NATIONAL BUREAU OF STANDARDS REPORT

10 455

PROGRESS REPORT

ON

APATITES

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

311.95-11-3110189 July 29, 1971 10 455

PROGRESS REPORT

ON

APATITES

W. E. Brown

Director, American Dental

Association Research Unit

National Bureau of Standards

This investigation was supported in part by research grant

D E—00572 to the American Dental Association from the

National Institute of Dental Research and is part of the

dental research program conducted by the National Bureau

of Standards, in cooperation with the American Dental

Association; the United States Medical Research and

Development Command; the Dental Sciences Division of the School of Aerospace Medicine, USAF; the National

Institute of Dental Research; and the Veterans Administration.

IMPORTANT NOTICE NATIONAL BUREAU OF STA

for use within the Government. B and review. For this reason, the whole or in part, is not authoriz Bureau of Standards, Washington

the Report has been speciticaily p

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

; accounting documents intended

ubjected to additional evaluation

listing of this Report, either in Office of the Director, National

the Government agency for which

pies for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

APATITES

W. E. Brown

Director, American Dental

Association Research Unit

National Bureau of Standards

Washington, D. C. 20234

APATITES

The term apatite refers to a structural type that can be represented by

the formula A4BS (M04) s X2. In their most common forms, A and B are

calcium ions in two kinds of sites, M04 is a P04 , and X is OH“, F~, or

Cl”, but a rich variety of combinations of other elements are known to have

the apatite structure. The unit-cell dimensions for the three principal apatites are given in Table 1. More complete lists are given elsewhere (1 through 5);

Wondratschek (3,4) has discussed the general characteristics of this group

in some detail. The apatites, in their calcium phosphate forms, are

important constituents of tooth (6), bone (7, 8), and a large number of minerals (9), and are of great commercial importance in fertilizers (11), fluorescent lamp materials (12), lasers, and chromatographic absorbents.

Fluorapatites . Most mineral apatites contain considerable fluorine,

presumably because the fluoride ions suppress solubility. Fluorapatite,

Ca10 (P04) 6 F3 , may be considered to be the prototype which provides the

basis for describing the structures of the other end -member apatites and

their solid solutions. The space group of fluorapatite is P63 /m; the unit

cell contains one formula weight, Ca10 (P0 4) 6 F2 ; and its structure ( 13 , 14)

is shown projected on the c face in Fig. 1. The structure of fluorapatite

can be visualized in three dimensions from the one given for hydroxyapatite

in Fig. 2a and 2b. The origin of the unit cell was chosen to display the

four groups of ions conveniently: (i) six calcium ions (Ca(II) in Fig. 2)

comprising a triangular group of three on the mirror at z = i and a centro

symmetrically related group on the mirror at z = f; (ii) two sets of three
2

P04 groups similarly arranged on the mirrors at z = J and f; (iii) two

pairs of calcium ions (Ca(I) in Fig. 2) at z = 0 and one such pair being on

each of the triads near the acute corners of the cells in Fig. 1 and 2, and

(iv) two fluoride ions at the centers of the Ca(n) triangles. The fluorine

positions are slightly above those of the hydroxyl oxygen atoms shown in

Fig. 2b.

Hydroxyapatite

Tooth and bone crystallites and some minerals, although

they contain many impurity substituents, approach hydroxyapatite, Ca10 (PO4 ) 6 -

(OH) g , in their compositions. It is immediately obvious that hydroxyapatite

cannot truly fulfill the requirements of space group P63 /m except in a

statistical sense because the symmetry of the OH" group is less than that of

the F” ion site in fluorapatite. A substantial step in understanding the

chemical and physical properties of hydroxyapatite was made when it was

discovered (13, 15, 16) that the OH" oxygens of hydroxyapatite lie about 0. 37 A

above or below the planes at z = \ and f which in fluorapatite are mirrors.

This introduces possible variability in structure -dependent properties

resulting from (i) differences in the ordering of OH groups along a given

column in which the orientation of the groups may reverse at vacancies or

at sites containing impurities such as F" or O

2 *, and (ii) polar interactions

between adjacent ordered columns, which could lower the symmetry to

monoclinic in micro -regions of the crystal.

These features could have significant effects on the thermodynamic

properties (including solubility) of apatites and the physiological properties

of tooth and bones. For example, a polar character imparted to bone crystallites by a predominant orientation of the OH" columns in one direction along c could have important effects on the organization and properties of bone.
3

Chlorapatite . Ca10 (PO4) 8 Cl2 , the third principal calcium -phosphate

apatite, occurs ii two forms. It is monoclinic, P2!b, below about 200° C,

where it transforms into an apparently hexagonal phase (17, 18). Because

of the nearly hexagonal symmetry of the monoclinic crystal, it shows

prominent twinning (18). When heated to about 1, 000° C chlorapatite tends

to lose CaCl2 , thereby lowering the transition temperature (18).

