.4rrh. oral Bid. Vol.10, pp.421-431, 1965. Pergamon Press Ltd. Printed in Gt. Britain.

CRYSTAL CHEMISTRY OF HYDROXYAPATITE ITS RELATION TO BONE MINERAL

DUNCAN MCCONNELL.

College of Dentistry, Ohio State University, Columbus, Ohio, U.S.A.

Summary-The properties of a natural hydroxyapatite with Ca/P= 1.75 and a synthetic hydroxyapatite with Ca/P== 1.67 are considered with respect to (i) the non- stoichiometric properties of most synthetic calcium-phosphate precipitates, (ii) the crystal chemistry of carbonate-containing mineral apatites (francolite and dahllite) and (iii) the interpretation of analyses of teeth and bones. It is concluded that “tetra- hedral hydroxyls” substitute for phosphate groups, that their occurrence is not necessarily dependent upon the presence of carbonate groups within the structure and that some structural positions, which would normally contain (OH) or F, may contain H,O. The assigned structural formula for the mineral from northern Mexico is

The effects of isomorphic substitution, as well as the sizes of the crystallites, on the characteristics of X-ray diffraction patterns are considered. It is concluded that some carbonate apatites are certainly not hexagonal, and others, including those of tooth and bone mineral, may be merely pseudohexagonal. Inasmuch as “line broadening” cannot be distinguished, in general, from incipient “line splitting”, investigations which purport to determine the “degree of crystallinity” are not based on sound crystallographic premises. The size of the unit cell of an isomorphic variant involving merely fluorine, carbonate, and structurally bound hydrogen cannot be predicted because the effects are not independently assessable.

INTRODUCTION

THE compositions, structures and crystal chemistry of simple hydroxyapatite and

carbonate hydroxyapatite (dahllite) have received considerable attention in recent years because of their relationships to the inorganic substance of bone and teeth

(MCCONNELL, 1962a).

Some interpretations have assumed that certain synthetic apatites have atomic

ratios Ca/P below the theoretical value 1.667. POSNER and PE~LOFF (1957) reported

the preparation of such “calcium-deficient” apatites. Elsewhere POSNER, PERLOFF and DIORIO (1958) attempted to refine the structural parameters of hydroxylapatite on the basis of unanalysed synthetic material.

In contradistinction to POSNER’S conclusions are many reliable data which indicate

that the Ca/P ratio for teeth and bones is greater than the theoretical value, and that the same is true for numerous natural crystalline apatites.

CALCIUM -PHOSPHORUS RATIOS

Elsewhere it has been indicated (MCCONNELL, 1962b) that the lattice parameters of apatites depend on at least three variables: the contents of fluorine, carbon dioxide and water, Also it has been demonstrated (MCCONNELL, 1960b) that, although the

421

422 D. MCCONNELL

natural substances dahllite and francolite show appreciable ranges of carbon dioxide and of Ca/P, there is no straightforward relationship between these components.

Fossil teeth show Ca/P ratios greater than 1.667 according to BROPHY and HATCH (1962) and MCCONNELL (196Oc), as also do reliable analyses of materials from living vertebrates (KLEMENT, 1938; MCCONNELL, 1962b; STRANDH, 1960). In fact, the entire concept that a true apatite structure can have Ca/P<1*667 appears to be based solely upon extremely fine-grained synthetic materials, which are almost unquestionably mixtures, and a few analyses of bones.

In order to explain the Ca/P ratios > 1.667 it has been necessary to deduce that carbonate groups or “tetrahedral hydroxyls” substitute for phosphate groups in accordance with the following relations

3PO,+Ca+lCO,+H,O 9e-2e = 8e-e

and PO,+OH+H,O,+H,O

3e+e = 4e

where e is one negative electronic charge. The justifications for such conclusions arise from several other structures (MCCONNELL and VERHOEK, 1963).

Earlier work on minerals (MITCHELL et al., 1943) leads to ambiguous conclusions. Although both analyses of materials from Holly Springs, Cherokee County, Georgia, show Ca/P > 1.68, both show insufficient water to compensate for the deficiency of (PO& groups, and the analysis of fluor-hydroxyapatite shows slightly less than the amount of water required to fill the F plus (OH) positions when the number of Ca positions is assumed to be ten. Rather than assuming the presence of vacancies among the (OH) or F positions, this phenomenon is usually attributed to the presence of voelckerite (oxyapatite). The analysis of the material from Kemmleten, Hospenthal, Kt. Uri, Switzerland, while showing the theoretical ratio for Ca/P, shows (OH) plus F in excess of the required 2.0 by at least 0.5. There does not seem to be a theoretical explanation for Ca/P=1.667 and a simultaneous excess of water, although this enigma is readily resolved if unreported Na,O or K,O were present.

