The synthesis of hydroxyapatite nanoparticles starts with a mixture of Calcium sources and plant extracts. In this step, the Calcium ion interacts with plant extract metabolites. Previous works showed calcium could interact with tannins and other organic molecules through p-track conjugation to form a tannin-Ca complex . However, the Euclea natalensis root extracts are rich with naphthoquinones, polyphenolics, and terpenoids that can interact with calcium using their functional groups, such as -OH, and C=O, as FTIR analysis shows, to form metabolites (phenolic compounds and terpenoids)-Ca complex . This complex interacts with phosphoric acid in the second step to form hydroxyapatite nanoparticles. The formation of hydroxyapatite nanoparticles/Euclea natalensis roots was confirmed by XRD, and FTIR was present previously. Suggesting that hydroxyapatite formed is surrounded by organic molecules. These molecules act as capping agents, preventing the formation of larger particles and agglomeration. This probably explains why the hydroxyapatite nanoparticles synthesized with Euclea natalensis root extracts present a smaller particle size than those obtained without Euclea natalensis root extracts. In the latest step, the hydroxyapatite/Euclea natalensis root extracts are submitted to calcination, resulting in particles with higher crystallinity due to the elimination of organic compounds from Euclea natalensis root extracts (see Figure ).