Aragonite formation through precipitation of calcium carbonate monohydrate

Mat. Res. Bull. Vol. 12, 1095-1102, 1977. Pergamon Press, Inc. Printed in the United States.

ARAGONITE FORMATION THROUGH PRECIPITATION OF CALCIUM CARBONATE MONOHYDRATE

Kanlchi KAMIYA, Sumio SAKKA and Katsuyuki TERADA Faculty of Engineering, Mie University Kamihamacho, Tsu, Mie-Ken, Japan, 514

(Received September 8, 1977; Communicated by M. Nakahira)

ABSTRACT Formation of aragonite from amorphous, gelatinous CaCO~ and CaCO~'H~O precipitated in the solution containing Mg 2÷ ion was investigated. Amorphous CaC03 was converted to aragonite via CaC03"H20 formation on drying in air. The stability of CaC03"H20 in air depended on how long it has been immersed in the mother solution. Effect of Mg 2+ ion on the formation of aragonite was discussed on the basis of surface energy of a small particle.

Introduction Mg 2~ ions of low concentration in a mother solution, where precipitation of CaC03 occurs, favor the formation of thermodynamically metastable aragonite under normal pressure and temperature (i). On the other hand, those of high concentration favor the formation of calcium carbonate monohydrate, CaCO3"H20

(2). Natural and synthesized crystals of CaC03"H20 are less stable relative to calcite and aragonite (3). In pure water or in air the CaCO~'H20 is reported to change into aragonite in some cases (4, 5) and to calcite in other cases (6), indicating the complexity of the role of CaCO3'H20 in aragonite formation. Previous works suggest that at low concentrations of Mg 2+, aragonite is formed directly through a reaction of Ca ~÷ with C0~ion. It is also suggested that at high concentrations of Mg ~ a process via CaCO3-H20 is preferred. In the present

work,

major

attention

1095

was directed

towards

the

1096

K. KAMIYA, et al.

Vol. 12, No. 11

formation of various CaC03 polymorphs in relation to the Mg 2+ concentration. The Mg ~+ concentration was varied in a wide range. X-ray d i f f r a c t i o n and optical microscopic observations made it possible to follow the kinetics of the various changes of those polymorphs. Finally, the m e c h a n i s m of aragonite formation in presence of Mg 2+ ions was discussed. Experimental i) Preparation

and mixin~ of the solutions

CaCI2, (NH~)~C03 and MgCI2 of special reagent grade were used. Further p u r i f i c a t i o n of (NH~)~CO] was made by adding an appropriate amount of NH40H in order to remove NH4COaNH2 and N H 4 H C O ~ included as impurities. 50 ml of 0.4 M CaCI 2 solution was mixed with the same amount of 0.4 M (NH4)2CO ~ solution in a test tube. I00 ml of solution containing CaCI~ and (NH~)2CO~ both in concentration of 0.2 M was thus obtained. Seven such solutions, containing MgCI2 in concentration of 0, 0.01, 0.05, 0.I, 0.2, 0.5, and 1.0 M respectively, were prepared and named sample No.l through No.7 as listed in Table I. Other six test tubes were prepared which contained CaCI 2 and (NH4)zC03 both in concentration of' 0.I M, and MgCI2 in varying amounts as in the above series. These were named sample No.ll through No.16. The solutions were allowed to stand in a laboratory for two months with the seal. During that period, the temperature of the solutions varied b e t w e e n 1 9 ~ 29°C, but this variation did not affect the qualitative conclusion drawn in this study. 2) Observation

of c r y s t a l l i z a t i o n

of CaCO3"H~O

and CaC03

The phase change of the precipitates with standing time was investigated for the soluticn consisting of 0.i M CaCI2, 0.i M (NH4)2C03 and 0.5 M MgCI2. This solution was equivalent to the sample No.15 in the concentrations of starting chemicaIs. Ten test tubes were filled with i00 ml of this solution, sealed and allowed to stand in a laboratory. Ten different precipitates, filtered out after I, 2, 3, 4, 6, 8, I0, 15, 20 and 27 days, respectively, were thus obtained. 3) X-ray diffraction

and optical microscopy

Identification of the crystals in the precipitate was made by X-ray diffraction. For semi-quantitative estimation of the amount of crystal, the diffraction intensities of I01 line of CaCO~.H20 and iii line of aragonite, were employed. Measurement of diffraction intensities was conducted for the constant weight of sample with constant tube current and voltage. Microscopic observation was made using a Nippon Kogaku optical microscope model POH II. 4) Chemical

analysis

of MS 2~ and Ca 2+ ions in the filtrate

The chemical analysis of Ca ~* and Mg 2+ ions in the filtrate was made by the conventional method of titration using EDTA. Results