DATA ON SYNTHETIC HYDROXYAPATITE Many attempts have been made to synthesize hydroxyapatite but the products

obtained are far from uniform. If one compares the lattice periodicities (MCCONNELL, 1960a) one finds, for example, that POSNER and STEPHENSON (1952) obtained for a 9.45 A, whereas POSNER et aZ. (1958) obtained 9.43,. COLLIN (1959) prepared a series of Ca-Sr hydroxyapatites, but his Ca-containing preparation later was found to contain one per cent of carbon dioxide according to his letter of 21 February, 1961.

Two measurements of synthetic “hydroxyapatites” which cannot be attributed to differences in methods of calibration of apparatus are those recorded by ALDEN and LINDQVIST (1964) :

44 4% Hydroxyapatite I 9.428 :k 0.008 6.877 f 0.005 HydroxyapatIte II 9.384 & 0.006 6.892 & 0.004

CRYSTAL CHEMISTRY OF HYDROXYAPATITE 423

Although even the dimensions for c cannot be explained by experimental error, the

differences in the reported u dimensions exceed experimental errors by an appreciable

factor, departing from the average by +0.022 A. The probable explanation, which will be discussed below, is: The material with the smaller a periodicity contains sodium substituting for calcium or carbonate substituting for phosphate or both, whereas the material with Q = 9.428 contains H,O, substituting for PO, or chlorine substituting

for OH ions or both. Surely both substances cannot be stoichiometric hydroxyapa- tite, and it seems probable that neither one is.

“Reagent tricalcium phosphate” is predominately hydroxyapatite, although it does

not have a consistent Ca/P ratio and although, without known exception, it will

liberate bubbles of carbon dioxide when placed in normal HCl. Unless special pre- cautions are taken to exclude carbonate contamination it appears to be virtually

impossible to obtain apatite crystals that do not contain carbonate groups. POSNER ef

al. (1958) fail to mention any precautions to exclude carbonate from their precipitates, however.

The low Ca/P ratios of the “calcium deficient” apatites produced by POSNER and

PERLOFF (1957) readily can be explained by assuming that more than one phase was present, and the same explanation accounts for variation of the Ca/P of the “reagent

tricalcium phosphate,” in part. Low ratios (below 1.667) are to be expected because of

the simultaneous presence of brushite (CaHP0,.2H,O), monetite (CaHPO,), octa-

calcium phosphate [Ca,H(PO,), ‘24 H,O] or whitlockite [/3-Ca,(PO,),]. The system CaO-P,O,-H,O seems to be one of extreme complexity, involving the persistence of

metastable compounds. According to NEWESELY (1961): “The specific course of the reaction is largely determined by small quantities of accompanying ions.”

POSNER and PERLOFF (1957) also reported the preparation of a lead hydroxyapatite on which they made some elaborate-though meaningless+alculations of diffraction

intensities. Their calculations fail to take into account the intensities which would be

expected if the positions, which they presume to be vacant, contained sodium ions.

A recent conclusion of WONDRATSCHEK (1963) is interesting in this connection : “Discrepancies in the literature concerning the precipitation of lead phosphates from

aqueous solution could be explained through replacement of lead by alkali.” He also states : “No indication could be found for missing cations or tetrahedral anions or for

interstitial cations.” However, we cannot agree with his statement that the halogen

positions “can be vacant or partly or fully occupied,” inasmuch as large “holes”

(involving halogens, oxygen, hydroxyl or water) are so unlikely as to require far better evidence for their existence.

Nevertheless, through special precautions, the Fundamental Research Branch of the Tennessee Valley Authority has prepared a substance which appears to be essen- tially pure hydroxyapatite and which contains very little, although a trace, of car-

bonate. It “was prepared by a thermal method and extracted with neutral ammonium citrate to remove non-apatitic constituents,” according to Dr. Kelly L. Elmore (letter dated 30 August 1963). The Ca/P mole ratio is 1.67 and it contained (by spectroscopic analysis) the following constituents: SrO, O-01; MgO, 0.02; CuO. 0.002; Al,O,<:O.Ol ; Fe,O,, 0.03 ; and SiO,, 0.01 (weight per cents).

