Proto‐Calcite and Proto‐Vaterite in Amorphous Calcium Carbonates

Abstract

Crystallization of calcium carbonate, typically, progresses sequentially via metastable phases. Amorphous CaCO 3 (ACC) generally forms initially, both in vitro and in vivo, and is the precursor of the predominant anhydrous polymorphs (calcite, aragonite, and vaterite). [1][2][3][4][5][6][7][8][9][10][11][12][13] A new picture of the crystallization of calcium carbonate is emerging, which involves transformations of clusters to ACC and eventually to crystalline polymorphs. [14,15] This stepwise manner has implications for the understanding of biomineralization [16] and of crystallization. ACCs that contain additives display order over atomic length scales that are related to crystalline polymorphs; [1][2][3] ACC synthesized at high supersaturation levels without additives, [17][18][19][20] on the other hand, show no distinct short-range order. [21,22] Herein, we analyze proto-crystalline features of two amorphous intermediates, ACCI and ACCII, [15] and discuss their relevance for crystallization of CaCO 3 . We rationalize the identification of ACCI with pc-ACC (proto-calcite ACC) and ACCII with pv-ACC (proto-vaterite ACC), respectively. These ACCs were precipitated from metastable solutions of calcium carbonate at different pH values by destabilization in excess ethanol.TEM (Figure 1) reveals the ACCs as spherical particles with a diameter of approximately 20 nm. Small-angle X-ray scattering (SAXS) data support this characteristic size

Distinct Short‐Range Order Is Inherent to Small Amorphous Calcium Carbonate Clusters (<2 nm)

“…and 1399/1462 cm À1 (double band) are very close to that of proto calcite ACC at 862 cm À1 and 1392/1462 cm À1 , [16] respec tively. This indicates the small ACC clusters exhibit a proto calcite short range order in the CaCO 3 core.…”

“…3.8 ppm for the ACC clusters is even larger than that of proto calcite ACC (3.6 ppm). [16] This indicates that the CaCO 3 core of the ACC clusters indeed exhibits a proto calcite structure but with a relatively high degree of disorder that likely arises from the very small size of the ACC cluster core. Despite the discovery of several proto crystalline forms of short range order in ACC (or polya morphism), [16 18, 27, 28] the here reported finding of the existence of short range order in very small ACC clusters less than 2 nm has never been reported, which also supports the relevance of the preformed short range order in prenucleation clusters.…”

“…[26] More impor tantly, the n 1 n 4 bands can be used to relate the structure of the ACC cluster to distinct amorphous forms. [16,17] The n 1 and n 4 bands are partially superimposed by vibrational modes of PCDA, but the n 2 and n 3 modes of ACC cluster at 860 cm…”

“…As shown in Figure 4 a, a broad resonance centered at 168.5 ppm can be observed in the solid state 13 C NMR spectrum of the ACC clusters, which agrees with that of both calcite and proto calcite ACC (168.7 ppm) within experimental accuracy. [16] The full width at half maximum (FWHM) of ca. 3.8 ppm for the ACC clusters is even larger than that of proto calcite ACC (3.6 ppm).…”

State between Liquid and Crystal: Locally Crystalline but with the Structure Factor of a Liquid

“…[4][5][6] The structures of Amorphous Calcium Carbonate (ACC), [7][8][9][10][11][12][13][14][15][16] Amorphous Calcium Phosphate (ACP), 17 and Amorphous Calcium Sulphate (ACS) [18][19][20] are also poorly understood. The properties of ACC clearly depend on how it is prepared, 9,[21][22][23][24] and Gebauer et al 23 refer to 'protovaterite' and 'proto-calcite' forms of ACC, which * To whom correspondence should be addressed although amorphous have NMR and EXAFS spectra that have (broadened) features in common with vaterite, and with calcite, respectively. Vaterite and calcite are two of the crystal polymorphs of calcium carbonate.…”