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
The simplified formula for calcium-deficient carbonated hydroxyapatite is:$$\ce{Ca_{$10-x$}Na_{$x$}(PO4)_{$6-y$}(CO3)_{$z$}(OH)_{$2-u$}F_{$u$}}$$
The perfect formula is:$$\ce{Ca10(PO4)6(OH)2}$$
So why does the $10-x$ on the simplified formula show that it is calcium deficient?
On the article, ...