The existence of two forms and the variability in transition temperature

have been explained on the basis of the following structural considerations

(16, 17). The positions of the Ca2 + and P04

3 " ions in chlorapatite are

approximately the same as those in hydroxyapatite and fluorapatite. In

comparison to fluorapatite where the F” ions are on the mirror planes at z = i, f, the Cl" ions in chlorapatite are at z = 0. 44, or about 1. 27 A away

from the mirror. Thus, a given Cl" will be closer to the calcium ions on

one mirror than it is to those on the other. As a result, the greater

attractive forces will bring the Ca2+ ion in the nearer triangle closer together than they are in the triangle slightly farther away. This difference

in the Ca2+ triangles causes slight tilting of the P04

3 ~ groups, thus

destroying the mirror on which these ions lie in fluorapatite. These displacements are propagated from one Cl" column to the next, thereby converting the mirror plane in fluorapatite into a glide plane in chlorapatite.

When the temperature is sufficiently high, thermal motion or disorder in

the Cl" positions apparently removes the ordering in the Cl" positions, thus

producing the hexagonal form. Vacancies in the Cl" positions are thought

to have a similar effect, thereby lowering the transition temperature.
4

Infrared & Raman Spectra

Infrared spectra of apatites have been studied extensively (19 through 25).

The bands in the spectra of powder samples have been assigned (19) with

considerable reliability on the basis of (i) correlations between the spectral

frequencies of calcium, strontium, and barium hydroxyapatites, fluorapatite,

and hydroxyapatite -fluorapatite solid solution, (ii) isotopic shifts produced by

S H and ls O, (iii) comparisons with other phosphates, and (iv) various

theoretical considerations such as site symmetry. These assignments (19)

for hydroxyapatite are given in Table 2. The hydrogen stretching band,

3572 cm*" 1

, and librational bands, 630 cm"* 1

, are of special interest because

(i) they are missing, as one would expect, in fluorapatite, (ii) they break

up into several bands in fluorapatite -hydroxyapatite solid solutions in a manner

one would expect if weak hydrogen bonds are formed between the OH” and F”

ions on the hexads, and (iii) they are sensitive to the presence of impurities

in the crystal, to the extent that they are missing in the spectra of bone,

enamel, many mineral hydroxyapatites, and synthetic preparations containing

Cl” and C03

2 ". The band at 474 cm" 1 was particularly difficult to assign,

but now appears reliably attributed to a v2 mode of the P04 group (19).

Polarized infrared and Raman spectral studies of single crystals

have separated the peaks found in the spectrum of fluorapatite powders into a

larger number of peaks which depend on the orientations of the beam and the

plane of polarization relative to the crystal axes. Factor-group analysis (20),

based on the unit-cell contents and the space -group symmetry of fluorapatite,

has provided an internally consistent assignment of specific symmetries to

the observed peaks. Thus although the structures of the apatites and their
5

infrared and Raman spectra are relatively complex, the state of understanding

of these spectra are at a relatively high level.

Carbonate -containing apatites . Mineralogical and synthetic apatites can be

divided generally into two classes, high -temperature (non-aqueous) and lowtemperature (precipitated) products. (9, 26 through 30). Carbonate is ubiquitous in both types of apatites, but appears to be differently situated in them.

The carbonate -containing mineral apatites are further subdivided into those

relatively low in fluorine (dahllite) and those high in fluorine (francolite).

The mineral francolite frequently contains more F than the 3. 77% found in

stoichiometric FAp (26). The presence of C03 is easily detected by

characteristic peaks in the infrared spectra, those for the high -temperature

apatites being distinct from those of the low -temperature apatites (27,28).

In both types of apatites, the C03

2 ~ ions are thought to be in specific

sites within the apatite structure because the unit-cell dimensions, the

optical properties, and the content of phosphate and various other ions

(coupled substitutions) appear to vary more or less monotonically with

carbonate content. It is generally held (i) that in most low -temperature

apatites, the C03

2 " occupies mostly the sites of P04

3 " ions, the smaller

size of the C03

2 " ion accounting for the observed shrinkage in the a axis,

and (ii) that in most high -temperature apatites the C03

2 " ion is in the

vicinity of the sixfold axis, where it replaces two OH ions; here its large

size compared to two OH ions is thought to cause the increase in a dimension

found in high-temperature carbonate apatites. Many other proposals have

been put forth to explain the carbonate -apatite problem—presence of

extraneous phases, adsorption on internal and external surfaces, various
6

sites for the C03

2 ~ group—which may still have partial validity, but most

of the present emphasis is on these two models even though unambiguous

structural proof for either is lacking.