424 D. MCCONNELL

X-ray powder diffraction data for this substance are contained in Table 1, and are shown graphically as Fig. 1, where they are compared with data on fossil enamel (dahllite). The fundamental periodicities of the structure are a = 9.416 & O-002 A and c = 6.883 f 0.002 A, which yield a calculated density of 3.156. These measure- ments agree within stated limits with those of COLLIN (1959) for material heated to

950°C (a = 9.418 and c = 6.883, both f OGO3), but do not agree with a = 9.43, given by POSNER et al. (1958).

TABLE 1. X-RAY POWDER DIFFRACTION DATA FOR SYNTHETIC HYDROXYAPATITE

(Essentially free from carbonate) Cu Ka radiation-Philips goniometer

hk.1 I d (observed) d (c&d.) h&.1 I d (observed) d (calcd.)

IO.0 10 8.147 8.154 13.1 5 2.148 2.149 20.0 5 4.079 4.077 11.3 4 2.060 2.063 11.1 5 3.892 3.896 20.3 5 1.998 1.996 00.2 35 3441 3442 22.2 35 1.943 1.948 10.2 10 3.175 3.171 13.2 12 1.891 1.890 12.0 20 3.083 3.082 23.0 5 I.871 1.871 12.1 100 2.812 2.813 12.3 45 1.845 1.840 11.2 65 2.778 2.780 23.1 20 1.805 1.805 30.0 75 2.718 2.718 14.0 20 1.779 1.779 20.2 25 2.628 2.630 40.2 15 1.755 1.754 30.1 5 2.525 2.528 30.3 1.753 21.2 4 2.297 2.296 00.4 20 1.721 1.721 13.0 22 2.262 2.262

--

CRYSTAL CHEMISTRY OF HYDROXYAPATITE 425

Only infrequently are refractive indices reported. For this synthetic hydroxy- apatite they are: l-651 and 1647 for w and E, both 50.002. Considering the experi- mental error, these values are in perfect agreement with 1.652 and 1.648 (both & 0.002) reported by SIMPSON (1964), but are far from agreement with the “average” 164 reported by POSNER and STEPHENSON (1952). The a dimension reported for the unit cell by POSNER and STEPHENSON (9.45 A) likewise is far from agreement with the measurement obtained in the present work (a = 9.416 3. 0.002 A).

Nevertheless, as will be indicated below, 164 can be obtained for the refractive index of hydroxyapatite with Ca/P > 1.667, and a as large as 9.45 A can be explained on the same basis. But these measurements cannot be correct for Ca,O(OH),(PO,),i.

DATA ON NATURAL HYDROXYAPATITE

Good, complete analyses of natural, well-crystallized. hydroxyapatite are scarce, so that the material reported by CADY et al. (1952) from Mercedes mine, Sabinas Hidalgo, Nuevo Leon, Mexico, is of considerable interest, particularly because of the excellent reputation of the analyst. The late W. L. Hill had indicated (his letter of 19 August 1963) that all constituents were determined under his supervision.

Table 2 contains the results of chemical analysis and certain data calculated there- from. The reason for the exceptionally high determination of water at 105°C (3.26 %) is not apparent, but all of it has been excluded from the re-calculated results which sum to 100%.

On the basis of the premise that all structural positions of the apatite are filled (i.e., there are no “holes”) the structural formula becomes

Ca,,[F,.,(OH),.,(H,O),.,1 KP0JS.1(H404)0.31. Its calculated composition shows excellent agreement with the recalculated analysis with the exception of the Hz0 (+ 1OsOC). This minor discrepancy may arise because

TABLE 2. CRYSTAL CHEMISTRY OF HYDROXYAPATITE FROM MEJUCO

Oxides wt. % s -= looo/. Theoretical

composition* Ions per unit cell

Ionic charges

paos 38.61 40.71 40.50 PO, 5.7 -17.1 CaO 53.36 56.27 56.15 Ca 10.0 -120.0

Lo+ Ha,O-

0.35 2.51 0.37 2.65 0.38 2.97 OH F 0.2 2.7 - - 0.2 2.7 3.26 t - H,O 0.3 0.0

Othersf 0.14 - -

98.23 lOO.OO$j 100.00g

Notes: Analyst: R. M. Magness, W. L. Hill et al.

