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Some observations on the crystal chemistry of carbonate-containing apatites
SOME OBSERVATIONS
ON THE CRYSTAL
OF CARBONATE-CONTAINING
CHEMISTRY
APATITES
J. C. ELLIOTT The London Hospital Medical College, Turner Street, London, E.1. Abstract-The relationship between some carbonate-containing apatites and dental enamel is discussed in the light of X-ray diffraction studies and new observations using infra-red absorption methods. The application of infra-red absorption methods allows the possible distinction between different carbonate-containing substances such as calcite and aragonite (both calcium carbonates) because the exact shape and position of the absorption bands depends on the local environment of the ion. These methods have been principally applied to carbonate-containing apatites prepared by the reaction between calcium carbonate and alkaline phosphate solutions and to carbonate-apatites prepared by the reaction between carbon dioxide and hydroxyapatite at 1OOO’C. Resume-Les relations existant entre certaines des carbonate-apatites et le mineral adamantin sont discut&s & la lumiere des don&es obtenues par la diffraction des rayons X et d’observations nouvelles sur les spectres d’absorption en lumi&re infrarouge de tels minCraux. Cette spectrophotomttrie permet de distinguer des corps aussi semblables que la calcite et l’aragonite (tous deux des carbonates de calcium) parce que la forme et la position exacte des bandes de leurs spectres d’absorption en infra-rouge dtpendent des conditions de voisinage immtdiat de l’ion carbonate dans le cristal. Ces mtthodes ont CtCappliqu6es principalement B des carbonate-apatites pr&par6es, soit par reaction du carbonate de calcium sur une solution alcaline de phosphate, soit par l’action & 1OOO’Cde I’anydride carbonique sur l’hydroxy-apatite. Zusammenfassung-Die zwischen gewissen Karbonat-Apatiten und dem Zahnschmelz bestehenden Beziehungen werden im Lichte der durch Rantgendiffraktion erhaltenen Resultate diskutiert sowie gestiitzt auf neue Beobachtungen im Infrarot-Absorptionsspektrum. Letztere Methode erlaubt sogar, einander sehr nahe stehende Verbindungen wie Aragonit und Calcit (beides Calciumkarbonate) zu unterscheiden. Dieses Procedere wurde im Besonderen auf Karbonat-Apatite angewandt, die durch Reaktion von Calciumkarbonat auf eine alkalische PhosphatlGsung oder durch die Einwirkung von Kohlenstoffdioxyd auf Hydroxylapatit bei 1000’ Celsius synthetisiert wurden. INTRODUCTION
WHEN enamel is treated with dilute acids it dissolves and about 3 per cent by weight of carbon dioxide is evolved. It has been assumed that the carbon dioxide is in the form of carbonate ions in the enamel. Since the presence of carbonate may possibly be correlated with the susceptibility of enamel to caries, an important problem which must be solved is the form of combination of the carbonate in enamel. However, it is not possible to consider this problem with reference to enamel only, as many other carbonate-containing apatites have been described. 12771
278
J. C.
ELLIOTT
Various theories have been proposed for the form of combination of the carbonate which have been applied to enamel and the whole subject has remained highly controversial for many years. It is the purpose of this paper to discuss the additional insight to the problem which can be gained from a study of the infra-red absorption spectrum of carbonate-containing apatites. Infra-red methods are particularly useful because they provide precise information about the covalent bonds present, as it is these,which are responsible for the absorption. For instance, it is possible to distinguish between the carbonate absorption bands in calcite and aragonite, two crystalline forms of calcium carbonate. We can, then, apply this method to an examination of the various types of carbonate-containing apatites and ascertain whether the form of combination of the carbonate is the same in each. MATERIAL AND METHODS The infra-red spectra of the apatites were recorded with the samples as finely dispersed powders on plates of barium fluoride using a model 137 NaCl Infracord Spectrophotometer, or as thin sections using a modified Grubb Parsons monochromator fitted with a rock-salt prism and a reflecting microscope giving a magnification of x 8. An instrument similar to the one used has been described by FORD et al. (1958). The thin sections were examined using polarized radiation, the direction of polarization being either along the direction of the c-axis of the sample or at rightangles to it. The radiation was polarized