How Does A Transient Amorphous Precursor Template Crystallization

Proc Natl Acad Sci U Due south A. 2010 Sep 21; 107(38): 16438–16443.

Chemistry

Transformation and crystallization energetics of synthetic and biogenic baggy calcium carbonate

A. V. Radha

^aPeter A. Rock Thermochemistry Laboratory and Nanomaterials in the Surround, Agriculture, and Technology Organized Research Unit (NEAT ORU), University of California at Davis, One Shields Avenue, Davis, CA 95616; and

Tori Z. Forbes

^aPeter A. Rock Thermochemistry Laboratory and Nanomaterials in the Surround, Agriculture, and Engineering Organized Research Unit of measurement (NEAT ORU), University of California at Davis, One Shields Avenue, Davis, CA 95616; and

Christopher East. Killian

^bDepartment of Physics, Academy of Wisconsin at Madison, 1150 University Avenue, Madison, WI 53706

P. U. P. A. Gilbert

^bSection of Physics, University of Wisconsin at Madison, 1150 Academy Artery, Madison, WI 53706

Alexandra Navrotsky

Supplementary Materials: Supporting Data

GUID: 8BF74B6D-2C7C-4D41-AA35-B6907D90F358

GUID: FA58AB6C-4D65-48A1-A59C-D3DEC539958B

Abstract

Baggy calcium carbonate (ACC) is a metastable phase oftentimes observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with fourth dimension crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California imperial sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acrid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC → less metastable hydrated ACC⇒anhydrous ACC ∼ biogenic anhydrous ACC⇒vaterite → aragonite → calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a serial of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during aridity is exothermic and irreversible. Dehydrated constructed and anhydrous biogenic ACC are like in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the beginning phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may exist a first pace in the precipitation of calcite nether a broad variety of conditions, including geological CO₂ sequestration.

Keywords: amorphous calcium carbonate (ACC), calorimetry, crystallization enthalpy, ocean urchin larval spicules, synthetic and biogenic ACC

Calcium carbonate is ubiquitous in nature and establish in fresh and saline waters, soils, sediments, mineral dusts, and geologic formations (i–iii). Calcium carbonate occurs in five different crystalline polymorphs at ambient pressure, anhydrous phases (calcite, aragonite, and vaterite), and hydrated phases (monohydrocalcite CaCO₃·H₂O and ikaite CaCO₃·6 H₂O), and various amorphous forms as well (1, iii). Baggy calcium carbonate (ACC) is transient and transforms into one of the crystalline forms in presence of water or when heated.

ACC can form by both biological and abiotic means. In organisms every bit diverse as mollusks, sea urchins, sponges, ascidians/bounding main squirts, and crustaceans, ACC is a precursor to complex calcite and aragonite structures in shells, spines, teeth, and spicules (3–12). Abiotically, ACC is hands formed by mixing aqueous solutions at or below room temperature. These solutions can exist totally inorganic or they can contain small-scale organic species which affect the formation and persistence of ACC (13–20). Increasing interest in CO₂ sequestration has led to proposals and pilot projects in which large plumes of supercritical carbon dioxide would be introduced into deep saline aquifers (21–25). ACC may be produced as an initial reaction production between the CO₂ and the aqueous phase, with mineral surfaces and small pores interim as possible nucleation sites (26).

The ACC formed by these various processes is a variable and still poorly understood textile. Two forms, anhydrous transient ACC and a hydrated ACC containing about 1 mol of water that persists for longer time periods exist in biogenic sources (4, ix, 10, 27). The chemically precipitated ACC is also variable in particle size and water content and transforms to a less hydrated or anhydrous form upon heating before crystallization sets in (18, 19, 28). A fundamental question, which the present work addresses, is the nature of the compositional, structural, and energetic similarities and differences among unlike ACC samples (both chemically and biologically produced) and their transformations with heating.

Several computational and experimental studies on germination mechanism and structural characteristics of ACC have recently appeared. A metadynamics report including ciphering of energy suggests preference for an amorphous structure during the early on stages of crystal growth (29). Molecular dynamics studies indicate that the initial binding of calcium to carbonate and subsequent growth of amorphous clusters in h2o is not but energetically favorable, but besides almost gratis from thermodynamic barriers (30). Nuclear magnetic resonance, Ten-ray assimilation spectroscopy, and infrared spectroscopy accept studied the structure of ACC (3, 7, 31, 32). High-energy X-ray total scattering studies on ACC indicate no structural coherence beyond 1.5 nm distance (20).