* Composition of Ca,,[F,.,(OH),.,(H,O),.,l KP01),.1~H404)0.31.

t Water lost at 105°C is excluded.

0.0

$ Others are AllOa + Fe,O, 0.09; MgO < @05; MnO< 001; SO, 0.00; CO, not determined but supposedly less than 0.02.

0 The sums have not been corrected for the oxygen equivalent of fluorine, the correction being fairly small in this case.

426 D. MCCONNELL

the H,O substituted for (OH) begins to leave the structure at 105°C in which case some of the H,O( - 105°C) should be included as an essential constituent of the mineral. (According to W. L. Hill, the loss on ignition was determined at 1400°C so the ignition temperature was adequately high.)

It is important that the Ca/P ratio for this mineral hydroxyapatite is 1.75, appreciably above 1667. The permissible assumptions concerning analytical error (e.g., the pre- sence of Na,O or K,O) cannot account for this increase of Ca/P. The only reasonable explanation for Ca/P ratios > 1.667 that has ever been presented for good, crystalline materials is the one involving isomorphic substitution of something for phosphorus atoms. The “adsorption” theories are completely untenable under the circumstances; contamination by another calcium phosphate could only lower the Ca/P ratio; and there is no evidence of the presence of a high-calcium mineral (such as, calcite) be- cause there is no other anion with which the calcium could be associated as a stable compound.

Investigation by X-ray diffraction of a sample obtained at a later date from Mer- cedes mine (supplied by W. L. Hill) indicated that this sample was predominately whitlockite, a mineral found in association with hydroxyapatite by CADY et al.

(1952). (It must be recalled, nevertheless, that the analysed material (see Table 2) could not have been similarly contaminated because the presence of whitlockite could only lower the Ca/P ratio.) Although four diffraction maxima were identifiable as belonging to apatite [(maxima for (12-l), (11.2) (30.0) and (20.2)] these data are insufficient to obtain precision measurements for the dimensions of the unit cell. The mean refractive index of this apatite was approximately 164, expectedly somewhat lower than that of the synthetic hydroxyapatite, and coincidentally the same as the material reported by POSNER and STEPHENSON (1952) to have a = 9.45 A.

DISCUSSION

Figure 1 compares the diffraction pattern of the synthetic hydroxyapatite with that of a natural carbonate hydroxyapatite (dahllite) which has been described in detail elsewhere (MCCONNELL, 1960~).

Attention is directed to the sharpness (narrowness) of the diffraction maxima of the synthetic material as compared with those of dahllite. This difference does not imply that the dahllite necessarily has smaller crystallites.

It should be emphasized that, in the hexagonal class of apatite, the diffraction plane (hk. l) has the “multiplicity factor” twelve, which means that there are eleven other symmetrical planes having identical spacings [(e.g. (?zk.l), @.1), (kh.Z), (i&Z), etc.)]. Now, if the misfit produced by substitution of CO, groups for PO,, groups is suffi- ciently great to cause angular distortion of the lattice, the structure does not remain hexagonal. Francolite (MCCONNELL and GRUNER, 1940) from Magnet Cove, Arkan- sas, is triclinic, for example, as it is reasonable to assume that many carbonate apatites, including those of teeth and bones, are merely pseudohexagonal.

With a reduction in symmetry, the planes of the family (hk.f) cease to have identical spacings and “line splitting” occurs. In as much as incipient line splitting cannot, in general, be distinguished from line broadening, the recent conclusions of POSNER

CRYSTAL CHEMISTRYOF HYDROXYAPATITE 427

et al. (1963) concerning the sizes of crystallites of bone, are not based on valid crystal-

lographic premises. (It should be clearly indicated that their measurements cannot be based on (00-2); they say (p. 568): “The results of the X-ray diffraction studies on bone apatite show no change in the c-axis direction due to fluoride”. Thus they are

concerned with spacings which intercept axes that are virtually perpendicular to c.

All of these interplanar spacings [(except (00.2)] are capable of splitting, and this fundamental principle-though completely disregarded by them-could account for

all of the observed line broadening on which their conclusions are based.)