by passing it through an inclined stack of thin selenium plates as described by ELLIOTTand AMBROSE(1947). Samples of carbonate-containing apatites were prepared by refluxing calcium carbonate with an alkaline solution of ammonium phosphate for three hours. This material, dried at llO°C for several hours, gave an X-ray diffraction pattern indistinguishable from hydroxyapatite and had a carbon dioxide content of about 2 per cent. Another series of samples of carbonate-containing apatites were prepared by passing dry carbon dioxide over hydroxyapatite at 1000°C for three days, by which time the carbon dioxide content had risen to 3 per cent. The X-ray diffraction pattern of this was markedly different from hydroxyapatite, which is in agreement with the results of WALLAEYS(1954). The enamel sections used were 20,~ thick, they were longitudinal bucco-lingual ground sections through the crowns of unerupted lower third molar teeth from four human subjects. The section of francolite was cut from a singIe crystal (Fowey Consols Mine, St. Blazey, Cornwall, England. British Museum Number 55548). This crystal was an ill-formed hexagonal plate 3.5 mm in diameter with the optic axis and hexagonal axis coincident. A section 60,~ thick cut in the plane of the optic axis was used. This section showed a constant extinction direction throughout, but the magnitude of the birefringence varied. Powder samples from various human teeth and samples of francolite and dahllite were prepared by grinding the material under water with a diamond impregnated disc held in a dental handpiece. The absorption
RESULTS spectra of the various powder samples are shown in Fig. 1 with
SOME
OBSERVATIONS
ON
THECRYSTALCHEMISTRY OF CARBONATE-CONTAINING APATITES 279
the frequencies and origin of the bands in Table
1. The polarized
of the section of enamel and francolite in the region 1000 cm-‘-700 in Figs. 2 and 3 respectively. the powder
infra-red spectra cm-’ are shown
No marked differences were observed either between
spectra of the various enamel samples or between the powder
of the various
spectra
samples of francolite.
DIOXIDE
CAREON
3000
1500
1ooo*c +HYDROXYAPATITE
1000
700
FREQUENCY CM -’ FIG. I. Absorption spectra of various samples dispersed as fine powders on plates of
barium fluoride. DISCUSSION Examination to the carbonate
of Table
1 shows that the frequencies
of the absorption
bands
due
ion in the various samples differ, and that this means, therefore,
that the carbonate ions must enter the structures in different ways. It is not possible, therefore, to discuss the properties of one of these materials in terms of a proposed structure of another. They all show splitting of the band at about 1450cm-‘, which is also a feature found in basic carbonates. single band due to the C-O group in more complicated
asymmetrical
In simple carbonates this appears as a stretching.
Distortion
of the carbonate
structures will lower its symmetry and the degeneracy
of this mode will be lost. (GATEHOUSE, et al. 1958). POSNER and DUCKAERTS(~~~~) studied the infra-red absorption spectra of francolite and enamel and concluded that a chemical bond exists between calcium and carbonate in these materials which is identical to the bond in calcite. I have found no evidence for the presence of an appreciable
amount of calcite, on the basis of
280
J. C.
ELLIOTT
I
960
880
FREQUENCY
CM -I
FIG. 2. Polarized infra-red absorption spectrum of a longitudinal ground section of human enamel. Full line-electric vector perpendicular to the enamel rods. Broken linedlectric vector parallel to the enamel rods.
I
I
960
FREQUENCY FIG.
880
CM -’
3. Polarized infra-red absorption spectrum of a section of francolite cut in the plane of the optic axis. Full line-electric vector perpendicular to the optic axis. Broken line-electric vector parallel to the optic axis.
SDME 03jERVATIONS ON THE CRYSTAL CHEMISTRY OF CARBONATE-CONTAINING
APATITES
281
infra-red measurements, because uj the C-O asymmetrical stretching mode occurs as a single peak in the spectrum of calcite, whereas this is always doubled in enamel and francolite. TABLE ~.THEFREQUENCIESOFTHEAESORPTIONBANDSSHOWN ~I Origin of band
VI co,_
Substance Carbon dioxide + : Hydroxyapatite 1000°C. I
1535 1458
____~ Calcium carbonate + Ammonium phosphate solution -Francolite
Human
Enamel
* The assignment
i
vz co,
I ’ -PI
880
1465
~
IN FIGS. l-3 INCM.-1 I
PO,_
I SO
ca. 1050 960
I i 1640 !
ca.1050
’
ca. 870 1410
1640 960
: 1453 1427
,
1542* 1454 1410
’
868 868
~ ca. 1050
880 873
ca.1050 960
960
i
1640
/ ,
of this band to a carbonate
frequency isdue to EMMER~ON
and FISCHER(~~~~).