It has been reported (31) that as much as 22% C03 (about 3 C03

2 ~ ions

per unit cell) can be incorporated into the structure of carbonate -hydroxyapatite,

and that the unit-cell dimensions change linearly with C03 content in this

range of compositions, with a decreasing about 0. 006 A and c increasing

about 0.0045 A, respectively, for each 1% increase in C0 3 content. These

variations in unit -cell dimensions are quite graphic, but it remains to be

known how the loss of negative charge is compensated, and it seems incredible that the apatite structure can be maintained when three of the six

P04

3 ~ ions per unit cell are replaced by C03

2 " ions.

Dry COs at 900 °C is taken up by hydroxyapatite to produce a material

with an expanded a axis and with an infrared absorption spectrum characteristic of high -temperature carbonate apatite. The COs

2 ~ ion is thought to lie

on the sixfold axis with its plane making an angle <27° with the c axis (27,

29). In contrast to this, when fluorapatite is heated with CaO and dry C02 , the product has a shortened a axis and the infrared absorption spectrum

characteristic of low-temperature apatites in which the C03

2 " is thought to

be in the P04

3 ~ site (27, 32, 33). Dichroic ratios derived from polarized

infrared absorption data (27,2$) suggest that the plane of the C03

2 ~ makes

an angle of 37° with the (00*1) face of the crystal; this is in accord (i)

with earlier calculations (27,29) based on birefringence measurements and,

(ii) with a proposed model (34) in which the C0 3

2aa ion occupies one of the

two "sloping” faces of the P04

3 ~ site.
7

Water in apatitea Stoichiometric hydroxyapatite contains constitutional

water in the form of OH" ions; this water can be driven off at high temperatures

(circa 1200° C), first producing a partially dehydrated hydroxyapatite (35

through 39) (which presumably contains one 0

2 ~ ion for each water molecule

that has been lost, and then disproportionating according to the overall

reaction (40)

Ca10 (P0 4 ) s (OH) 2 = Ca40(P0 4) 2 + 2Ca3 (P0 4) 2 + Hs O.

A large variety of proposals have coupled the presence of water protons,

and extra hydroxyl ions with observed deviations from the ideal stoichiometry

of apatites. These proposals include Hs O in place of OH” (41), H2 0 or H s O

+

in place of Ca2+

(8,42), water as (C0 3 + H2 0)

2 ~ substituting for P0 4

3 ~ (9),

(H 40 4) 4 ~ in place of P04

3 ~ (43), interstitial H

+ (as hydrogen bonds between

P04

3 " oxygens) (44 through 50), and water of hydration in octacalcium

phosphate interlayered with hydroxyapatite (51, 52). Water may be present

also in the form of acidic or hydrated calcium phosphates present as separate

phases. In addition, there is NMR evidence (53, 54) for the presence of

’’liquid" water, which is supported by the observation that enamel crystals

explode when heated (55). The infrared spectra of nearly all hydroxyapatite

precipitates show the presence of strong bands in the hydrogen stretching

region. The a dimension of synthetic hydroxyapatite usually decreases about

0. 01 A and the indexes of refraction increase (from about 1. 630 to 1. 645) when

synthetic hydroxyapatite is heated. These are all indications that water is

within the crystal in some form, but it is not necessarily there as H

+

, H2 0,

or H3 0

+ substituents in specific crystallographic sites.
8

Non-stoichiometry of apatites . A long-time, somewhat -controversial problem

in apatite chemistry has related to the large variations in Ca/P ratios in

products which otherwise appear to be pure hydroxyapatite (42, 56, 57). Many

proposals have been put forth (57, 58) to account for this phenomenon, but

the finely -divided state of most hydroxyapatite preparations has prevented an

assessment of these proposals in the vast majority of the materials that have

been studied for this purpose. Adsorption of calcium or phosphate ions is

undoubtedly an important factor in many very finely -divided apatites. The

presence of secondary phases (e. g. , CaHP04 , CaHP0 4 *2H a O, Ca8 H2 (P0 4) 6

• 5H g O, CaC03 ) can sometimes be demonstrated by microscopic methods

even when X-ray diffraction patterns do not indicate their presence. Intracrystalline mixtures of hydroxyapatite and octacalcium phosphate have been

shown to occur in some instances by X-ray diffraction and optical studies of

individual crystals. It is widely held (44 through 48, 50, 59), however, that

low Ca/P ratios also result from the presence of calcium vacancies with

charge compensation achieved by incorporation of protons interstitially, as

H3 0

+ ions, or by protonating OH" ions to form Hg O. It has been suggested

(43), also, that high Ca/P ratios may result from the substitution of (0H) 4

4 ~

for P04

3 ~. Although definitive evidence for these mechanisms is lacking,

it is possible that under highly irreversible conditions of precipitation,

appreciable concentrations of vacancies, interstitial ions and substituents

may become frozen into the structure.

