Hydroxyapatite

Hydroxyapatite
Hydroxyapatite in hexagonal-prismatic formation from Cerro Huañaquino, Departamento Potosí , Bolivia (size: 1.3 × 0.5 × 0.4 cm)
General and classification
other names	formerly apatite (CaOH)
chemical formula	Ca 5 [OH | (PO 4 ) 3 ]
Mineral class (and possibly department)	Phosphates, Arsenates and Vanadates - Anhydrous phosphates with foreign anions
System no. to Strunz and to Dana	8.BN.05 ( 8th edition : VII / B.39) 08.41.01.03
Similar minerals	Chlorapatite , fluoroapatite
Crystallographic Data
Crystal system	hexagonal
Crystal class ; symbol	hexagonal dipyramidal; 6 / m
Space group	P 6 3 / m (No. 176)
Lattice parameters	a  = 9.42  Å ; c  = 6.87 Å
Formula units	Z  = 2
Frequent crystal faces	{10 1 0}, {0001}, {10 1 1}
Physical Properties
Mohs hardness	5
Density (g / cm 3 )	measured: 3.14 to 3.21; calculated: 3.16
Cleavage	indistinct after {0001} and {10 1 0}
Break ; Tenacity	shell-like, brittle
colour	white, gray, yellow, green, brown, black
Line color	White
transparency	transparent to opaque
shine	Glass shine, fat shine, earthy
radioactivity	contains traces of uranium and rare earths
Crystal optics
Refractive indices	n ω  = 1.651 n ε  = 1.644
Birefringence	δ = 0.007
Optical character	uniaxial negative
Pleochroism	green apatite slightly yellow, blue apatite very strong blue and colorless
Other properties
Chemical behavior	soluble in ENT 3
Special features	after heating phosphorescence

Hydroxyapatite (also hydroxyapatite , formerly apatite (CaOH) ) is a mineral from the mineral class of " phosphates , arsenates and vanadates ", which can be abundant in various locations, but is generally not very common.

Hydroxyapatite crystallizes in the hexagonal crystal system with the chemical composition Ca ₅ [OH | (PO ₄ ) ₃ ] and usually develops short to long prismatic crystals of up to 30 cm in length. But it is also found in the form of low-grape to massive mineral aggregates , stalactitic forms and crusty coatings. In addition, hydroxyapatite forms the basis of the hard substance (bones, teeth) of all vertebrates .

In its pure form, hydroxyapatite is colorless and transparent. However, due to multiple light refraction due to lattice construction defects or polycrystalline formation, it can also appear white and, due to foreign admixtures, take on a gray, yellow, green, brown or black color, the transparency decreasing accordingly. However, its line color is always white.

Hydroxyapatite is a member of the apatite group and forms a seamless mixed series with chlorapatite (formerly apatite (CaCl) ) and fluorapatite (formerly apatite (CaF) ) .

Etymology and history

The name hydroxylapatite indicates, on the one hand, its close relationship and chemical similarity with the other members of the apatite group , and on the other hand, the hydroxide ion , which is characteristic in its chemical composition , but which eluded detection for a long time after the apatite was first formulated (unnoticed water formation during Ashing and when dissolving the samples in acid).

In 1873 Robert Warington described the formation of a "hydrated oxygen apatite" as a product of the hydrolysis of calcium phosphate. A naturally occurring mineral corresponding to the formula of this hydrated calcium oxide apatite was later named "hydroapatite" by Damour and named "hydroxyapatite" in 1912 by Waldemar Theodore Schaller .