The reaction between hydroxyapatite and carbon dioxide at high temperatures was first studied in detail by WALLAEYS(1954), although probably a similar product was made by EITEL (1924) by fusing calcium carbonate and calcium phosphate together under a pressure of carbon dioxide. From chemical evidence, Wallaeys suggested that there is a partial replacement of hydroxyl groups in the lattice which resulted in an appreciable change in the unit cell dimensions. The use of polarized radiation enables much more detailed information to be obtained about the carbonate groups, such as a preferred orientation with respect to the c-axis of the apatite crystal. The most useful mode to examine is the out-ofplane deformation of the carbon atom at about 870 cm-‘. This mode will only absorb when the direction of the electric vector of the incident radiation is perpendicular to the plane of the carbonate group. Examination of this mode in francolite, Fig. 3, allows the following deductions to be made. First, there is some orientation of the carbonate; this is in agreement with the collected birefringence data by GEIGER (1950). Second, only one absorption band is seen. This is not the result which would be expected from the structure proposed by MCCONNELL (1952), which involves carbonate groups in two different environments. However, this result is not conclusive because it would be possible for the carbonate groups in the two different environments to have the same absorption frequencies. Lastly, by measuring the actual magnitude of the absorption in the two directions of the polarizer, it is possible to calculate the angle 8 between the perpendicular from the plane of the carbonate ion and the c-axis, provided, of course, that the absorption 19
282
J. C. ELLIOTT
is due to one type of carbonate group at a constant inclination to the c-axis.This angle is 38’ (estimated error 5O), calculated from the equation, dichroic ratio= 2cor2B given by ELLIOTT et al. (1948), which is applicable in this case. This is to be compared with the only other estimate of this angle, namely 42’ calculated by TRAUTZ (1960) from the optical constants of francolite and calcite. The two results are in agreement within the limits of the experimental errors of the methods used. Examination of the same spectral region in the section of enamel, Fig. 2, shows that there are two absorption bands, and because this is a non-degenerate mode, this has been interpreted as indicating the presence of carbonate groups in two different environments (ELLIOTT (1961), EMERSONand FISCHER(1962)). From the actual dichroic measurements the carbonate ions would be approximately parallel and perpendicular to the c-axis. It should be emphasized however, that, although this is the most likely interpretation of this doubling, it is not the only one possible. It cannot be deduced from these measurements whether the carbonate groups are within or absorbed on the crystal lattice, as both would give orientation. It is very unlikely that the observed orientation could arise from carbonate groups present in an amorphous phase. Acknowledgements-The author would like to acknowledge the permission given by Dr. C. H. Bamford, Head of the Courtaulds Research Laboratory, Maidenhead, for the use of the infra-red spectrometers and the help given by Dr. E. M. Bradbury in making the spectra of the sections. Thanks are also due to Dr. G. F. Claringbull and Mr. P. G. Embrey, Department of Mineralogy, The British Museum (Natural History) for supplying the mineral samples used. This work has been supported by a grant from the Medical Research Councii. REFERENCES EITEL, W. 1924. ilber Karbonatphosphate der Apatitgruppe. Schr. kdnigsb. gelehrt. Ges. Naturw. KI. 1, 159. ELLIOTT,A. and A~~BROSE, E. J. 1947. Nature, Lond. 159, 641. ELLIOTT,A., AMBROSE,E. J. and TEMPLE,R. B. 1948. J. Chem. Phys. 16,877. ELLIOTT,J. C. 1961. The Infra-red Spectrum of the Carbonate Ion in Carbonate-Containing Apatites. Abstract in J. dent. Res. 40, 1284. EMERSON,W. H. and FISCHER,E. E. 1962. Infra-red Absorption Spectra of Carbonate in Calcified Tissues. Arch. oral Biol. (In press). FORD, M. A., PRICE, W. C., SEEDS,W. E. and WILKINSON,G. R. 1958. J. Opt. Sot. America48,249. GATEHOUSE,B. M., LIVINGSTONE, S. E. and NYHOLM,R. S. 1958. J. Chem. Sot. p. 3137. GEIGER, TH. 1950. Beitrsge zum Problem der Karbonatapatite. Schweiz. mineralog. petrog. Mitt. 30, 161. MCCONNELL, D. 1952. The crystal chemistry of carbonate apatites and their relationship to the composition of calcified tissue. J. dent. Res. 31, 53. POSNER,A. S. and DUYCKAERTS, G. 1954. Infra-red study of the carbonate in bone, teeth and francolite. Experentia. 10, 424. TRAUTZ,0. R. 1960. Crystallographic studies of calcium carbonate phosphate. Annals of the New York Academy of Sciences. 85, 145. WALLAEYS,R. 1954. Silicon, Sulphur, Phosphates. Colloquium of the International Union of Pure and Applied Chemistry. Miinster. Verlag Chemie.