It is noticeable (Fig. 1) that there is broadening of (00.2) for the carbonate hydroxy- apatite. Although this phenomenon cannot be explained by line splitting, this fact

does not in any way support the argument by POSNER et al. (1963). It merely demon-

strates that, in addition to the theoretical reasons which controvert their conclusions, there are practical (experimental) difficulties which preclude the measurement of

either an average size or a range of particle dimensions. Attention should be called to the conclusion of SLATKINE (1962) who studied the

alteration of the a dimension associated with changes in the fluorine content in VOW: “ . . . proof that the modifications of the crystalline structure do not relate to a trans-

formation of hydroxylapatite into fluorapatite, but to that of a carbonate-hydroxyl- apatite into a carbonate-fluorapatite”.

Hydroxyapatite (free from carbonate) seems to occur very rarely in nature. Only a few occurrences of the mineral are known, and it is unknown as an inorganic com-

ponent of teeth and bones. The conclusion of BACHRA and TRAUTZ (1962) that

carbonate ions “interfere” with the crystalline development of apatite in aqueous

systems may be correct, but it cannot deny the simple fact that carbonate is present in

virtually all apatites that form in aqueous systems below about 4O”C, including

various fossils (both vertebrate and invertebrate), sedimentary rock phosphates, teeth

and bones of living vertebrates, and the shell of Lingula (MCCONNELL, 1963).

It appears to be axiomatic that low-temperature apatites, formed at temperatures ranging from physiologic to oceanic, have a strong affinity for carbonate ions and

incorporate such ions in their structures. Furthermore, it should be noted that although it is possible to obtain precipitates with extremely poor crystalline charac-

teristics in the laboratory (TRAUTZ, 1960) a naturally occurring calcium phosphate

that is truly amorphous has never been reported. The dimensions of the unit cells for dahllite and for synthetic hydroxyapatite

compare as follow:

a&Q co(W) Hydroxyapatite (synthetic) 9.416 & 0.002 6.883 f O-002 Dahllite (fossil enamel) 9.454 + 0.004 6.892 $ 0.0025

Although the difference between the two measurements for c is not much greater than

the experimental errors, an appreciable difference for a is indicated, with the dimension for dahllite being larger. It is true that substitution of CO, for PO, causes a decrease in the a dimension of francolite---as it theoretically should (MCCONNELL, 1952). Tn the case of dahllite, however, any tendency toward reduction of the a dimension, is more than offset by the substitution of Hz0 or H,O+ for Ca and/or H,O, for PO,.

428 D. MCCONNELL

Thus, while there can be no straightforward correlation between Ca/P ratios and a dimensions of carbonate hydroxyapatites, there exist two opposing tendencies: (i) when the increase in Ca/P is caused by increase in carbonate, a tends to decrease, but (ii) when the increase in Ca/P is caused by substitution of H,O, for POI, a tends to increase. The effect of such substitutions on the c dimension is not so clear, but appears to be far less significant. [Another factor tending to increase the dimensions of the dahllite is the presence of 0.42 per cent of chlorine. Although fluorine substitution for OH has the opposite effect, the amount of fluorine in the dahllite sample is quite small-merely 0.03 weight per cent (MCCONNELL, 196Oc).]

The substitution of CO, for (OH), in bone mineral-a concept revived by WALLAEYS (1954) after its having been completely disproved as a possible type of substitution for F by GRUNER and MCCONNELL (1937)-is not supported by the work of WINAND (1963), NEWESELY (1963) or SIMPSON (1964). All of these authors conclude that the carbonate is within the structural lattice and that it substitutes for phosphate. The recent discovery of substitution of carbonate for sulphate in ettringite (MCCONNELL and MURDOCH, 1962) has an important crystallochemical significance.

SIMPSON (1964) in his study of sodium and potassium carbonate apatites, finds that these substances uniformly contain an excess of water (above what is required for hydroxyapatite). He states: “This water probably is present in the crystals as hydro- nium ions and (H40& substituting for Ca2 1 and (P04)3- respectively”.

The crystallochemical interpretation of tooth and bone mineral is indeed a complex matter, and depends upon the recognition that carefully demonstrated principles of structural crystallography are no less applicable here than they are for other micro- crystalline materials. There exists an appreciable mass of knowledge, both empirical and theoretical, on the crystallography of the clay mineral montmorillonite, for example, and yet, except as a shadow on the screen of an electron microscope, nobody has ever seen a single crystal of montmorillonite.

