Apatite
| Apatite group | |
|---|---|
 |
|
| General | |
| Category | Phosphate mineral |
|
Formula (repeating unit) |
Ca5(PO4)3(F,Cl,OH) |
| Strunz classification | 8.BN.05 |
| Crystal system | Hexagonal |
| Crystal class | Dipyramidal (6/m) (same H-M symbol) |
| Identification | |
| Color | Transparent to translucent, usually green, less often colorless, yellow, blue to violet, pink, brown. |
| Crystal habit | Tabular, prismatic crystals, massive, compact or granular |
| Cleavage | [0001] indistinct, [1010] indistinct |
| Fracture | Conchoidal to uneven |
| Mohs scale hardness | 5 (defining mineral) |
| Luster | Vitreous to subresinous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 3.16–3.22 |
| Polish luster | Vitreous |
| Optical properties | Double refractive, uniaxial negative |
| Refractive index | 1.634–1.638 (+0.012, −0.006) |
| Birefringence | 0.002–0.008 |
| Pleochroism | Blue stones – strong, blue and yellow to colorless. Other colors are weak to very weak. |
| Dispersion | 0.013 |
| Ultraviolet fluorescence | Yellow stones – purplish-pink, which is stronger in long wave; blue stones – blue to light-blue in both long and short wave; green stones – greenish-yellow, which is stronger in long wave; violet stones – greenish-yellow in long wave, light-purple in short wave. |
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl−ions, respectively, in the crystal. The formula of the admixture of the four most common endmembers is written as Ca10(PO4)6(OH,F,Cl)2, and the crystal unit cell formulae of the individual minerals are written as Ca10(PO4)6(OH)2, Ca10(PO4)6(F)2 and Ca10(PO4)6(Cl)2.
The mineral was named apatite by the German geologist Abraham Gottlob Werner in 1786, although the specific mineral he had described was reclassified as fluorapatite in 1860 by the German mineralogist Karl Friedrich August Rammelsberg. Apatite is a mineral that is often mistaken for other minerals. This tendency is reflected in the mineral's name, which is derived from the Greek word απατείν (apatein), which means to deceive or to be misleading.
Apatite is one of a few minerals produced and used by biological micro-environmental systems. Apatite is the defining mineral for 5 on the Mohs scale. Hydroxyapatite, also known as hydroxylapatite, is the major component of tooth enamel and bone mineral. A relatively rare form of apatite in which most of the OH groups are absent and containing many carbonate and acid phosphate substitutions is a large component of bone material.
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