[deepika16]

I could not understand the structure of hydroxyapatite and arrangement of calcium, phosphate and OH. Molecular formula  shows 10 calcium ions but not seen in its structure. some one plzz explain.

[pgk]

Hydroxyapatite is a mixed salt with the formula:
Ca(OH)2 . 3 Ca3(PO4)2
That derives from 1 molecule of Ca(OH)2 and 3 molecules of the salt Ca3(PO4)2  that co-crystallize in the same lattice.
Stoichiometrically, hydroxyapatite can be represented as: Ca10((PO)4)6(OH)2 or Ca5(PO4)3(OH)
Not to be confused with a double salt that are two different salts that share the same cation (or anion) and co-crystallize in the same lattice, e.g. alum: K2SO4 . Al2(SO4)3 . 24 H2O
Also, not to be confused with a complex salt, e.g. potassium ferrocyanide:  K4[Fe(CN)6] that is ionized to 4K+ and the complex anion, [Fe(CN)6]4- .

[deepika16]

thank you..plz explain this image comparing to molecular formula...

[pgk]

This a 3D representation of the crystal lattice, where six Ca atoms of Ca3(PO4)2 form an hexagone that surrounds 3 Ca atoms of Ca3(PO4)2 that form an internal triangle and they are chelated by the OH group of the Ca(OH)2. Then tenth Ca atom that belongs to Ca(OH)2 is not represented in that scheme, because it does not belong to the same planar level. The so formed hexagons are superposed one over the other and connected by the internal chelation. This schematic 3D representation explains why hydroxyapatite is a so hard material and why it is not water soluble (and ionizable) due to the co-crystallization of Ca(OH)2. 
Note that fluroapatite has a similar structure, meaning the co-crystallization of Ca3(PO4)2 with CaF2 instead of Ca(OH)2.

[deepika16]

Thank you very much sir.. 

[pgk]

As mentioned above, fluroapatite has a similar structure, meaning the co-crystallization of Ca3(PO4)2 with CaF2 instead of Ca(OH)2.
Question: Is fluroapatite a mixed or a double salt and why?