CONCLUSIONS

So-called “calcium-deficient apatites” have been demonstrated only as extremely fine-grained (poorly crystalline) synthetic materials which readily can be explained by assuming that they are mixtures of more than one phase, or that they are isomorphic variants containing sodium or potassium. These precipitates probably contain ad- mixed brushite, monetite, whitlockite and/or octacalcium phosphate, any of which would contribute to a Ca/P ratio below 1.667. Certainly the diversity of lattice periodicities of synthetic materials is appreciable; it is adequate to demonstrate that most preparations of so-called hydroxyapatite are not uniform in composition.

High Ca/P ratios ( ;‘. 1.667) can be explained only on the basis of isomorphic substitution. The presence of some other Ca-containing compound as a possible explanation can be eliminated in the present circumstance by the absence of another anion (such as carbonate) which might otherwise account for a high Ca/P ratio.

Although the compositions of teeth and bones always contain carbonate and show Ca/P ratios greater than the theoretical value for hydroxyapatite (Ca/P= l-667), these superior ratios do not necessarily arise solely from substitution of carbonate for

CRYSTAL CHEMISTRY OF HYDROXYAPATITF. 429

phosphate. The Ca/P ratio also may exceed 1.667 because of substitution of “tetra- hedral hydroxyls” for phosphate groups. Furthermore, it is shown that the sites of the apatite structure which normally contain F or (OH) may contain H,O as a concomitant effect of the substitution of (H404) for (PO.,).

In the case of natural hydroxyapatite (CajP =- 1.75) from Mexico the “deficiency” of phosphorus atoms is 0.3 among 6.0. leading to the following structural formula

Ca,,[F,.,(OH),.,(H,O),.,1 P04MWbhI. The unit-cell dimensions (particularly a) depend upon complex functions of (i)

the carbonate content, (ii) the content of structurally bound hydrogen (as OH, H,O, H,O and/or H,O,) and (iii) the fluorine (and/or chlorine) content. However, the quantitative effects of these types of isomorphic substitution cannot be assessed because they operate in opposing directions and because two or more types usually operate simultaneously.

The size of crystallites (sometimes referred to as the “degree of crystallinity”) cannot be determined from the phenomenon called “line broadening” because this effect cannot be distinguished from incipient “line splitting” for any X-ray diffraction reflections other than pinacoidal ones (in the hexagonal case (OO*l), (0@2), etc.). Although it might be possible, theoretically at least, to detect a reduction of crystal- line size in the c direction in terms of the half-height broadening of the pinacoidal reflections, the interpretation of such measurements (in a practical situation) seems to involve insurmountable difficulties.

Acknowledgements-This investigation was supported by Research Grant D-141 1, N.I.D.R., N.I.H., U.S. Public Health Service.

I am indebted to Dr. KELLY L. ELMORE, Chief, Fundamental Research Branch, Tennessee Valley Authority, for the synthetic preparation of pure hydroxyapatite. The late W. L. HILL, Director of the U.S. Fertilizer Laboratory, U.S. Department of Agriculture, supplied natural material from Sabinas Hidalgo, Nuevo Leon, Mexico, and supplemental information. ALI.EN CICHANSKI measured the refractive indices and made instrumental recordings of the X-ray diffraction data.

R&sum&-Les propriitts d’une hydrowyapatite naturelle avec un rapport Ca/P = 1.75 et celles d’une hydroxyapatite synthttique avec un rapport Ca/P - 1.67 sont analyskes en fonction: (I) de leurs proprittks non-stoichomktriques de la plupart des prkipitts de phosphate de\ calcium synthttique, (2) de la chimie cristalline d’apatites minkraux contenant des carbonates (francolite et dahlite) (et 3) de I’interprttation des analyses des dents et des OS. On en conclue que des “hydroxyles tetrahkdriques” sont sub- stituks k des groupes phosphates, que leur diveloppement n’est pas like nkcessairement ti la prknce de groupes carbonates dans la structure et que certaines positions struc- turales, qui contiendraient normale-ment (OH) ou F, peuvent contenir de I’H, 0. La formule structurale indiqutk pour un mirkral provenant du Mexique du Nord est de: Ca,” [F,., (OH) 1.b (H,O) 0.31 [(PO,) 5.7 (H&A) 0.31.