ON THE CRYSTAL
OF CARBONATE-CONTAINING
CHEMISTRY
APATITES
J. C. ELLIOTT The London Hospital Medical College, Turner Street, London, E.1. Abstract-The relationship between some carbonate-containing apatites and dental enamel is discussed in the light of X-ray diffraction studies and new observations using infra-red absorption methods. The application of infra-red absorption methods allows the possible distinction between different carbonate-containing substances such as calcite and aragonite (both calcium carbonates) because the exact shape and position of the absorption bands depends on the local environment of the ion. These methods have been principally applied to carbonate-containing apatites prepared by the reaction between calcium carbonate and alkaline phosphate solutions and to carbonate-apatites prepared by the reaction between carbon dioxide and hydroxyapatite at 1OOO’C. Resume-Les relations existant entre certaines des carbonate-apatites et le mineral adamantin sont discut&s & la lumiere des don&es obtenues par la diffraction des rayons X et d’observations nouvelles sur les spectres d’absorption en lumi&re infrarouge de tels minCraux. Cette spectrophotomttrie permet de distinguer des corps aussi semblables que la calcite et l’aragonite (tous deux des carbonates de calcium) parce que la forme et la position exacte des bandes de leurs spectres d’absorption en infra-rouge dtpendent des conditions de voisinage immtdiat de l’ion carbonate dans le cristal. Ces mtthodes ont CtCappliqu6es principalement B des carbonate-apatites pr&par6es, soit par reaction du carbonate de calcium sur une solution alcaline de phosphate, soit par l’action & 1OOO’Cde I’anydride carbonique sur l’hydroxy-apatite. Zusammenfassung-Die zwischen gewissen Karbonat-Apatiten und dem Zahnschmelz bestehenden Beziehungen werden im Lichte der durch Rantgendiffraktion erhaltenen Resultate diskutiert sowie gestiitzt auf neue Beobachtungen im Infrarot-Absorptionsspektrum. Letztere Methode erlaubt sogar, einander sehr nahe stehende Verbindungen wie Aragonit und Calcit (beides Calciumkarbonate) zu unterscheiden. Dieses Procedere wurde im Besonderen auf Karbonat-Apatite angewandt, die durch Reaktion von Calciumkarbonat auf eine alkalische PhosphatlGsung oder durch die Einwirkung von Kohlenstoffdioxyd auf Hydroxylapatit bei 1000’ Celsius synthetisiert wurden. INTRODUCTION
WHEN enamel is treated with dilute acids it dissolves and about 3 per cent by weight of carbon dioxide is evolved. It has been assumed that the carbon dioxide is in the form of carbonate ions in the enamel. Since the presence of carbonate may possibly be correlated with the susceptibility of enamel to caries, an important problem which must be solved is the form of combination of the carbonate in enamel. However, it is not possible to consider this problem with reference to enamel only, as many other carbonate-containing apatites have been described. 12771
278
J. C.
ELLIOTT
Various theories have been proposed for the form of combination of the carbonate which have been applied to enamel and the whole subject has remained highly controversial for many years. It is the purpose of this paper to discuss the additional insight to the problem which can be gained from a study of the infra-red absorption spectrum of carbonate-containing apatites. Infra-red methods are particularly useful because they provide precise information about the covalent bonds present, as it is these,which are responsible for the absorption. For instance, it is possible to distinguish between the carbonate absorption bands in calcite and aragonite, two crystalline forms of calcium carbonate. We can, then, apply this method to an examination of the various types of carbonate-containing apatites and ascertain whether the form of combination of the carbonate is the same in each. MATERIAL AND METHODS The infra-red spectra of the apatites were recorded with the samples as finely dispersed powders on plates of barium fluoride using a model 137 NaCl Infracord Spectrophotometer, or as thin sections using a modified Grubb Parsons monochromator fitted with a rock-salt prism and a reflecting microscope giving a magnification of x 8. An instrument similar to the one used has been described by FORD et al. (1958). The thin sections were examined using polarized radiation, the direction of polarization being either along the direction of the c-axis of the sample or at rightangles to it. The radiation was polarized by passing it through an inclined stack of thin selenium plates as described by ELLIOTTand AMBROSE(1947). Samples of carbonate-containing apatites were prepared by refluxing calcium carbonate with an alkaline solution of ammonium phosphate for three hours. This material, dried at