1192 Chonghong Zhang et al.


Figure 4. a: Scanning electron microscopy (SEM) image of samples on day 50; (b,c) SEM images of fractured spherulites in a;(d) SEM image of a 30th day spherulite covered by partial film; d1 and d2 are the magnifications of uncovered and covered areas in d, respectively.


Figure 5. Scanning electron microscopy image of spherulites after treating with 30% H2O2 for 4 h.


discovered a unique Mg-calcite spherulite (Fig. 4d), one half of which was coated with a film (Fig. 4d2) while the other half displayed rhombic and trigonal structures (Fig. 4d1) that are typical of the classical calcite phase morphology. The film appeared to spread to eventually cover the entire spherulite. In most cases, biogenic carbonates are representative of organic–mineral complexes, and different combinations of minerals and organic molecules alter their physicochemical properties (Wohlrab et al., 2005). To investigate the interior structure and composition of the spherulites, samples were etched in 30% H2O2 for 4 h to remove organic matter, then washed with deionized water and dried in air prior to observation (Blanco-Ameijeiras et al., 2012). As shown in Figure 5, there are many holes on the outside surface of the spherulites, which are discontinuous and unevenly dis- tributed. However, the intermediate region maintains a tight structure. Surprisingly, the spherulites have a hollow central


Figure 6. X-ray diffraction spectra of precipitates isolated from growth solution.


area around 2−10 μm in diameter. These results suggest the organic matter is mainly distributed at the center and the outer surface of the spherulites. By contrast, the remaining intermediate region of spherulites is tightly filled with needle-like subunits without any organic matter. The spherulites are therefore Mg-calcite crystals pack-


aged with small amounts of organic matter because no crystalline phases other than Mg-calcite were detected in the XRD patterns (Fig. 6). Of course, we cannot rule out the possibility that ACC is present in spherulites, since ACC would not diffract when present in the calcite phase. To further clarify the phase composition of Mg-calcite spher- ulites, FT-IR analysis was performed. FT-IR bands (Fig. 7) centered at ~1,075cm−1 (ν1) were observed in samples collected at different time intervals, revealing the presence of ACC (Zhong & Chu, 2010). The intensity ratio of the ν2 absorption band around 875cm−1 and the ν4 absorption


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