Vol. 12, No. 11

ARAGONITE FORMATION

i) Effect

of concentration

substance

and formation

of M~CI2

1(}97

on persistency

of ara~onite

of ~elatinous

and CaCO~.H~O

In both series of solutions as described in (i) of the previous section, which contained different amounts of MgCI2, a gelatinous substance was formed just after mixing the reactants together. As shown in Table I, the gelatinous substance disappeared after about 22 min and fine particles (calcite) were observed in No.l containing no Mg 2÷ ion, while it persisted almost one day in No.2 through No.5. Its stability increased with further increase of Mg 2. concentration. After two months, the crystalline species of the precipitates in the solutions were identified by X-ray diffraction as shown in Table i. The precipitate No.2 through No.4 was a mixture of calcite and aragon~te. The aragonite content increased with increasing Mg ~4 ion as stated in the literature (I). Aragonite was the only crystal found in No.5. However, CaC03.H~O crystallized out besides aragonite in No.6 and 7 containing larger amount of Mg ~2~ ion. It can be said that the formation of CaCO~.H20 is favored when a ratio of [Mge*]/[Ca zt] is higher than 2.5. Almost the same result as above was obtained in No.ll through No.16 with smaller concentration of Ca 2. and CO{2- ions. 2) Phase chan~e

of the precipitate

with time in the solution

containin$ M~ ~÷ ion In order to examine the c r y s t a l l i z a t i o n process of precipitate in the solution, several different precipitates obtained in the solution described in (2) of the previous section were examined by both X-ray d i f f r a c t i o n and optical microscopic observation. TABLE i Persistency of the gelatinous substance in the solution and crystalline species observed after 2 month standing of the solutions solution

series

No. i 2

3 4

1

5 6 7 ii 12

13 2

14

15 16 .C; calcite,

content species

of chemical in M

cacl 2 (m{4)2c03

MgCl2

persistency period of gelatinous substance

crystalline* product

0.2 0.2 0.2 0.2 0.2 0.2 0.2

0.2 0.2 0.2 0.2 0.2 0.2 0.2

0.00 0.01 0.05 0.I 0.2 0.5 1.0

22 i i i i i 2

min day day day day week weeks

C A, A, A, A A, A,

0.I 0.I 0.I 0.i 0.i 0.I

0.I 0.i 0.i 0.i 0.i 0 .i

0.01 0.05 0.I 0.2 0.5 1.0

< i i i I i 12

day day day day week days

C, A, A, A A,

A; aragonite,

V; vaterite,

CH; CaC03.H20

C, V C C CH CH A C C CH

A, CH

1098

K. KAMIYA, et al.

Vol. 12, No. 11

Fig.l shows the change of diffraction intensities representing the contents of CaCOg"H20 and aragonite in the precipitate as a function of standing time. The X-ray measurements were made within one day at the latest after removing the precipitate from the solution. It has been confirmed that almost no change occurred w i t h i n one day. The gelatinous substance obtained after standing for I, 2 and 3 days was substantially amorphous, including a small amount of n o n - i d e n t i f i e d crystals. After 4 days the precipitates contained CaCO~'H20 as a crystalline phase besides the amorphous phase. After 6 to 20 days the amorphous phase disappeared completely and only CaCOa.H20 was observed. Its diffraction intensity reached the m a x i m u m for about ten-day precipitate, followed i0 by a gradual decrease • i01 line of for the precipitates 40 with longer standing time. A Iii line of Q aragonite was found 40 besides those of CaCO~" H20 after 27 days, indi~vq cating that a part of o I CaCO3"H20 transformed iii line of to aragonite. aragonite cO For comparison, crystallization of the o c..) 3o precipitates in the solution containing 0.I M of Ca 2+, 0.i M CO~FIG.1 and 0.I M Mg2+ (characV a r i a t i o n of d i f f r a c t i o n intensities terized by lower content of CaC03"H20 and aragonite with of Mg 2+) was investistanding time gated. In this solution no CaCO3"H20 was formed, 1.3 as contrasted with the solution with high con1.2 tent of Mg 2+, and the i o~ . @ _amounts of calcite and i. i Mg2+ aragonite remained constant throughout the standing time. + 1.O

°0

Standing time, (days)

U:

C~J hO

0.2

~

t~

"~joc 0 . o o

Ca2÷ 0

I0 Standing

20

time,

(clays)

FIG.2 V a r i a t i o n of the content of Mg 2~ and Ca 2+ ions in the solution with standing time

In order to examine the variation of Mg 2~ and Ca 2t concentrations in a solution with progress of crystallization, the solution corresponding to No.15 was again investigated. The results are shown in Fig.2. Originally, 1215 mg of Mg 2+ ion was contained in the solution. After one day 170 mg was absorbed by the gelatinous substance. However,