classification

In the 8th edition of the mineral systematics according to Strunz , which is now outdated, but still in use , hydroxyapatite belonged to the mineral class of "phosphates, arsenates, vanadates" and there to the department of "anhydrous phosphates with foreign anions ", where together with Belovit (Ce) , Belovit- (La) , carbonate-fluoroapatite ( carbonate-apatite- (CaF) ), carbonate-hydroxylapatite ( carbonate-apatite- (CaOH) ), chlorapatite ( apatite- (CaCl) ), Fermorite , fluorapatite ( apatite- (CaF ) ), Fluorcaphit , Hedyphan , Johnbaumit , Klinomimetesit , Kuannersuit- (Ce) , Mimetesit , Morelandit , Pyromorphit , strontium apatite , Svabit , Turneaureit and Vanadinit the stand-alone "apatite Pyromorphit group" with the system number. VII / B.39 .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also assigns hydroxyapatite to the class of “phosphates, arsenates and vanadates” and there to the department of “phosphates etc. with additional anions ; without H ₂ O “. However, this section is further subdivided according to the size of the cations involved and their molar ratio to the phosphate, arsenate or vanadate complex RO ₄ , so that the mineral according to its composition is classified in the subsection “With only large cations; (OH etc.): RO ₄  = 0.33: 1 “can be found where it is found together with Alforsit , Belovit- (Ce), Belovit- (La), carbonate-fluorapatite, carbonate-hydroxylapatite, chlorapatite, fluorapatite, fluorostrophite , Hydroxylapatit-M, Deloneit- (Ce), Fermorit, Fluorcaphit, Hedyphan, Hydroxyl-Pyromorphit, Johnbaumit, Klinomimetesit, Kuannersuit- (Ce), Mimetesit, Morelandit, Phosphohedyphan , Pyromorphit, Svabit, Turneaureit, Vanadinit the pyrom "Apatit" Group "with the system no. 8.BN.05 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns hydroxyapatite to the class of "phosphates, arsenates, vanadates" and there to the category of "anhydrous phosphates, etc., with hydroxyl or halogen". Here it is together with chlorapatite, carbonate-fluorapatite, carbonate-hydroxydapatite, Belovit- (Ce), Belovit- (La), Kuannersuit- (Ce), fluorapatite, fluorostrophite, fluorocaphite, deloneit- (Ce), stronadelphite, fluorophosphohedyphane and phosphohedyphane in the "apatite group" with the system no. 41.08.01 to be found within the subsection of " Anhydrous phosphates etc., with hydroxyl or halogen with (A) ₅ (XO ₄ ) ₃ Z _q ".

Crystal structure

Hydroxyapatite crystallizes hexagonally in the space group P 6 ₃ / m (space group no. 176) with the lattice parameters a  = 9.42  Å and c  = 6.87 Å as well as 2 formula units per unit cell .

Education and Locations

Several small, green hydroxyapatite crystals on orthoclase from the Sapo Mine, Minas Gerais , Brazil (size: 11.4 × 9.0 × 2.4 cm.)

White, massive hydroxyapatite and salmon-colored triplite from the Morefield Mine near Winterham , Amelia County , Virginia, USA

Hydroxyapatite forms either metamorphically in serpentinite and talc shale or hydrothermally in pegmatite . In addition, it is built up in different rock layers through biogenic sedimentation . Accompanying minerals include brushite , calcite , montebrasite , muscovite , crandallite , serpentine slate and talc .

In total, hydroxyapatite has so far (as of 2011) been found at around 250 sites. Noteworthy due to extraordinary hydroxyl finds are, among others, Snarum in the Norwegian province of Viken , Hospental in the Swiss canton of Uri and Eagle in the US state of Colorado , where crystals with a diameter of up to 3 cm were discovered.

In Germany, the mineral was found in the Bavarian Fichtelgebirge and Spessart , near Neuhof , in the Odenwald , near Waldgirmes and Wiesbaden-Naurod in Hesse, Bad Harzburg in Lower Saxony, Neheim-Hüsten in North Rhine-Westphalia, near Rheinbreitbach in Rhineland-Palatinate several places in the Saxon Ore Mountains , near Barmstedt in Schleswig-Holstein and near Ilfeld in Thuringia.

In Austria, hydroxyapatite occurred near Badersdorf in Burgenland, on the Brandback in the Carinthian Koralpe , in several places Krieglachs in Styria and in the Bregenzerwald Mountains in Vorarlberg. In Switzerland , the mineral was found near Sils in Engadin / Segl in the canton of Graubünden, in Centovalli , on Lake Maggiore and near Sambuco in Ticino.

