Hydroxyapatite
| Hydroxyapatite | |
|---|---|
Hydroxylapatite crystals on matrix
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| General | |
| Category |
Phosphate mineral Apatite group |
|
Formula (repeating unit) |
Ca5(PO4)3(OH) |
| Strunz classification | 8.BN.05 |
| Crystal system | Hexagonal |
| Crystal class | Dipyramidal (6/m) H-M Symbol (6/m) |
| Space group | P63/m |
| Unit cell | a = 9.41 Å, c = 6.88 Å; Z = 2 |
| Identification | |
| Formula mass | 502.31 g/mol |
| Color | Colorless, white, gray, yellow, yellowish green |
| Crystal habit | As tabular crystals and as stalagmites, nodules, in crystalline to massive crusts |
| Cleavage | Poor on {0001} and {1010} |
| Fracture | Conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 5 |
| Luster | Vitreous to subresinous, earthy |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.14–3.21 (measured), 3.16 (calculated) |
| Optical properties | Uniaxial (-) |
| Refractive index | nω = 1.651 nε = 1.644 |
| Birefringence | δ = 0.007 |
| References | |
Hydroxylapatite, also called hydroxyapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. Hydroxylapatite is the hydroxyl endmember of the complex apatite group. The OH−ion can be replaced by fluoride, chloride or carbonate, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxylapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis.
Up to 50% by volume and 70% by weight of human bone is a modified form of hydroxylapatite (known as bone mineral). Carbonated calcium-deficient hydroxylapatite is the main mineral of which dental enamel and dentin are composed. Hydroxylapatite crystals are also found in the small calcifications (within the pineal gland and other structures) known as corpora arenacea or 'brain sand'.
Hydroxyapatite can be synthesized via several methods such as wet chemical deposition, biomimetic deposition, sol-gel route (wet-chemical precipitation) or electrodeposition. Yagai and Aoki proposed the hydroxyapatite nanocrystal suspension can be prepared by a wet chemical precipitation reaction following the reaction equation below:
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