with precipitation, force the precipitates to crystallize more
388
S.L. Tracy et al. / Journal of Crystal Growth 193 (1998) 382–388
slowly than their control counterparts. Slower crystallization is a hallmark of spherulitic growth in several systems. The impurities could set up a rejected boundary layer and cause fibrillation of the crystal. This pathway also allows calcite to have a spherulitic morphology instead of being vaterite, as expected. Acknowledgements S.L.T. would like to thank the Department of Defense for awarding her a graduate fellowship,
and the USG Corporation for sponsoring this research.
References [1] S.L. Tracy, C.J.P. Franc7 ois, H.M. Jennings, J. Crystal Growth 193 (1998) 374. [2] R.B. Williamson, J. Crystal Growth 3—4 (1968) 787. [3] M.M. Reddy, G.H. Nancollas, J. Crystal Growth 35 (1976) 33. [4] H.D. Keith, F.J. Padden Jr., J. Appl. Phys. 34 (1963) 2409. [5] N. Goldenfeld, J. Crystal Growth 84 (1987) 601.