Vol. 12, No. i i

ARAGONITE F O R M A T I O N

1099

as can easily be seen in the figure, the amount of Mg ~ ions in the solution again began to increase and lebeled off after about one week. The time of leveling off nearly coincided with the persistency period of the gelatinous substance, indicating that this gelatinous substance discharged Mg 2+ ions into the solution upon crystallization. Regarding Ca 2~ ion in the solution, 400.8 mg of Ca 2+ ion was contained originally. It is seen from the figure that once CaCO 3. H~O was formed, only a small amount of Ca 2. ion remained in the solution. The high content of Ca 2t ion in the solution at the early stage of standing might be attributed to higher solubility of amorphous precipitates than that of the crystalline compound. 3) Phase chanse of precipitates in dry air Fig.3 shows the phase changes of the precipitates on drying

11--

i000 ~ 0

--"

i01 line of CaCO3.H20

4J

0 50O

i0

20

30

40 JJl'~0

4~

0 .0---, 0

0

Ill llne of a r a g o n l t e i ~ ! ! | I0 20 30 40 120 Period of drying in air, (days)

[email protected]

Variation of diffraction intensities of CaCO3.H20 and aragonite with drying in air. O; removed from the solution after 3 days, O; 4 days, ~; 6 days, ~; 15 days

1100

K. KAMIYA, et al.

Vol. 12, No. 11

in air at 19-~30°C. The precipitates were taken out of the solution after standing for different days. In the figure, O and • show the results on the precipitates which were subjected to 3 and 4 day standing in the solution and were substantially amorphous at the beginning of drying. The others, [] and ~, correspond to 6 and 15 day standing in the solution. As evident from the plots, O and 0, in the figure, the amorphous substance exhibited a rapid crystallization to CaC03"H20 at the early stage of drying, followed by a subsequent conversion to aragonite. The plots, [] and ~, represent the further change of CaCO3"H20 which already crystallized from the gelatinous substance in the solution. Upon drying, it gradually changed to aragonite. Although this process of aragonite formation was already suggested by previous authors, the present experiment has shown the detailed process clearly and proved the suggestion. It is noted that CaCO3"H20 removed from the mother solution after 15 day standing (~) exhibits higher resistance to transformation to aragonite than that of 6 day standing (O). When the gelatinous substance with 3 or 4 day standing was dried in air, the X-ray diffraction lines similar to those of (NH4)2Ca(C03) 2 reported by Towe and Malone (5) were observed at the early stage of drying. This substance disappeared on further drying, followed by crystallization of CaCO3"H20 as described above. A diffuse line was sometimes observed at d = 2 . 9 8 ~ 2 . 8 8 A besides those of CaCO~'H20 which might be ascribed to either dolomite, huntite or magnesian calcite. The intensity of this peak remained constant with increasing drying time, while CaCO3.H20 decreased gradually as shown in Fig.3. 4) Observation

under optical

microscope

In order to show the conversion of CaCO3.H20 clearly, sectioned spherulites of CaCO3.H20 were observed under a transmission optical microscope. The diameter of the spherulite was about 0.3 mm after 6 day standing in the mother solution and did not increase any more on further standing. A concentric thin transparent ring was observed along the periphery of the spherulite as shown in Fig.4a). The spherulites

(a)

0.3 mm

(b)

FIG.4 Microphotographs of sectioned CaC03"}~O spherulite before (a) and after immersion in distilled water for 9 days (b)

Vol. 12, No. 11

ARAGONITE FORMATION

1101

having such ring was found to contain aragonite by X-ray diffraction. It is seen in Fig.4b) that t r a n s p a r e n c y of whole spherulite increased on holding the sectioned spherulite in distilled water. Such a treatment was confirmed to cause the transformation of CaC09"H20 to aragonite. These indicate that the transparent region in a spherulite consists of aragonite. These also suggest that the t r a n s f o r m a t i o n of CaCOg"H20 into aragonite starts at the periphery of spherulite and proceeds into the interior. Discussion I) Stability

of CaCO3"H20

Considerable discrepancies have been found among the previous literatures on the stability of CaCO3"H20. The only agreement among them is that it transforms to calcite on heating in air. Brooks et al (2) reported that synthesized CaCO3-H20 was dehydrated to aragonite at 20°C in air. Marschner's CaCO3'H20 (4) obtained within an a i r - c o n d i t i o n e r remained unchanged over a period of three months in a dry state. This material, however, transformed readily to aragonite in pure water. A natural CaC03" H20 which Taylor (6) obtained was not dehydrated for 3 years. In his case, it slowly transformed to calcite in pure water. In the present experiment, the CaCO3"H20 was converted to aragonite in air at room temperature, as contrasted with the last two examples described above. It should be noted, however, that the stability of the CaCO3"H20 depended on the duration of its immersion in the mother solution in the present experiment (compare ~ with [] in Fig.3). This latter result would explain the stability of the CaCO3"H20 in the above examples. That is, the CaCO~'H~O stable in air might have been kept in the mother solution fer a long time in nature or in an air-conditioner. 2) Effect