Other sites are Argentina , Ethiopia , Australia , the Bahamas , Bolivia , Brazil , the small Antilles island of Anguilla , China , France , Greenland , Iran , Italy , Japan , Yemen , the Democratic Republic of the Congo , Canada , Cuba , Malta , Mexico , the Mongolia , Namibia , the Netherlands , Norway , Papua New Guinea , Poland , Puerto Rico , Romania , Russia , Saudi Arabia , Sweden , the Seychelles , Slovakia , Spain , South Africa , Thailand , the Czech Republic , Turkey , Uganda , Hungary , the Ukraine , Venezuela , the United Kingdom (Great Britain) and the United States of America (USA).

Occurrence in living things

Hydroxyapatite forms the basis of the hard substance of all vertebrates and is created in the body through biomineralization . It is contained in bones to a proportion of about 40%, in the calcification zone of articular cartilage , in the dentine to 70%, and in tooth enamel (enamelum) to 95%. According to this, the tooth enamel with a Mohs hardness of 5 is the hardest material in our body.

Tooth enamel is made up of adamantoblasts ( ameloblasts , enamel- forming cells). These cells first secrete a connective tissue substance ( prenamelum ). After the eruption of the teeth , the main part of the mineralization takes place: the enamel attains its final hardness through the storage of Ca ²⁺ and phosphates in the form of apatite.

Tooth enamel not only protects mechanically, but also chemically. However, if it is brought into solution at a pH <5.5, it demineralizes. This happens in the mouth mostly by bacterial acids and fruit acids :

${\displaystyle {\ce {Ca5(PO4)3OH + H3O+ -> 5 Ca^2+ + 3 PO4^3- + 2 H2O}}}$

(Under the influence of acids - represented here as oxonium ion H ₃ O ⁺ - ionic calcium, phosphate and water are formed from hydroxyapatite )

This can be prevented by substituting the hydroxide ion with a fluoride ion, for example by adding fluoride to toothpaste, table salt or drinking water (see fluoridation ).

${\displaystyle {\ce {Ca5(PO4)3OH + F^- -> Ca5(PO4)3F + OH-}}}$

Fluorapatite has a much lower solubility product at the same pH value, i.e. that is, far fewer fluoroapatite molecules dissociate in a solution than do hydroxyapatite molecules. That is the reason why fluoroapatite is more permanent than the body's own hydroxyapatite.

The naturally occurring calcium phosphate does not correspond to the chemically pure and 100% crystalline hydroxyapatite, but has substitutions in the crystal lattice. In the first place, when there is contact with carbonate ions , for example from blood and interstitial fluid , PO ₄ ^{3− is} substituted by CO ₃ ²⁻ . Further important substituents in vivo are mainly magnesium , but also sodium and zinc ions , as well as biological species such as citrate and proteins.

Bones, dentin and tooth enamel are not made entirely of mineral apatite. Rather, flat particles of carbonate-substituted hydroxyapatite are embedded in a matrix of proteins, primarily collagen , which gives the bone material the properties of a composite material .

Synthetic manufacture

Hydroxyapatite can be made in the laboratory; it forms as a very slow precipitate in the form of hexagonal, needle-shaped deposits of extremely dilute solutions, which are obtained with calcium nitrate , potassium dihydrogen phosphate and sodium hydroxide solution .

${\displaystyle \mathrm {5\ Ca(NO_{3})_{2}+3\ KH_{2}PO_{4}+7\ NaOH\longrightarrow } }$

${\displaystyle \mathrm {\ Ca_{5}(PO_{4})_{3}(OH)+3\ KNO_{3}+7\ NaNO_{3}+6\ H_{2}O} }$

Another possibility for production is the use of calcium hydroxide solution and phosphoric acid . The latter is titrated into the solution until a precipitate forms. The excess liquid is removed at approx. 1270 ° C ( calcination ). The resulting solid can then be shaped.

The solubility and pH stability of various calcium phosphates play a role in the preparation. In order to obtain hydroxyapatite from aqueous solution, a molar calcium-to-phosphate ratio of 1.67 must be present, and ideally a pH of 9.5 to 12.0 must be maintained. If very low concentrations are used, the nanoscale crystallization nuclei can be prevented from further agglomeration into larger particles by adding ionic species or polymers such as SDS , CTAB , PEI , PVP and others .

