= Hydroxyapatite
{title2=$\mathrm{Ca_{10}(PO_4)_6(OH)_2}$}
{wiki}
\b[Hydroxyapatite] is a crystalline <calcium phosphate> whose ideal composition is $\mathrm{Ca_{10}(PO_4)_6(OH)_2}$, equivalently $\mathrm{Ca_5(PO_4)_3OH}$. The formula describes an extended ionic solid, not a discrete hydroxyapatite molecule. Its ideal calcium-to-phosphorus molar ratio is $10/6=5/3$. NIST uses hydroxyapatite reference powder to evaluate physical and chemical measurements of apatites.<Hydroxyapatite Standard Reference Material 2910b Certificate of Analysis by NIST>[p. 1, purpose, formula and Ca/P ratio]{ref}
The ideal composition is electrically neutral: ten <calcium ions> contribute charge $+20$, six <phosphate> ions contribute $-18$ and two <hydroxide> ions contribute $-2$. Changing a subscript changes this composition; it is not the same operation as multiplying the whole <chemical formula> by a coefficient.
An original composition calculation uses approximate <molar masses> $40.078$ for calcium, $30.974$ for phosphorus, $15.999$ for oxygen and $1.008$ for hydrogen, in grams per mole. The full formula has molar mass about $1004.62\,\mathrm{g\,mol^{-1}}$, with ideal calcium mass fraction about $39.89\%$ and phosphorus mass fraction about $18.50\%$. Their mass-fraction ratio is about $2.157$, whereas their mole ratio is $1.667$. An elemental mass ratio cannot be compared directly with the stoichiometric mole ratio.
Identity also needs structural evidence. NIST characterized its reference powder by elemental analysis and <powder X-ray diffraction>, comparing diffraction positions with the hydroxyapatite pattern. Its certificate distinguishes certified composition values from supplementary diffraction information.<Hydroxyapatite Standard Reference Material 2910b Certificate of Analysis by NIST>[pp. 1–2, composition measurements; Appendix A, diffraction tables and apparatus]{ref} A measured Ca/P ratio near $5/3$ alone does not establish that every part of a specimen is a single hydroxyapatite phase: a mixture can reproduce an overall elemental ratio. Composition, phase identification and the properties of a biological <composite material> answer related but different questions.
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