Soluble clusters have been found in solution prior to the nucleation of calcium carbonate, such clusters are like in size to the structural coherence length of ACC. Based on equilibrium thermodynamics, Gebauer et al. (33) demonstrated qualitatively the germination of stable ∼2 nm clusters prior to nucleation. Pouget et al. observed the initial formation of 0.six to 1.1 nm prenucleation clusters using cryogenic transmission electron microscopy (34). On aggregation, these clusters pb to the formation of ACC nanoparticles approximately 30 nm in diameter. These observations suggest a new and more complex pathway for calcium carbonate formation through stable nanoscopic prenucleation clusters as opposed to the simple picture provided by classical nucleation theory (35, 36).

If prenucleation clusters, ACC, and nano-calcite are ofttimes encountered in the initial stages of calcite precipitation from aqueous solutions, then these materials and their transformations could be important in a broad ecology, biological, and technological context. To understand the part of ACC in biomineralization, CO_two sequestration, carbonate rock formation, and other processes, the energetics of this organization must exist characterized. A written report by Wan et al. investigated the dynamic process of calcium carbonate atmospheric precipitation with different amounts of Fifty-aspartic acid by microcalorimetry (37). Wan et al. observed 2 calorimetric peaks corresponding to formation of a transient ACC and its transformation to a crystalline phase. In improver, there are three reports of thermally induced crystallization of ACC to calcite at approximately 320 °C using differential scanning calorimetry (DSC) (18, 19, 38).

The present study investigates the enthalpies of transformation and crystallization in synthetic and biogenic ACC to provide a comprehensive movie of their relative stabilities. Synthetic ACC samples were freshly prepared in our laboratory without the employ of organic additives and biogenic-ACC samples were extracted from California imperial sea urchin (S. purpuratus) larval spicules. The transformation of hydrous synthetic and biogenic ACC to calcite at ambient temperature was measured using acid solution calorimetry. The aridity of hydrous ACC, followed by the crystallization of anhydrous ACC to calcite on heating, was analyzed using DSC. This study provides information on the metastability, aridity, and crystallization energetics, and a possible crystallization pathway for ACC. This written report besides includes the direct measurement of crystallization enthalpy of biogenic ACC. Major findings are that ACC is a highly metastable phase compared to all crystalline CaCO_iii polymorphs, that the dehydrated ACC produced by heating the chemically precipitated phase is energetically similar to biogenic ACC, and that the sequence of transformations, more than metastable hydrated ACC—less metastable hydrated ACC—anhydrous ACC ∼ biogenic anhydrous ACC—vaterite—aragonite—calcite, provides an energetically downhill reaction pathway for crystallization.

Results

The absence of a crystalline phase in ACC samples was confirmed by 10-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. The water content was estimated from the first weight loss pace (ambient to 200 °C) in the themogravimetric assay (TGA) bend. The thermal behavior of ACC on heating was studied using DSC. Enthalpies of solution at ambient conditions were done using isothermal acid solution calorimetry. The Materials and Methods section below and the deposited SI Text provide details.

Synthetic ACC.

To minimize the crystallization of initial materials, we studied five freshly prepared ACC samples (ACC-1 to 5) synthesized under similar conditions. All these samples are hydrated with limerick CaCO₃·northward H₂O. The absence of any Bragg reflections in the pulverization XRD patterns indicates the formation of an amorphous phase (Fig. 1). 2 lengthened maxima centered at 20 and 45° 2θ are common features for the ACC phase and were previously observed (19, twenty). The variable temperature pulverization XRD patterns of the synthetic ACC-1, 4, and v samples indicate that ACC persists up to 330 °C (see Fig. S1) and crystallization to calcite occurs in a higher place this temperature.

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Pulverization XRD patterns of more disordered (ACC-1 and -2), less disordered (ACC-iii to v) synthetic ACC, and calcite PDF: 60-7239.