Another manufacturing method is tissue engineering , in which osteoblasts are placed on a scaffold and exposed to sound. This is modeled on the appearance of the human foot, for example, so that the osteoblasts grow as if they were growing directly in the body.

use

In the chemical industry , hydroxyapatite is an important ore for the extraction of phosphorus and thus for the production of fertilizers and phosphoric acid .

In medicine it is used as biomaterial for bone replacement ( bone graft ) and partly in combination with β-tricalcium phosphate, or bioactive coating of titanium implants used to improve the bone replacement.

For the coating of implants with hydroxyapatite, there is the approach of incubating surfaces made of bioactive glass ceramic for several days in simulated body fluid . The concentration of calcium and phosphate ions in the solution exceeds the solubility product and calcium phosphate gradually precipitates. If the right conditions are met with regard to the pH value and composition of the simulated body fluid, a modification similar to or identical to that of hydroxyapatite is obtained. This procedure makes it possible to store other substances in the calcium phosphate formed as a coprecipitate. Inorganic constituents such as silicon, which can promote osteoconductivity by changing the surface properties, are suitable for this. Research is also being carried out into the storage of proteins, primarily growth factors such as BMPs , in order to achieve osteoinductivity .

In preparative protein biochemistry , hydroxyapatite is used as a stationary phase in the chromatographic separation of proteins , especially membrane proteins .

In genetics , the mineral is used in DNA-DNA hybridization (an older method of determining the degree of relationship in organisms). However, his property is used, that it is to DNA - duplexes attached, but not to single strands. In this way, double helices can be separated from single strands.

Hydroxyapatite is also contained in special toothpastes as an additive to prevent tooth decay and to counteract tooth sensitivity in so-called sensitive toothpastes.

See also

• List of minerals

literature

• Petr Korbel, Milan Novák: Mineral Encyclopedia (=  Villager Nature ). Nebel Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 171 .

Web links

Commons : Hydroxylapatite  - Collection of pictures, videos and audio files

• Mineral Atlas: Apatite- (CaOH) (Wiki)
• David Barthelmy: Hydroxyapatite Mineral Data. In: webmineral.com. Retrieved February 22, 2020 (English). 

