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Mineraly.sk - hydroxylapatit.jpg
Hydroxylapatite crystals on matrix
Category Phosphate mineral
Apatite group
(repeating unit)
Strunz classification 08.BN.05
Crystal symmetry Hexagonal 6/m – dipyramidal
Unit cell a = 9.41 Å, c = 6.88 Å; Z = 2
Formula mass 502.31 g
Color Colorless, white, gray, yellow, yellowish green
Crystal habit As tabular crystals and as stalagmites, nodules, in crystalline to massive crusts
Crystal system Hexagonal
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 [1][2][3]

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 7% by weight is a modified form of hydroxylapatite (known as bone mineral).[4] 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'.

Chemical synthesis of hydroxyapatite[edit]

Hydroxyapatite can be synthesized via several methods such as wet chemical deposition, biomimetic deposition, sol-gel route (wet-chemical precipitation) or electrodeposition.[5] Yagai and Aoki proposed the hydroxyapatite nanocrystal suspension can be prepared by a wet chemical precipitation reaction following the reaction equation below:[6]

10Ca(OH)2 + 6H3PO4 → Ca10(PO4)6(OH)2 + 18H2O

Several studies have shown that hydroxyapatite synthesis via wet-chemical route can be improved by power ultrasound. The ultrasonically assisted synthesis (sono-synthesis) of hydroxyapatite is a successful technique to produce nanostructured hydroxyapatite at high quality standards. The ultrasonic route allows to produce nano-crystalline hydroxyapatite as well as modified particles, e.g. core-shell nanospheres, and composites.[7]

Calcium deficient hydroxyapatite[edit]

Calcium deficient (non-stochiometric) hydroxyapatite, Ca10-x(PO4)6-x(HPO4)x(OH)2-x (where x is between 0 and 1) has a Ca/P ratio between 1.67 and 1.5. The Ca/P ratio is often used in the discussion of calcium phosphate phases.[8] Stoichiometric apatite Ca10(PO4)6(OH)2 has a Ca/P ratio of 10:6 normally expressed as 1.67. The non-stoichiometric phases have the hydroxyapatite structure with cation vacancies (Ca2+) and anion (OH) vacancies. The sites occupied solely by phosphate anions in stochiometric hydroxyapatite, are occupied by phosphate or hydrogen phosphate, HPO42–, anions.[8] Preparation of these calcium deficient phases can be prepared by precipitation from a mixture of calcium nitrate and diammonium phosphate with the desired Ca/P ratio, for example to make a sample with a Ca/P ratio of 1.6:[9]

9.6Ca(NO3)2 + 6(NH4)2HPO4 → Ca9.6 (PO4)5.6 (HPO4)0.4 (OH)1.6

Sintering these non-stoichiometric phases forms a solid phase which is an intimate mixture of tricalcium phosphate and hydroxyapatite, termed biphasic calcium phosphate:[10]

Ca10-x(PO4)6-x(HPO4)x(OH)2-x → (1-x)Ca10(PO4)6(OH)2 + 3xCa3(PO4)2

Medical uses[edit]

Flexible hydrogel-HA composite, which has a mineral-to-organic matrix ratio approximating that of human bone.

Hydroxylapatite can be found in teeth and bones within the human body. Thus, it is commonly used as a filler to replace amputated bone or as a coating to promote bone ingrowth into prosthetic implants. Although many other phases exist with similar or even identical chemical makeup, the body responds to them very differently. Coral skeletons can be transformed into hydroxylapatite by high temperatures; their porous structure allows relatively rapid ingrowth at the expense of initial mechanical strength. The high temperature also burns away any organic molecules such as proteins, preventing an immune response and rejection.

Many modern implants, e.g. hip replacements, dental implants and bone conduction implants, are coated with hydroxylapatite. It has been suggested that this may promote[11] osseointegration.[citation needed]. Porous hydroxylapatite implants are used for local drug delivery in bone.[12][13] It is also being used to repair early lesions in tooth enamel.[14]


Hydroxylapatite deposits in tendons around joints resulting in the medical condition calcific tendinitis.