The FTIR spectra of synthetic ACC are shown in Fig. S2A. The absence of the abrupt ν ₄ band at 713 cm^-1 in the FTIR spectra indicate the absence of meaning amounts of calcite in these samples (Table 1 and Figs. S2A, S3A). The O-H stretching vibration around 3,400 cm^-1 indicates the presence of h2o in the samples. The water content ranges from 1.twenty to 1.58 moles H_twoO per mole CaCO₃, which is consequent with the hydration state of persistent ACC (Fig. S4).

Table i.

Characterization of the synthetic ACC samples

Sample description		H_twoO, north (mol) from TGA	FTIR I(ν ₂)/I(ν ₄)	BET surface area (grandⁱⁱ/thousand)	Particle size (nm)
Type	ID	H_twoO, north (mol) from TGA	FTIR I(ν ₂)/I(ν ₄)	BET surface area (grandⁱⁱ/thousand)	BET	SEM
Constructed calcite	Calcite	0	1.4	0.99 ± 0.08	-

More matted constructed ACC (more metastable)	ACC-i	ane.20 ± 0.02	viii.0	19.iii ± 0.2	192 ± 3	-
More matted constructed ACC (more metastable)	ACC-ii	i.58 ± 0.01	9.v	9.0 ± 1	412 ± 53	∼200

Less disordered Synthetic ACC (less metastable)	ACC-3	i.46 ± 0.01	half dozen.8	12.1 ± 0.two	306 ± 6	-
	ACC-iv	i.36 ± 0.01	6.2	12.5 ± 0.one	296 ± 3	150–200
	ACC-5	1.xiii ± 0.02	vii.2	42.iv ± 0.2	86 ± 4	fifty–100

Both SEM and surface area measurement past the Brunauer-Emmett-Teller (BET) method were used to guess the particle size of the synthetic ACC (Table one). The observed particle size in the SEM images ranges from l to 200 nm, whereas BET analysis indicates particle sizes between 80 to 400 nm. Considering the uncertainties, the two sets of information are in reasonable agreement. We have some evidence that the ACC may take dehydrated and crystallized in the high vacuum of the SEM, but the main purpose of SEM was to cheque particle size, which should not take been affected.

The enthalpies of solution in 5 N HCl (ΔH_sln) range from -46.3 ± 0.9 to -53.3 ± 1.4 kJ/mol (Table two). The enthalpy of solution of calcite (Fisher Chemicals) is -28.8 ± 0.3 kJ/mol. The solution enthalpy values for ACC samples are more exothermic than that of calcite (Table 2). Using these values in a thermochemical bicycle (see SI Text: Thermochemical bicycle-1), we calculated the enthalpies for the crystallization (ΔH_cryst) of the hydrous ACC phase to the stable calcite phase at 26 °C. These vary from -17 ± 1 to -24 ± ane kJ/mol for the synthetic ACC samples (Table 2) and do not show whatever correlation with either water content or expanse (Fig. S5). This variation may be acquired by small differences in grooming conditions (mixing, filtration, or aging during storage). Broadly, the synthetic ACC samples fall into two distinct ranges of enthalpies of crystallization. The samples ACC-one and ACC-2 show more exothermic crystallization enthalpies (-24 ± 1 kJ/mol) and the other samples, ACC-3, -four, and -five, prove less exothermic values (-17 ± 1 kJ/mol).

Table 3.

Label of the spicule samples

Sample description		H₂O, n (moles) from TGA	I(ν _two)/I(ν ₄) FTIR
Type	ID	H₂O, n (moles) from TGA	I(ν _two)/I(ν ₄) FTIR
ACC-spicules (biogenic ACC)	Spicule-48-1	-	viii.3
	Spicule-48-2	-	-
	Spicule-48-three	-	3.3
	Spicule-48-4	0.25 ± 0.01	7.3

Calcite-spicules (biogenic calcite)	Spicule-48-v	0.35 ± 0.01	7.ii
	Spicule-48-6	-	five.six
	Spicule-72-1	0.16 ± 0.02	two.3
	Spicule-72-2	0.16 ± 0.02	4.0

