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Human enamel is the hardest tissue in our body (hardness~ 2-6 GPa), due to its high mineral content (96 wt%) and its complex 3D hierarchical structure across different length scales (Fig. 1). The characteristic microstructural unit of enamel‒the enamel rod (Fig. 1A)‒is comprised of thousands of lath-like crystallites (Fig. 1B) nominally composed of hydroxylapatite (OHAp; Ca5 (PO4) 3 (OH)(Fig. 1C & Fig. 1C, inset)). However, enamel is known to contain small quantities of other ions, such as Mg2+(0.2-0.6 wt%), Na+, CO3 2-, and F-, which can substitute in the OHAp lattice and are known to increase (Mg2+, Na+, CO3 2-) or decrease (F-) its solubility [1]. These minor components play a key role in how enamel is affected by exposure to acid in the oral cavity, which may lead to tooth decay (caries). Therefore, knowledge of the ion distribution on the length scale of individual crystallites is vital in understanding processes …