Polymorphic and morphological selection of CaCO3 by magnesium-assisted mineralization in gelatin: magnesium-rich spheres consisting of centrally aligned calcite nanorods and their good mechanical properties

Mg2+ ions and acidic proteins are both controlling elements in biomineralization. The present study combines the two types of additives (Mg2+ ions and a denatured collagen protein (gelatin)) to direct the mineralization of CaCO3. We find that the polymorphs and morphologies critically depend on gelatin concentration at a given Mg2+ concentration. Moreover, at a given gelatin concentration, the Mg molar percentages in the mother liquor, although not a determining factor for the polymorphs, can affect the crystal micro-and nano-structures. The controlled crystallization can be rationalized by the interplay between Mg2+ and gelatin, which mutually enhances their uptake and regulate the concomitant mineralization. We have obtained high magnesium calcite microspheres (HMCMs), which consist of vertically aligned single crystal nanorod building blocks. Such HMCMs show good mechanical properties (E-r = 28.91 +/- 2.97 GPa; H = 1.37 +/- 0.07 GPa), akin to the central stereo of the spine skeleton of Paracentrotus. lividus (E-r = 32.20 +/- 3.26 GPa; H = 1.76 +/- 0.70 GPa).(1) The remarkable mechanical properties should be ascribed to the hierarchical structure, the anisotropic arrangement, and the centrally aligned calcite nanorods separated by the cohesive gelatin matrix, which can effectively dissipate energy under force loading.