The DSC curves (Fig. S4) exhibit ii peaks. The first peak at 100 °C is endothermic and corresponds to dehydration of ACC. A crude integration of this DSC peak for different samples, normalized to the amount of water loss, gives a value close to the enthalpy of vaporization of liquid water at 100 °C (40.87 kJ/mol). The second sharp exotherm at 327 °C is due to the crystallization of the dehydrated constructed ACC phase and agrees with the calcite crystallization temperature observed past variable temperature XRD. The enthalpies of crystallization of anhydrous synthetic ACC at 327 °C (ΔH_{ahhydACC-cryst} at T _cryst) were calculated from the second exothermic DSC top (Tabular array 2) and vary from -12.eight ± 0.v to -fifteen.4 ± 0.iv kJ/mol for dissimilar synthetic samples. Other thermodynamic studies of synthetic ACC by Koga and Yamane and Koga et al. reported wider variations in thermally induced enthalpies of crystallization (-xiii.1 ± 0.ii and -19.i ± 0.2 kJ/mol (22); -12.3 ± 0.2 to -39.ix ± ii.ane kJ/mol (21)).

Biogenic ACC from Sea Urchin Larval Spicules.

Several biogenic-ACC samples extracted from 72 h (spicule-72-one and 72-2) and 48 h (spicule-48-ane to 48-6) body of water urchin embryo spicules (California purple sea urchin, S. purpuratus) were studied within 1 d of extraction. Similarly extracted textile has been reported to incorporate v mol% magnesium with the composition Ca_0.95Mg_0.05CO_three· north H₂O (27). We confirmed this composition for the spicule samples past inductively coupled plasma mass spectrometry. The pulverisation XRD patterns of the samples extracted after 48 h (spicule-48-1 to 48-4) show depression intensity Bragg reflections corresponding to calcite. In contrast, the powder XRD patterns of the samples extracted afterwards 72 h show well defined Bragg reflections of calcite (see Fig. S6).

The FTIR spectra of spicules are shown in Fig. S2B. The FTIR spectra propose spicules-72-1, and 72-2 have crystallized to calcite, whereas spicules-48-1 to 48-6 are mixed calcite and ACC phases (Table 3 and Figs. S2B and S3B). The well defined Bragg reflections in the XRD blueprint and the absence of whatever "amorphous bump" in the diffraction pattern argue strongly that these samples are mostly calcite, with little ACC left (meet Fig. S6). The measured enthalpies are consistent with mainly crystalline calcite in spicule-48-five and 48-6 (Table 2). The low intensity wide O-H stretching vibration around 3,400 cm^-i indicates the presence of a small corporeality of water in these samples. The TGA studies indicate the spicule samples incorporate less water than synthetic ACC, namely 0.16–0.35 mol H₂O per formula unit of measurement (Table three). Based on these measurements, nosotros can allocate spicule samples into ii groups every bit biogenic ACC (spicule-48-one to 48-4) and biogenic calcite (spicules-72-1, 72-2,-48-5, and 48-6).

Tabular array 2.

Calorimetric data

Sample description		Acrid calorimetry in 5 N HCl, 26 °C		DSC
Type	ID	ΔH_sln (kJ/mol)	ΔH_cryst at 26 °C (kJ/mol) (ACC to calcite)	T _cryst (°C)	ΔH_{anhydACC-cryst} at T _cryst (kJ/mol) ^* (anhydrous ACC to calcite)
Synthetic calcite	Calcite	−28.75 ± 0.26 (7)	-	-	-

More disordered synthetic ACC( more metastable)	ACC-1	−50.lxx ± 1.78 (5)	−21.47 ± 1.80	326	−15.4 ± 0.4
More disordered synthetic ACC( more metastable)	ACC-2	−53.33 ± one.38 (iii)	−23.95 ± i.40	327	−13.three ± 0.seven

Less disordered Synthetic ACC (less metastable)	ACC-3	−46.33 ± 0.93 (4)	−17.00 ± 0.97	329	−fifteen.2 ± 0.3
	ACC-4	−46.38 ± 0.60 (5)	−17.09 ± 0.65	327	−14.8 ± 0.9
	ACC-5	−46.81 ± 0.85(4)	−17.61 ± 0.89	313	−12.8 ± 0.five

ACC-spicules (biogenic ACC)	spicule-48-1 to 48-three	−42.50 ± 0.47 (3) (−43.84 ± 0.68) ^†	−12.86 ± 0.76 ^‡ (−14.29 ± 0.97) ^‡ ^†

