Nucleation and Growth of Mg-Calcite Spherulites 1193


band around 712cm−1 (Iν2/Iν4) can be used to identify the ACC percentage in the calcite phase (Jayaraman et al., 2007; Yan et al., 2008; Mantilaka et al., 2014). The value is 3.0 for pure calcite, but it increases with increasing ACC content (Jayaraman et al., 2007). Our results showed an obvious decreasing trend as mineralization progresses (Fig. 7). Thus, the Mg-calcite spherulites likely originated from ACC. The ACC phase is usually unstable and gradually transforms into a more stable crystalline phase as mineralization progresses. The solubility of ACC is higher than that of the crystalline phases (Beck & Andreassen, 2010b), hence it is easy to dif- ferentiate the ACC phase from the crystalline phase. Etching the crystals inKOH solution has proven useful for dissolving the ACC phase without affecting the calcite (Beniash et al., 1997). After etching in 1MKOH for 12 h, the central area of the spherulites was more heavily dissolved than the outer region, and a few corroded fractions were visible near the outer surface of spherulites, which confirmed the presence of the aforementioned radial needle-like subunits (Fig. 8a). The enlarged image of the central area in Figure 8a suggests the presence of unaffected ACC particles in the cavity, which are enveloped in surrounding organic matter (Fig. 8b). Thus, a large number of ACC particles appear to be present in the core, and remain stable, while only a few ACC particles precipitate on the surface film. In addition, EDS analysis (insets in Figs. 8, 9) revealed a small signal from


phosphorous (P) in the central and surface regions of the spherulites, while no P signal was found in the intermediate region. These results indicate that the central core and outer surface are much richer in organic matter than the inter- mediate region, consistent with the results of the etching experiments following treatmentwithH2O2 described above. This distribution pattern inlaid with organic matter may be attributed to low initial supersaturation, resulting in insuffi- cient phase transition of the amorphous phase to the crys- talline phase, and domination of nucleation in the earlier stages. Thus, more bacteria and biomolecules were stably preserved in the center of spherulites, where they interact with amorphous phases. Increased levels of supersaturation therefore forced phase transition of the amorphous phase to Mg-calcite particles and promoted the rapid growth of Mg-calcite subunits. Accordingly, incorporation of Mg2+ into calcite crystals resulted in lattice strain and altered crystal orientation. Moreover, the film on the surface of spherulites caught our attention, and we observed numerous calcified bacteria on the surface of spherulites (inset in Fig. 9) where organic biomolecules were present. Based on the above observation, we conclude that a


mature biogenic spherulite can be divided into three parts: a core of organic matter and adjacent ACC particles with a


Figure 7. Fourier transform infrared spectra of samples collected at different time.


Figure 9. Scanning electron microscopy image of mature spherulite absolutely covered by thin film, the top-right and bottom-right insets are the magnification of square area and energy dispersive spectroscope results of +, respectively.


Figure 8. a: Scanning electron microscopy image of spherulite after etching in 1M KOH solution for 12 h, inset in a is the energy dispersive spectroscope (EDS) results of + in a;(b) is the magnification of the center area in a, the top-right and bottom-right insets in b are the magnification and EDS results of + in b, respectively.


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