Occlusion of naphthazarin molecules within calcite crystals through the crystallization of amorphous calcium carbonate precursors

Sea urchin spines are colorful biogenic calcite crystals formed after the crystallization of amorphous calcium carbonate (ACC) nanoparticles initially stabilized by organics that incorporate the final mineral phase together with organic pigments, the polyhydroxylated naphthoquinone (PHNQ) molecules. Intra-crystalline organic molecules are known to induce lattice distortions in biogenic and synthetic CaCO3. However, our previous HR-XRD measurements (HC_4779) showed that naphthazarin, part of the PHNQ family do not induce such lattice distortion when occluded into calcite via ACC precipitation and crystallization in water (dissolution/reprecipitation mechanisms). To complete our understanding of the mechanism of naphthazarin occlusion we here measure calcite-naphthazarin samples obtained after ACC crystallization induced by heating (solid-state transformation).