Calcite-spicules (biogenic calcite)	Spicule-72-ane and 72-2	−29.60 ± 0.60 (5) (−29.54 ± 0.sixty) ^§	-
Calcite-spicules (biogenic calcite)	spicule-48-five and 48-half-dozen	−29.81 ± 0.48 (four)(−29.67 ± 0.48) ^§	-

The measured enthalpies of solution are summarized in Table 2. For biogenic calcite with v mol% Mg (spicules 72-1, 72-2, 48-5, and 48-6), the enthalpy of solution is -29.v ± 0.6 kJ/mol (Tabular array 2). The enthalpies of solution of spicule-48-4 and 48-5 confirm that these samples had crystallized to calcite since this enthalpy is very close to that of pure calcite (-28.eight ± 0.3 kJ/mol). For enthalpies of solution of hydrated phases at 26 °C (ΔH_sln-spicule) at ambient weather, the corrections for contributions of water have been made (SI Text: Thermochemical cycle-2). For the biogenic-ACC samples (spicule-48-1 to 48-3), which independent calcite impurities, a correction was also practical assuming the sample to exist 90% biogenic ACC and ten% biogenic calcite (SI Text: Thermochemical cycle-3). The enthalpies of crystallization from biogenic ACC to biogenic calcite at 26 °C (ΔH_cryst-bio) are then calculated using the corrected enthalpies of solution (SI Text: Thermochemical cycle-iv). The enthalpy of crystallization at 26 °C (ΔH_cryst-bio) from anhydrous biogenic ACC to biogenic calcite is -12.nine ± 0.viii kJ/mol. Applying the correction for ten% calcite in the biogenic ACC modifies the enthalpy of crystallization to -fourteen.3 ± 1 kJ/mol. Unlike synthetic ACC, the biogenic-ACC spicules are almost anhydrous (0.25 mol of h2o), which crystallizes hands to calcite.

DSC measurements of the calcite spicules and synthetic calcite samples showed no exothermic crystallization peaks. The DSC results confirms that the spicules 72-1, 72-2, and 48-v are crystalline calcite and do not comprise meaning residuum ACC. The DSC on biogenic ACC (spicule-48-4) was carried out with a very small (∼0.seven mg) sample and no top could be resolved.

Discussion

Fig. 2 presents a stability diagram or energy landscape for the calcium carbonate arrangement, including the crystalline and diverse baggy phases. The stability expressed as the enthalpy relative to calcite likely as well represents the order of free energy because the TΔS term is probably small at room temperature. The figure illustrates several of import points. (i) An energetically downhill sequence can be inferred: more than metastable hydrated ACC—less metastable hydrated ACC—anhydrous ACC ∼ biogenic anhydrous ACC—vaterite—aragonite—calcite. (ii) Hydrated and dehydrated ACC are much higher in enthalpy than whatever of the crystalline polymorphs, which are all similar in energy. This tendency suggests that ACC is a suitable precursor for all CaCO₃ polymorphs, and which phases really are formed in the transformation sequence above, and which are bypassed, can be kinetically controlled. (iii) The formation of anhydrous ACC from hydrated ACC is exothermic. Thus anhydrous ACC is less metastable than the hydrated material and its germination is a thermodynamically irreversible procedure. (iv) Dehydrated synthetic and biogenic ACC take similar enthalpies. (v) There appear to be two energetically distinct groups of hydrated synthetic ACC, more than metastable and less metastable, come across discussion below.

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Relative energetic stabilities of dissimilar calcium carbonate phases with respect to calcite. The enthalpy values of vaterite and aragonite are taken from (48) and (49) respectively.

These data support previous studies (18, 19) indicating that ACC is indeed metastable with respect to any of the crystalline calcium carbonate phases. Both hydrated and dehydrated ACC are high-energy metastable phases and in that location is a big energetic driving force for their crystallization. This big energy difference between hydrated amorphous and dehydrated crystalline phases is like to the energetics of amorphous oxides such every bit zirconia and silica (39, xl). The surface energies of the amorphous oxide materials are significantly smaller than those of their crystalline counterparts, suggesting that small particle size could atomic number 82 to thermodynamic stabilization of baggy relative to crystalline nanoscale materials (41, 42). The surface energies of amorphous and crystalline calcium carbonates are currently unknown, but may also affect the energetics of the arrangement.