Microcrystalline hydroxylapatite (MH) is marketed as a "bone-building" supplement with superior absorption in comparison to calcium.[15] It is a second-generation calcium supplement derived from bovine bone.[15] In the 1980s, bone meal calcium supplements were found to be contaminated with heavy metals,[15] and although the manufacturers claim their MH is free from contaminants, people are advised to avoid it because it has not been well-tested.[15] However, the limited tests seem to show positive results. A 1995 randomized placebo-controlled study of 40 people in Europe found that it was more effective than calcium carbonate in slowing bone loss.[15] A 2007 randomized double-blind controlled study of an MH supplement called the Bone Builder found significant positive effects in bone mineral density (BMD) compared to control.[16] Hydroxylapatite has been used by Noel Fitzpatrick to facilitate bionic development in animals, by coating steel rods in hydroxylapatite to encourage natural growth of skin around it. As a component of nanocomposites, hydroxylapatite is finding uses as a potential new bone replacement materials.


The mechanism of hydroxylapatite (HA) chromatography is complicated and has been described as "mixed-mode" ion exchange. It involves nonspecific interactions between positively charged calcium ions and negatively charged phosphate ions on the stationary phase HA resin with protein negatively charged carboxyl groups and positively charged amino groups. It may be difficult to predict the effectiveness of HA chromatography based on physical and chemical properties of the desired protein to be purified. For elution, a buffer with increasing phosphate concentration is typically used for application.

Use in archaeology[edit]

In archaeology, hydroxylapatite from human and animal remains is analysed in order to reconstruct ancient diets. The mineral fractions of bone and teeth act as a reservoir of trace elements, including strontium. It has been established that the ratio of strontium to calcium in bone hydroxylapatite broadly reflects an animal's diet during the period before its death when the bone was being formed (5–10 years in the case of human remains), or in the case of dental mineral in childhood. In either case analysis of Sr/Ca ratios allows an individual's diet to be classified as carnivorous, herbivorous or omnivorous and either predominantly marine or terrestrially based. However the difficulty of compensating for post-mortem contamination of archaeological samples through interaction with groundwater continues to cast doubt on the reliability of the method. Stable isotope analysis is considered a more viable alternative, although strontium and other trace mineral analyses of dental samples are commonly used in situations where this is impossible because the collagen content of the bone has completely decayed (i.e. for palaeolithic samples).[17][18]

Animal structures and potential uses in materials science[edit]

The clubbing appendages of the Odontodactylus scyllarus (peacock mantis shrimp) are made of an extremely dense form of the mineral which has a higher specific strength and toughness than any synthetic composite material; these properties have led to its investigation for potential synthesis and engineering use.[19] Their dactyl appendages have excellent impact resistance due to the impact region being composed of mainly crystalline hydroxyapatite, which offers significant hardness. Crack propagation of the impact region is reduced by thin layers of chitosan in between the highly organized crystal hydroxyapatite structures. Once a hydroxyapatite prism fractures, the chitosan inter-layers aid in preventing further cracks. This form of layered crack retardation is also seen in teeth enamel, where the hydroxyapatite prisms that make up an enamel rod are padded by thin layers of protein which fulfill the same function.[20] A periodic layer underneath the impact layer composed of hydroxyapatite with lower calcium and phosphorus content (thus resulting in a much lower modulus) inhibits crack growth by forcing new cracks to change directions. This periodic layer also reduces the energy transferred across both layers due to the large difference in modulus, even reflecting some of the incident energy.[21] Armor models based on nanoscale composites of hydroxyapatite designed in a similar fashion would help optimize impact resistance, reducing the weight needed to produce armor.[22]

The addition of hydroxyapatite particles to a magnesium alloy in the form of a metal matrix composite stabilized the corrosion rate of the alloy, resulting in a cyto-compatible, biodegradable material with adjustable corrosion rates and mechanical properties.[23]

Experimental nanostructure composite air filters containing hydroxyapatite were found to be efficient in absorbing and decomposing CO which could eventually lead to utilization in reducing automotive exhaust pollutants.[24]

In 2014 an alginate/nano-hydroxyapatite composite was synthesized and field-tested as an adsorbent for fluoride. The biocomposite removes fluoride through an ion-exchange mechanism, and is both biocompatible and biodegradable.[25]

Research into hydroxyapatite's proton conductivity resulted in the diffusion path and conduction mechanism of the mineral, allowing for potential use as developmental material for energy conversion devices.[26]