Individual evidence

1. ↑ ^{a b c d e} Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  466-467 .
2. ↑ ^{a b c} Hydroxyapatite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 66  kB ; accessed on April 16, 2018]).
3. ↑ ^{a b c d} Mindat - Hydroxylapatite
4. ↑ S. Gabriel: Chemical studies on the minerals of the bones and teeth . In: Hoppe Seyler's journal for physiological chemistry . tape 18 , no. 3–4 , 1894, pp. 257–303 ( available online at archive.org [accessed July 25, 2017]).
5. ^ Robert Warington: On the decomposition of tricalcic phosphate by water . In: Journal of the Chemical Society . tape 26 , January 1, 1873, p. 983–989 ( limited preview in Google Book Search).
6. ↑ A. Lacroix: Sur la constitution minéralogique des phosphorites francaises . In: Comptes Rendus hebdomadaires des séances de l'Académie des sciences . tape 150 , 1910, pp. 1213 ( In: New Yearbook for Mineralogy, Geology and Paläontologie , 1911; available online at archive.org [accessed July 25, 2017]).
7. ^ Waldemar T. Schaller: Mineralogical notes, Series 2 . In: Mineralogy of the French phosphorites, US Geological Survey Bulletin . tape 509 , 1912, pp. 89–100 (English, rruff.info [PDF; 431 kB ]).
8. ↑ Find location list for hydroxyapatite in the Mineralienatlas and Mindat
9. ^ W. Herzog, S. Federico: Articular Cartilage . In: BM Nigg, W. Herzog (Ed.): Biomechanics of the Musculo-skeletal System . 3. Edition. Wiley, Chichester 2007, pp. 95-109 .
10. ↑ Brigitte Wopenka, Jill D. Pasteris: A mineralogical perspective on the apatite in bone . In: Materials science & engineering. C, Materials for Biological Applications . tape 25 , no. 2 , 2005, p. 131-143 , doi : 10.1016 / j.msec.2005.01.008 .
11. ↑ Racquel Z. LeGeros, Besim Ben-Nissan: Introduction to Synthetic and Biologic apatite . In: Besim Ben-Nissan (Ed.): Advances in Calcium Phosphate Biomaterials (=  Springer Series in Biomaterials Science and Engineering ). tape 2 . Springer, Berlin, Heidelberg 2014, ISBN 978-3-642-53979-4 , pp. 1-17 , doi : 10.1007 / 978-3-642-53980-0_1 .
12. ↑ Pierre Layrolle, Guy Daculsi: Physicochemistry of Apatite and Its Related Calcium Phosphates . In: Betty León, John A. Jansen (Eds.): Thin Calcium Phosphate Coatings for Medical Implants . Springer, New York, NY 2009, ISBN 978-0-387-77718-4 , pp. 9-24 , doi : 10.1007 / 978-0-387-77718-4_2 .
13. ^ Rainer Deutzmann, Peter Bruckner: Cartilage and bone tissue . In: Peter C. Heinrich, Matthias Müller, Lutz Graeve (Eds.): Löffler / Petrides Biochemistry and Pathobiochemistry . Springer, Berlin, Heidelberg 2014, ISBN 978-3-642-17972-3 , pp. 952-960 , doi : 10.1007 / 978-3-642-17972-3_72 .
14. ↑ Huipin Yuan, Klaas de Groot: Calcium Phosphate Biomaterials: An Overview . In: RL Reis, S. Weiner (Ed.): Learning from Nature How to Design New Implantable Biomaterials: From Biomineralization Fundamentals to Biomimetic Materials and Processing Routes: Proceedings of the NATO Advanced Study Institute, held in Alvor, Algarve, Portugal, 13 -24 October 2003 (=  NATO Science Series II ). tape 171 . Springer Science + Business Media, Dordrecht 2005, ISBN 978-1-4020-2647-8 , p. 37-57 , doi : 10.1007 / 1-4020-2648-X_3 .
15. ^ Hydroxyapatites . In: Georg Brauer (Ed.): Handbook of Preparative Inorganic Chemistry . 2nd Edition. tape  1 . Academic Press, New York, London 1963, pp. 545-546 ( available online at archive.org [accessed July 25, 2017]).
16. ↑ Patent DE4232443C1 from September 23, 1993: Process for the production of hydroxyapatite from aqueous phosphoric acid and calcium hydroxide solution
17. ↑ Thea Welzel, Wolfgang Meyer-Zaika, Matthias Epple: Continuous preparation of functionalized calcium phosphate nanoparticles with adjustable crystallinity . In: Chemical Communications . No. 10 , 2004, p. 1204-1205 , doi : 10.1039 / B402521K .
18. ↑ T. Kokubo, H. Kushitani, S. Sakka, T. Kitsugi, T. Yamamuro: Solutions able to reproduce in vivo surface-structure changes in bioactive glass-ceramic A-W3 . In: Journal of Biomedical Materials Research . tape 24 , no. 6 , 1990, pp. 721-734 , doi : 10.1002 / jbm.820240607 .
19. ↑ Jianguo Li, Hailhong Liao, Malena Sjöström: Characterization of calcium phosphates precipitated from simulated body fluid of different buffering capacities . In: Biomaterials . tape 18 , no. 10 , 1997, pp. 743-747 , doi : 10.1016 / S0142-9612 (96) 00206-2 .
20. ↑ ^{a b} Akiyoshi Osaka: Self-Assembly and Nano-layering of Apatitic Calcium Phosphates in Biomaterials . In: Besim Ben-Nissan (Ed.): Advances in Calcium Phosphate Biomaterials (=  Springer Series in Biomaterials Science and Engineering ). tape 2 . Springer, Berlin / Heidelberg 2014, ISBN 978-3-642-53979-4 , pp. 97-169 , doi : 10.1007 / 978-3-642-53980-0_5 .
21. ↑ Population genetics and evolution ( Memento from June 13, 2007 in the Internet Archive ) (PDF 40.5 kB)