The formation of anhydrous ACC from the hydrated form is exothermic, and this ways the hypothetical formation of hydrated ACC from the dehydrated grade would exist endothermic. In other words, ACC dehydration is an irreversible process, with the dehydrated grade more stable than the hydrated. This behavior is in contrast to the exothermic hydration (with respect to liquid water) of oxides to form oxyhydroxides (43, 44), and of dehydrated zeolites to form hydrated ones (45). The DSC measurements indicate that the vaporization of water from constructed hydrated ACC is endothermic. The dehydration superlative near 100 °C is diffuse in both DSC and TGA, a rough integration of DSC tiptop, normalized to the amount of h2o loss, suggests that the enthalpy of dehydration is roughly equal to the enthalpy of vaporization of liquid h2o at 100 °C (40.87 kJ/mol). This enthalpy value suggests that the initial dehydration is essentially loss of physisorbed h2o. Further loss of more strongly jump water and restructuring of ACC occurs up to the crystallization elevation at 327 °C. This phenomenon implies that the structural reorganization during aridity of ACC is exothermic enough to more compensate for the endothermic removal of H_twoO.

Both hydrated and dehydrated ACC are much college in enthalpy relative to majority calcite than any of the crystalline polymorphs (aragonite, vaterite, or calcite). The major stabilization in enthalpy (energy) occurs during crystallization, and non during aridity or during possible transformation among crystalline polymorphs.

Unlike constructed ACC, biogenic ACC is well-nigh anhydrous (0.25 mol of water), and is a transient stage which crystallizes easily to calcite. The crystallization enthalpy measured for the amorphous spicule samples at ambient atmospheric condition is -14.three ± 0.97 kJ/mol. This value is like to that seen in the DSC measurement around 327 °C of dehydrated constructed ACC fabric (-12.8 ± 0.v to -15.4 ± 0.4 kJ/mol). This similarities in the enthalpy values suggests that the crystallization of anhydrous ACC from synthetic and biogenic sources takes place nether different conditions with similar energetics. Notwithstanding, the mechanism of crystallization may be different for synthetic and biogenic anhydrous ACC. The similarity of crystallization energetics for the constructed Mg-gratuitous and biogenic Mg-containing samples also suggests that the upshot of 5 mol% Mg in biogenic ACC on the enthalpy of crystallization is negligible.

Politi et al. (viii) and Killian et al. (11) observed three spectroscopically distinct mineral phases in forming calcitic biominerals (from ocean urchin spicules and teeth). The offset phase (termed type 1) is rarely found, the second (type ii) is the almost abundant phase observed during biomineral germination, and finally in the mature parts of the biominerals the merely stage is biogenic calcite (type 3). By comparing with synthetic hydrated ACC and calcite, the authors identified the spectra of type 1 to stand for to hydrated ACC and blazon 3 to calcite. In that location was, however, no synthetic reference material to assign the spectrum of type two. But the authors inferred that blazon two is anhydrous ACC. To our knowledge the nowadays results show the total energetic label of anhydrous synthetic and biogenic ACC. The similarity of water contents and crystallization enthalpies of biogenic and dehydrated synthetic ACC strongly suggest that the bulk of biogenic ACC is indeed anhydrous. This ascertainment suggests a general trend in the development of spicules and probable other biominerals, where the freshly deposited material is composed of hydrated ACC, which apace transforms into a transient anhydrous phase, and finally to biogenic calcite (46).

The enthalpies of crystallization for diverse hydrous synthetic ACC samples were not constant. Instead two groups were observed: more metastable and less metastable hydrated phases. There was no correlation between the measured enthalpies and the water content or particle size. The only noticeable qualitative differences were found in the XRD patterns (run into Fig. 1). We observed much lower intensity of the broad lengthened maxima in the XRD patterns of the samples with more than exothermic crystallization enthalpies (more metastable hydrated ACC-1 and ii). The XRD results suggest that these samples are more disordered and contain less short-range order than those with less exothermic crystallization enthalpies (less metastable hydrated ACC-iii to 5). Still these differences are difficult to quantify, most of the samples were consumed for calorimetry, and whatever leftover materials crystallized with aging.