Recent developments have allowed the synthesis of ceramic microspheres made from hydroxyapatite with a diameter of 1.5 micrometers; the microspheres can be utilized in a variety of fields including filters, grinding media, and light-weight concrete.[27][28]

See also[edit]


  1. ^ Hydroxylapatite. Mindat
  2. ^ Hydroxylapatite. Webmineral
  3. ^ Hydroxylapatite. Handbook of Mineralogy
  4. ^ Junqueira, Luiz Carlos; José Carneiro (2003). Foltin, Janet; Lebowitz, Harriet; Boyle, Peter J., eds. Basic Histology, Text & Atlas (10th ed.). McGraw-Hill Companies. p. 144. ISBN 0-07-137829-4. Inorganic matter represents about 50% of the dry weight of bone ... crystals show imperfections and are not identical to the hydroxylapatite found in the rock minerals 
  5. ^ M. P. Ferraz , F. J. Monteiro , C. M. Manuel (2004): Hydroxyapatite nanoparticles: A review of preparation methodologies. Journal of Applied Biomaterials & Biomechanics 2004; 2: 74-80.
  6. ^ Bouyer E, Gitzhofer F, Boulos MI (2000): Morphological study of hydroxyapatite nanocrystal suspension. J Mater Sci Mater Med 2000; 11: 523-31.
  7. ^ Sono-Synthesis of Nano-Hydroxyapatite
  8. ^ a b Rey, C.; Combes, C.; Drouet, C.; Grossin, D. (2011). "1.111 - Bioactive Ceramics: Physical Chemistry". In Ducheyne, Paul. Comprehensive Biomaterials 1. Elsevier. pp. 187–281. doi:10.1016/B978-0-08-055294-1.00178-1. ISBN 978-0-08-055294-1.   – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  9. ^ Raynaud, S., Champion, E., Bernache-Assollant, D., Thomas, P. (February 2002). "Calcium phosphate apatites with variable Ca/P atomic ratio I. Synthesis, characterisation and thermal stability of powders". Biomaterials 23 (4): 1065–1072. doi:10.1016/S0142-9612(01)00218-6. ISSN 0142-9612.   – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  10. ^ Vallet-Regí, M.;Rodríguez-Lorenzo, L.M. (November 1997). "Synthesis and characterisation of calcium deficient apatite". Solid State Ionics. 101–103, Part 2: 1279–1285. doi:10.1016/S0167-2738(97)00213-0.   – via ScienceDirect (Subscription may be required or content may be available in libraries.)
  11. ^ Jeong, KI. "Experimental Study of Osseointegration and Stability of Intentionally Exposed Hydroxyapatite Coating Implants". Journal. Journal of the Korean Maxillofacial Reconstructive Surgery. Retrieved 15 July 2012. 
  12. ^ Kundu, B; Soundrapandian C; Nandi SK; Mukherjee P; Dandapat N; Roy S; Datta BK; Mandal TK; Basu D; Bhattacharya RN (2010). "Development of new localized drug delivery system based on ceftriaxone-sulbactam composite drug impregnated porous hydroxylapatite: a systematic approach for in vitro and in vivo animal trial". Pharm. Res. 27 (8): 1659–76. doi:10.1007/s11095-010-0166-y. PMID 20464462. 
  13. ^ Kundu, B; Lemos A; Soundrapandian C; Sen PS; Datta S; Ferreira JMF; Basu D (2010). "Development of porous HAp and β-TCP scaffolds by starch consolidation with foaming method and drug-chitosan bilayered scaffold based drug delivery system". J Mater. Sci. Mater. Med. 21 (11): 2955–69. doi:10.1007/s10856-010-4127-0. PMID 20644982. 
  14. ^ http://www.nature.com/bdj/journal/v215/n4/full/sj.bdj.2013.741.html
  15. ^ a b c d e Straub, D.A. (2007). "Calcium Supplementation in Clinical Practice: A Review of Forms, Doses, and Indications". NCP- Nutrition in Clinical Practice 22 (3): 286–96. doi:10.1177/0115426507022003286. PMID 17507729. 
