Hydroxyapatite
Hydroxyapatite (IMA name: hydroxylapatite) (Hap, HAp, or HA) is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), often written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. It is the hydroxyl endmember of the complex apatite group. The OH− ion can be replaced by fluoride or chloride, producing fluorapatite or chlorapatite. It crystallizes in the hexagonal crystal system. Pure hydroxyapatite powder is white. Naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis.
| Hydroxyapatite | |
|---|---|
Hydroxyapatite crystals on matrix | |
| General | |
| Category | Phosphate mineral Apatite group |
| Formula (repeating unit) | Ca5(PO4)3OH |
| IMA symbol | Hap |
| 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 | |
Up to 50% by volume and 70% by weight of human bone is a modified form of hydroxyapatite, known as bone mineral. Carbonated calcium-deficient hydroxyapatite is the main mineral of which dental enamel and dentin are composed. Hydroxyapatite crystals are also found in pathological calcifications such as those found in breast tumors, as well as calcifications within the pineal gland (and other structures of the brain) known as corpora arenacea or "brain sand".