Crystallographic Control of the Hydrothermal Conversion of Calcitic Sea Urchin Spine (Paracentrotus lividus) into Apatite

Abstract

We analyzed the crystallographic relationships during hydrothermal conversion of a calcitic sea urchin spine into apatite. We identified a pseudomorphic mineral replacement mechanism involving a superficial dissolution of calcite and a subsequent overgrowth of oriented carbonated hydroxylapatite (HA) nanocrystals. Cross-section images of these converted spines show that the dimensions of the HA crystals increase the further they are from the outer surface. This replacement process is favored by an increase in porosity, which enables both fluid and mass to be transported by diffusion, thereby allowing the replacement reaction to progress toward the interior of the spine. These recrystallization reactions take place on the surface of the calcite single crystal, which acts as a substrate for the epitaxial nucleation of HA crystals. The epitaxial relationship observed between the parent calcite crystal and the newly formed apatite crystals can be defined as (0001) apatite//(011̅8) calcite and [10.0] apatite//[4̅4.1] calcite. The apatite crystals are related by the 3-fold axis arising from the trigonal symmetry of the parent calcite crystal. There is therefore a strong structural control which favors the conversion of calcite into apatite. This process coexists with the formation of apatite crystals which are not structurally related to the calcite crystal and which may precipitate within the porosity of the material. The analysis of crystallographic relationships is a fundamental step toward understanding mineral replacement reactions, which can be used for the synthesis of artificial materials with predefined shapes and microstructural characteristics, a technique that may have interesting technological applications.