If ACC is formed from aggregated prenucleation clusters, then the size of the cluster could determine the short-range social club of the sample. Studies on calcium carbonate solutions signal an initial size of approximately 0.6–i nm, merely later nucleation, 6 nm clusters were notwithstanding observed in solution (34). Small variations in the temperature, mixing, filtration, and crumbling of the initial ACC material may affect the size and aggregation of the initial prenucleation clusters. The nature of such curt-range ordering was not examined further in this report, merely suggests an interesting topic for future investigations.

The enthalpies reported herein suggest that the crystallization of hydrous and anhydrous ACC to the other polymorphs (aragonite, vaterite) or to calcite is energetically downhill (run into Fig. 2). The actual mechanisms of transformation along this energy landscape may not be straightforward since the possible part of prenucleation clusters, nanodomains, and reactions involving water make this system potentially very complex. One of these complex mechanisms, secondary nucleation, has recently been proposed for the crystallization and coorientation of biogenic-calcite nanoparticles (11). Our results clearly confirm that both synthetic and biogenic ACC are metastable phases with respect to crystalline forms of calcium carbonate, and that the major energy release comes with crystallization.

These results shed some calorie-free on the possible initial stages of crystallization of a calcium carbonate phase when CO_ii is injected underground. Near room temperature, rapid atmospheric precipitation of ACC when a CO₂ feather encounters aqueous brine containing dissolved calcium appears likely, followed by the dehydration and transformations described above. At higher temperature and a more h2o-poor surroundings, the initial hydrous ACC phase may be bypassed, with the dehydrated course of ACC forming directly. The roles of temperature, salinity, and surface nucleation under such geologic weather remain to be explored, but the energy landscape among phases adult by this study should provide useful constraints on possible reaction sequences.

Conclusions

To understand the crystallization pathway for ACC, the aridity and crystallization enthalpies of ACC were measured using both DSC and isothermal solution calorimetry. Both hydrated and anhydrous ACC were plant to be metastable with respect to crystalline forms. The dehydration of ACC is exothermic, implying that the structural reorganization during aridity is exothermic enough to more compensate for the endothermic release of H_iiO and confirming that such dehydration is irreversible. The enthalpy of crystallization at ambient conditions for hydrated synthetic ACC ranges from -17 ± ane to -24 ± 1 kJ/mol for several separately prepared samples. This variation may reflect differences in short-range order and/or nanodomain germination, with samples falling roughly into 2 groups: less ordered and more metastable vs. more ordered and less metastable. The enthalpy of crystallization of biogenic ACC at ambient conditions is -14.3 ± 0.97 kJ/mol, similar to that of dehydrated synthetic ACC effectually 327 °C (-12.8 ± 0.5 to -15.4 ± 0.4 kJ/mol for different samples). Previous studies in biogenic ACC inferred that the first phase formed is hydrated ACC, the second phase anhydrous ACC, and the final phase is calcite. The present data confirm that indeed the intermediate phase is anhydrous ACC, and that the transformations proceed through a sequence of phases of decreasing energetic metastability.

Materials and Methods

ACC samples were synthesized using previously reported methods (xix, twenty). A 0.02 One thousand sodium carbonate solution was prepared by mixing 0.424 g of Na_iiCO₃ (99.5%, Alfa Aesar) in 20 mL of 2 M NaOH solution (Alfa Aesar) and and so diluting it to 200 mL with deionized water. A second solution containing 0.02 M calcium chloride was likewise prepared by dissolving 0.588 one thousand of CaCl₂·2H₂O (99%, Alfa Aesar) in 200 mL of deionized h2o. Both these solutions were cooled in an ice bathroom for six h, then apace mixed with abiding stirring and immediately filtered nether vacuum. The precipitate was washed with acetone several times and vacuum dried overnight at 25 °C. All samples were equilibrated in the calorimetry suite, which is always maintained at ∼24 °C and ∼50% relative humidity, for 24 h before characterization and calorimetric measurements. The calcite sample was obtained from Fisher Chemicals (99.7%) and dried at 250 °C for twenty h.

Sea Urchin Spicules.