Les effets de la substitution isomorphique, ainsi que la taille des cristallites, sur les caractkristiques des diagrammes de diffraction par rayons X sont envisagks. On en conclue que certains carbonate-apatites ne sont certainement pas hexagonales et d’autres, comprenant celles des dents et de I’os, peuvent &re simplement pseudo- hexagonales. Comme “l’klargissement des raies” ne peut &tre distingud en gkn6ral d’une “division debutante des raies”, les recherches cherchant A dtteiminer le “degrk de cristallinitt” ne reposent pas sur des bases cristallographiques saines, La taille de la

430 D. MCCONNELL

maille cristalline d’un produit variable isomorphique comportant du fluor, du carbonate et de I’hydrogkne 1% structurellement ne peut Btre dCterminCe. car les effets ne peuvent &tre Cvalub indkpendamment.

Zusammenfassung-Die Eigenschaften eines natiirlichen Hydroxylapatits (Ca/P = 1,75) und eines synthetischen Hydroxylapatits (Ca/P = 1,67) werden hinsichtlich (i) der nichtstiichiometrischen Eigenheiten der meisten synthetischen Calciumphosphat- Niederschlgge, (ii) der Kristallchemie karbonathaltiger Mineralapatite (Francolit und Dahllit) und (iii) der Interpretation von Zahn- und Knochenanalysen betrachtet. Es wird der Schluss gezogen, dass “tetraedrische Hydroxyle” an Stelle von Phosphat- gruppen treten, dass ihr Vorkommen nicht notwendigerweise von der Gegenwart von Karbonatgruppen innerhalb der Struktur abhangt, und dass einige strukturelle Posit- ionen, die normalerweise (OH) oder F enthalten, H,O enthalten kiinnen. Dem aus Nordmexikao stammenden Mineral wir folgende strukturelle Formel zugeschrieben: CaIo (F,,, (OH) I,5 (Ha%3 ((PO,),,, (H@&).

Es wurden die Wirkungen isomorpher Substitution wie such der Kristallitgrijssen auf die Charakteristika von RGntgendiffraktionsbildern betrachtet. Daraus wird geschlossen, dass einige Karbonatapatite sicherlich nicht hexagonal sind, und dass andere- einschliesslich jener aus Zahn- und Knochenmineral- lediglish pseudohexagonal sein mijgen. Weil die “Linienverbreiterung” im allgemeinen nicht von der beginnenden “Linientrennung” unterschieden werden kann, beruhen solche Untersuchungen, die den “Grad der Kristallinittit” zu bestimmen vorgeben, nicht auf vertretbaren kris- tallographischen Voraussetzungen. Die Grijsse einer Zelleinheit einer isomorphen Variante, die lediglich Fluor, Karbonat und strukturell gebundenen Wasserstoff einschliesst, kann nicht vorausgesagt werden, weil die Effekte nicht unabhlngig abschatzbar sind.

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Science 137, 337. BROPHY, G. P. and HATCH, T. M. 1962. Recrystallization of fossil horse teeth. Amer. Min. 47,

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phosphate in northern Mexico. Amer. Min. 37, 180-183. COHEN-ADDAD, C. DUCROS, P. DURIF, A. BERTAUT E. F. and DELAPALME. A. 1964. D&termination

de la position des atomes d’hydrogkne dans l’hydrogrenat Alz03, 3Ca0, 6Ha0 per rCsonance magnetique nucltaire et diffraction neutronique. J. P/I_vs. Radium 25, 478-483.

COLES. J. L. 1963. A study of some synthetic apatites: Thesis, Univ. of Utah, Salt Lake City, Utah COLLIN, R. L. 1959. Strontium-calcium hydroxyapatite solid solutions: preparation and lattice

constant measurements. J. Amer. them. Sot. 81,5275-5278. [See also ibid. 82,5067-5069 (1960).] GRUNER, J. W. and MCCONNELL, D. 1937. The problem of the carbonate apatites. The structure

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ADDENDUM

It is now possible to cite further data on H404 groups (“tetrahedral hydroxyls”) as

a result of the investigation of this atomic configuration by nuclear magnetic resonance

and neutron diffraction by COHEN-ADDAD et al. (1964). These authors supply measure-

ments on the H-H, H-O, and O-O distances. Some of the interpretations on carbonate hydroxyapatites that have been obtained

by infrared absorption spectroscopy are inconsistent with numerous data obtained

by other methods. Nevertheless, according to COLES (1963): “The synthetic carbonate- apatites have infrared spectra very different from those of other apatites. Distinguish-

ing features are the strong carbonate band at about 1450 cm-’ and lack of resolution or the main P0,e3 band into a doublet.”