  16. ^ Tucker, L.A.; Nokes, N.; Adams, T. (2007). "Effect of a Dietary Supplement on Hip and Spine BMD: A Randomized, Double-blind, Placebo-controlled Trial: 1515: Board# 5 May 30 2: 00 PM-3: 30 PM". Medicine & Science in Sports & Exercise 39 (5): S230. doi:10.1249/01.mss.0000273874.34214.2e. Retrieved 2008-06-09. 
  17. ^ Pollard, M.; Heron, C. (2008). "Chapter 10: The Chemistry of Human Bone: Diet, Nutrition, Status and Mobility". Archaeological Chemistry (2nd ed.). Cambridge: Royal Society of Chemistry. pp. 346–382. doi:10.1039/9781847558299. ISBN 978-0-85404-262-3. 
  18. ^ Sadat-Shojai, Mehdi (2010). Hydroxyapatite: Inorganic Nanoparticles of Bone (Properties, Applications, and Preparation Methodologies). Tehran, Iran: Iranian Students Book Agency (ISBA). ISBN 978-600-102-092-6. 
  19. ^ Weaver, J. C.; Milliron, G. W.; Miserez, A.; Evans-Lutterodt, K.; Herrera, S.; Gallana, I.; Mershon, W. J.; Swanson, B.; Zavattieri, P.; DiMasi, E.; Kisailus, D. (7 June 2012). "The Stomatopod Dactyl Club: A Formidable Damage-Tolerant Biological Hammer". Science 336 (6086): 1275–1280. doi:10.1126/science.1218764. 
  20. ^ Simmons, Lisa M.; Al-Jawad, Maisoon; Kilcoyne, Susan H.; Wood, David J. (December 2011). "Distribution of enamel crystallite orientation through an entire tooth crown studied using synchrotron X-ray diffraction". European Journal of Oral Sciences 119: 19–24. doi:10.1111/j.1600-0722.2011.00909.x. 
  21. ^ Tanner, K. E. (7 June 2012). "Small But Extremely Tough". Science 336 (6086): 1237–1238. doi:10.1126/science.1222642. 
  22. ^ Barthelat, Francois; Rabiei, Reza (April 2011). "Toughness amplification in natural composites". Journal of the Mechanics and Physics of Solids 59 (4): 829–840. doi:10.1016/j.jmps.2011.01.001. 
  23. ^ Witte, Frank; Feyerabend, Frank; Maier, Petra; Fischer, Jens; Störmer, Michael; Blawert, Carsten; Dietzel, Wolfgang; Hort, Norbert (May 2007). "Biodegradable magnesium–hydroxyapatite metal matrix composites". Biomaterials 28 (13): 2163–2174. doi:10.1016/j.biomaterials.2006.12.027. 
  24. ^ Nasr-Esfahani, Mojtaba; Fekri, Sahar (29 May 2012). "Alumina/TiO2/hydroxyapatite interface nanostructure composite filters as efficient photocatalysts for the purification of air". Reaction Kinetics, Mechanisms and Catalysis 107 (1): 89–103. doi:10.1007/s11144-012-0457-x. 
  25. ^ Pandi, Kalimuthu; Viswanathan, Natrayasamy (November 2014). "Synthesis of alginate bioencapsulated nano-hydroxyapatite composite for selective fluoride sorption". Carbohydrate Polymers 112: 662–667. doi:10.1016/j.carbpol.2014.06.029. 
  26. ^ Yashima, Masatomo; Kubo, Naoyuki; Omoto, Kazuki; Fujimori, Hirotaka; Fujii, Kotaro; Ohoyama, Kenji (13 March 2014). "Diffusion Path and Conduction Mechanism of Protons in Hydroxyapatite". The Journal of Physical Chemistry C 118 (10): 5180–5187. doi:10.1021/jp412771f. 
  27. ^ Li, Shan; Wu, Hua Hong; Xu, Guo Jie; Xiao, Xiu Feng (August 2014). "Facile Biomimetic Fabrication of Hollow Hydroxyapatite with Hierarchically Porous Microstructure Using Hyperbranched Gemini Surfactant as Template". Advanced Materials Research 1015: 355–358. doi:10.4028/www.scientific.net/AMR.1015.355. 
  28. ^ Korolev, Evgeniy Valerjevich; Inozemtcev, Alexandr Sergeevich (August 2013). "Preparation and Research of the High-Strength Lightweight Concrete Based on Hollow Microspheres". Advanced Materials Research 746: 285–288. doi:10.4028/www.scientific.net/AMR.746.285.