Eggs of the California majestic sea urchin, S. purpuratus, were fertilized and the embryos cultured at a concentration of 0.25% (egg volume/seawater volume) at fifteen °C in filtered natural seawater co-ordinate to the methods described by Foltz et al (47). Embryos were harvested for spicule isolation at prism stage (48 h postfertilization) and pluteus phase (72 h postfertilization). The spicules were extracted, cleaned from any external organic molecules, dried, and transported in dry ice. The dry spicules were analyzed within 24 h of extraction to avoid crystallization of ACC with aging. Detailed spicule preparation is described in the SI Text.

Powder 10-ray Diffraction.

The powder XRD patterns of the samples were nerveless using a Bruker AXS D8 Advance diffractometer (Bruker AXS, Inc.) with Cu One thousand_α radiation source. Data were nerveless in the 2θ range of 10–120 ° with a step size of 0.01° 2θ and a drove time of 0.five–2 s/step using a zero-background sample holder. The XRD detection limit is approximately 2%–3%. In situ variable temperature powder XRD patterns of the samples were collected using an INEL X-ray diffractometer (INEL Inc.). The samples were heated from 110–500 °C in air at 10 °C/ min using the Symphonix software (Symphonix Devices). XRD patterns were collected in steps of xx °C with a position sensitive detector covering 120° in 2θ using Co K _α radiation. The temperature of the heating stage furnace was calibrated using an Omega HH506R thermometer (OMEGA Engineering, Inc.) and the calibrated temperature range corresponds to 117–695 °C.

Solid-State Infrared Spectroscopy.

The IR spectra of the samples were recorded past the KBr pellet technique using a Bruker Equinox 55 FTIR spectrometer (Bruker Optics Inc., range 400–4,000 cm^-1). A background spectrum with a pure KBr pellet was nerveless before each sample.

Thermogravimetric Analysis and Differential Scanning Calorimetry (TGA/DSC).

TGA/DSC was carried out using a Netzsch STA 449 organization (NETZSCH-Feinmahltechnik GmbH). The sample in a platinum crucible was heated from 25 °C–1,200 °C in argon at x °C/ min. A buoyancy correction was fabricated. The water content was determined by the TGA weight loss curve. The DSC curves recorded simultaneously give the enthalpies associated with reactions such as dehydration, crystallization, and decomposition.

Surface area Measurement.

Surface area was measured by the BET method using a Micromeritics ASAP 2020 apparatus (Micromeritics Instrument Corporation). Five point isotherms in the P/P ^o relative pressure range (P ^o = saturation pressure) of 0.05-0.iii were measured past nitrogen adsorption at -196 °C. Each sample was vacuum degassed at room temperature for 18 h before the BET measurement.

SEM.

SEM studies were carried out using a FEI XL30-SFEG high resolution Scanning Electron Microscope (FEI / Philips) without whatever boosted blanket of the sample. The microscope was operated at i kV considering the ACC samples showed signs of beam damage at higher voltages. Every bit the samples may take transformed and/or crystallized on crumbling earlier the SEM was done, simply likely did not coarsen, we used the SEM mainly to confirm particle size (see SI Text for details).

Isothermal Acid Solution Calorimetry.

A CSC 4400 isothermal microcalorimeter (Calorimetry Sciences Corporation) operated at 26 °C was used for the measurements of enthalpy of dissolution. Five milligrams of each sample was hand pressed into a pellet and dropped into a 5 N HCl solvent (∼25 thousand) and taken in the calorimetric sample chamber. Mechanical stirring was used to aid dissolution. The calorimeter was calibrated past dissolving 15 mg pellets of KCl in water with stirring at 26 °C (see SI Text for details).

Supplementary Material

Acknowledgments.

We thank Fred Hayes (Materials Science Central Facilities, UC Davis) for the assistance with SEM. The early stages of this project were supported by DOE Grant DE-FG02-97ER-14749. The last stages were supported as office of the "Center of Nanoscale Control of Geologic CO₂," an Energy Frontier Inquiry Center funded past the US Department of Energy, Part of Science, Office of Basic Energy Sciences under Award Number DE-AC02-05CH11231. The spicule piece of work was supported by DOE Grant DE-FG02-07ER15899, and National Science Foundation (NSF) Grant CHE/BMAT—0613972 (to P.U.P.A.Thousand.).

Footnotes

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