Octacalcium phosphate. Although the formula of this compound, Ca8 H 2 (P0 4: ) 6• 5Ha O, and its symmetry, PI, differs drastically from those of hydroxyapatite,

it contains a layer in which the structure resembles very closely that of

hydroxyapatite (51, 60). Thus, the two salts can occur as intracrystalline,
9

interlayered mixtures as depicted in Fig. 3. This compatibility of the two

structures introduces complexities into the chemistry of hydroxyapatite

and gives insight into possible causes for some of its anomalous behavior.

The platy nature of tooth and bone crystallites, which is not consistent with

hexagonal symmetry, may relate to octacalcium phosphate having been a

precursor during nucleation and growth (58, 61). This is in accord with the

low Ca/P ratios of early tooth and bone mineral (62), and the concomitant

tendency to form pyrophosphate when heated (63, 64). The conversion of

octacalcium phosphate to an apatite (as well as the conversion of hydroxyapatite to fluorapatite) may be related to why fluoride in the drinking water

during the first few years of life reduces dental caries (58). At least some

of the apparent non-stoichiometry of hydroxyapatite must be attributed to the

presence of octacalcium phosphate, either as an interlayered mixture or as

intercrystalline mixture (52).

Solubility . The solubility of hydroxyapatite in the ternary system,

Ca(OH) g -H3 P04 -H3 0, has been measured at four temperatures in the

range 5° to 37 °C (65 to 67). The solubility, in terms of the ion activity

product, (Ca2+

) l0 (PO4

3 ~) 6 (OH”) 2

, and derived thermodynamic quantities

are listed in Tables 3 and 4. These quantities were calculated taking into

/ion

account the presence of pairs of CaHP04

° and CaH2 P04

+ (68). The stability

range of hydroxyapatite is very broad. In the ternary system at 25° C,

hydroxyapatite is the stable phase from pH 4. 3 (below which CaHP04

is

more stable) to above pH 12. However, the lower limit of the stability is

not the same for all solutions. The presence of other calcium salts in the

solution will generally extend the range of stability to values below 4. 3,
10

and the presence of other phosphate salts will restrict the range to values of

pH higher than 4. 3 as long as CaHP04 is the other salt limiting the stability

of hydroxyapatite. Very low concentrations of F" ion in solution cause

hydroxyapatite to be unstable relative to fluorapatite.
References

1. Donnay, J. D. H. and Donnay, G. Crystal Data, Determinative Tables.

2nd ed. American Crystallographic Association, 1963. Monograph No. 5.

2. Wyckoff, R. W. G. Crystal Structures, 2nd ed. 3:231. Interscience,

New York, 1965.

3. Wondratschek, H. Other compounds with the apatite structure. Chap. 3

In: Structural Properties of Hydroxyapatite and Related Compounds, ed. W. E. Brown and R. A. Young, Gordon & Breach, New York,

in preparation.

4. Wondratschek, H. Untersuchungen zur Kristallchemie der Blei-Apatite

(Pyromorphite). N. Jb. Miner. Abh. 99: 113-160, 1963.

5. Ito, J. Silicate apatites and oxyapatites. Amer. Min. 53:890-907, 1968.

6. Trautz, O. R. Crystalline organization of dental mineral. Chap. 16

In: Structural and Chemical Organization of Teeth, ed. A. E. W. Miles.

Academic Press, New York, 1967.

7. McLean, F. and Urist, M. Bone, An Introduction to the Physiology of

Skeletal Tissue. University of Chicago Press, 1961

8. Neuman, W. F. and Neuman, M. W. The Chemical Dynamics of Bone

Mineral. University of Chicago Press, 1958.

9. Carlstrom, D. Mineralogical carbonate -containing apatites. Chap. 10

In: Structural Properties of Hydroxyapatite and Related Compounds , ed. W. E. Brown and R. A. Young, Gordon & Breach, New York,

in preparation.

10. Palache, C. , Berman, H. and Frondel, C. Dana's System of Mineralogy,

7th ed. Vol. II, John Wiley & Sons, Inc. , New York, 1951.
12

11. Lehr, J. R. , Brown, E. H. , Frazier, A. W. , Smith, J. P. and

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14

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55. Moody, W. E.

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.

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