of MS 2t ion on aragonite

formation

It is well known that when CaCO 3 precipitates through the reaction between Ca ~* and CO~ions or decarbonation of Ca(HC03) 2, c o e x i s t i n g Mg 2t ions favor the formation of t h e r m o d y n a m i c a l l y metastable aragonite under normal pressure and temperature. It is also known that Sr 2~ ions form a crystal having the aragonite structure when combined with CO~ 2- ions. McCauley and Roy (7) d e m o n s t r a t e d experimentally that SrCO~ acts as a nucleus for epitax~al growth of aragonite. In that work they also proposed that M g C O ~ . 3 H 2 0 might be a nucleus for aragonite growth in the solution containing Mg 2~ lens. The present result was inconsistent with the latter proposal, because M g C O 3 " 3 H 2 0 was not found throughout the experiment in the present study. Possibility of epitaxial growth of aragonite on other related crystals was also denied. MgCO:~ dld not seem to act as a nucleus for aragonite growth, because it is not isomorphous with aragonite. Other crystals containing Mg 2÷ ion in the lattice, e.g. dolomite, huntite and m a g n e s i a n calcite did not seem to have any relation to the growth of aragonite as described in (3) of the previous section. Therefore, it was thought that explanation of the effect of Jn aragonite growth should be based on some other mechanism. A theory was proposed by Bischoff (8). He pointed out the signif-

b~g "~

1102

K. KAMIYA, et al.

Vol. 12, No. I i

cance of absorption of Mg ~ ions on the CaCO~ nucleus and suggested that Mg 2~ ions on the surface of the nucleus stabilize aragonite rather than calcite. The absorption of Mg 2+ ions on the surface of precipitates were also shown in the present experiment. The concentration of Mg ~ ions in the filtrate increased with growth of colloidal precipitate as shown in Fig.2, indicating that Mg 2~ ions were present on the surface of precipitates beforehand. The detailed effect of Mg ~÷ ions on the aragonite formation, however, have not been clearly shown. In the followings, therefore, further consideration on the m e c h a n i s m will be made. It is generally known that microscopic particles have a very large excess surface energy with respect to macroscopic particles. Many cases are knwon where m i c r o n - s i z e d particles have an extraordinarily high free energy and exhibit a m e t a s t a b l e structure. Since the free energy difference b e t w e e n calcite and aragonite is as small as 272 cal/mol under normal pressure and temperature (9), such a small energy could be easily gained by the excess surface energy due to fine particle size and aragonite becomes stable relative to calcite. Without Mg ~ ions, aragonite can be found in some cases, e.g. at high reaction temperature where high rate of n u c l e a t i o n and fine particles of CaCO are observed. When Mg ~t ions exist, their effect may be considered as follows. It is known that the surface energy of alkaline earth oxide is in the order of M g O ~ C a O ~ S r O ~ BaO, r e f l e c t i n g the order of covalency of M-O b o n d i n g (I0). This sequence could be applied to the corresponding carbonates, namely MgC0~ ~ C a C O ~ S r C O ~ BaCOn. It is assumed that Mg z~ ions absorbed on the surface of CaCO~ particle would form the layer c o n t a i n i n g Mg-C03 b o n d i n g and increase the surface energy. Accordingly, aragonite is more favorable than in the case of the absence of Mg 2t ion. It is further expected that the probability of aragonite growth becomes large as the content of absorbed Mg 2÷ ion increases. References i. Y. Kitano,

Bull.

Chem.

Soc. Japan,

2. R. Brooks, L.H. Clark and E.F. Soc. London, 243, 145 (1950) 3. H. Hull and A.G. (1973) 4. H. Marschner, 5. K.M.

G.F. Taylor,

7

J.W. McCauley

I0

Science,

Am. Mineral., and R. Roy,

J.L.

Bischoff and W.S.

J.C.

Jamieson,

J. Chem.

Geochim.

165,

Towe and P.G. Malone,

6

9

Turnbull,

35, 1973

Thurston.

1120

Cosmochim.

226,

60,

(1975)

690

Am. Mineral., Am. Jour.

Phys.,

Trans. Acta,

Roy. 37,

685

(1969)

Nature,

Fyfe,

(1962)

Phil.

21,

1385

348

59, Sci.,

(1970)

947

(1975)

266,

65

(1953)

K. Kubo, "FUNTAI, RIRON TO OHYO" (Powder, A p p l i c a t i o n s ) ( i n Japanese) p327, Maruzen,

Theory and 1962

(1968)