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

An array of magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy experiments is applied to explore the surface reactions of a mesoporous bioactive glass (MBG) of composition Ca0.10Si0.85P0.04O1.90 when subjected to a simulated body fluid (SBF) for variable intervals. Powder X-ray diffraction and 31P NMR techniques are employed to quantitatively monitor the formation of an initially amorphous calcium phosphate surface layer and its subsequent crystallization into hydroxycarbonate apatite (HCA). Prior to the onset of HCA formation, 1H → 29Si cross-polarization (CP) NMR evidence dissolution of calcium ions; a slightly increased connectivity of the speciation of silicate ions is observed at the MBG surface over 1 week of SBF exposure. The incorporation of carbonate and sodium ions into the bioactive orthophosphate surface layer is explored by 1H → 13C CPMAS and 23Na NMR, respectively. We discuss similarities and distinctions in composition−bioactivity relationships established for traditional melt-prepared bioglasses compared to MBGs. The high bioactivity of phosphorus-bearing MBGs is rationalized to stem from an acceleration of their surface reactions due to presence of amorphous calcium orthophosphate clusters of the MBG pore wall.

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 Figure 1. TEM images of pc-ACC and pv-ACC at various magnifications. Insets in (b) and (e) are selective area electron diffraction (SAED) patterns obtained from an area slightly larger than the particular image sections, SAED scale bars: 5 nm À1 . SAED patterns are shown as negatives to make weak features clear. Arrows in (f) indicate nanostructural features.