llO°C for several hours, gave an X-ray diffraction pattern indistinguishable from hydroxyapatite and had a carbon dioxide content of about 2 per cent. Another series of samples of carbonate-containing apatites were prepared by passing dry carbon dioxide over hydroxyapatite at 1000°C for three days, by which time the carbon dioxide content had risen to 3 per cent. The X-ray diffraction pattern of this was markedly different from hydroxyapatite, which is in agreement with the results of WALLAEYS(1954). The enamel sections used were 20,~ thick, they were longitudinal bucco-lingual ground sections through the crowns of unerupted lower third molar teeth from four human subjects. The section of francolite was cut from a singIe crystal (Fowey Consols Mine, St. Blazey, Cornwall, England. British Museum Number 55548). This crystal was an ill-formed hexagonal plate 3.5 mm in diameter with the optic axis and hexagonal axis coincident. A section 60,~ thick cut in the plane of the optic axis was used. This section showed a constant extinction direction throughout, but the magnitude of the birefringence varied. Powder samples from various human teeth and samples of francolite and dahllite were prepared by grinding the material under water with a diamond impregnated disc held in a dental handpiece. The absorption
RESULTS spectra of the various powder samples are shown in Fig. 1 with
SOME
OBSERVATIONS
ON
THECRYSTALCHEMISTRY OF CARBONATE-CONTAINING APATITES 279
the frequencies and origin of the bands in Table
1. The polarized
of the section of enamel and francolite in the region 1000 cm-‘-700 in Figs. 2 and 3 respectively. the powder
infra-red spectra cm-’ are shown
No marked differences were observed either between
spectra of the various enamel samples or between the powder
of the various
spectra
samples of francolite.
DIOXIDE
CAREON
3000
1500
1ooo*c +HYDROXYAPATITE
1000
700
FREQUENCY CM -’ FIG. I. Absorption spectra of various samples dispersed as fine powders on plates of
barium fluoride. DISCUSSION Examination to the carbonate
of Table
1 shows that the frequencies
of the absorption
bands
due
ion in the various samples differ, and that this means, therefore,
that the carbonate ions must enter the structures in different ways. It is not possible, therefore, to discuss the properties of one of these materials in terms of a proposed structure of another. They all show splitting of the band at about 1450cm-‘, which is also a feature found in basic carbonates. single band due to the C-O group in more complicated
asymmetrical
In simple carbonates this appears as a stretching.
Distortion
of the carbonate
structures will lower its symmetry and the degeneracy
of this mode will be lost. (GATEHOUSE, et al. 1958). POSNER and DUCKAERTS(~~~~) studied the infra-red absorption spectra of francolite and enamel and concluded that a chemical bond exists between calcium and carbonate in these materials which is identical to the bond in calcite. I have found no evidence for the presence of an appreciable
amount of calcite, on the basis of
280
J. C.
ELLIOTT
I
960
880
FREQUENCY
CM -I
FIG. 2. Polarized infra-red absorption spectrum of a longitudinal ground section of human enamel. Full line-electric vector perpendicular to the enamel rods. Broken linedlectric vector parallel to the enamel rods.
I
I
960
FREQUENCY FIG.
880
CM -’
3. Polarized infra-red absorption spectrum of a section of francolite cut in the plane of the optic axis. Full line-electric vector perpendicular to the optic axis. Broken line-electric vector parallel to the optic axis.
SDME 03jERVATIONS ON THE CRYSTAL CHEMISTRY OF CARBONATE-CONTAINING
APATITES
281
infra-red measurements, because uj the C-O asymmetrical stretching mode occurs as a single peak in the spectrum of calcite, whereas this is always doubled in enamel and francolite. TABLE ~.THEFREQUENCIESOFTHEAESORPTIONBANDSSHOWN ~I Origin of band
VI co,_
Substance Carbon dioxide + : Hydroxyapatite 1000°C. I
1535 1458
____~ Calcium carbonate + Ammonium phosphate solution -Francolite
Human
Enamel
* The assignment
i
vz co,
I ’ -PI
880
1465
~
IN FIGS. l-3 INCM.-1 I
PO,_
I SO
ca. 1050 960
I i 1640 !
ca.1050
’
ca. 870 1410
1640 960
: 1453 1427
,
1542* 1454 1410
’
868 868
~ ca. 1050
880 873
ca.1050 960
960
i
1640
/ ,
of this band to a carbonate
frequency isdue to EMMER~ON
and FISCHER(~~~~).
The reaction between hydroxyapatite and carbon dioxide at high temperatures was first studied in detail by WALLAEYS(1954), although probably a similar product was made by EITEL (1924) by fusing calcium carbonate and calcium phosphate together under a pressure of carbon dioxide. From chemical evidence, Wallaeys suggested that there is a partial replacement of hydroxyl groups in the lattice which resulted in an appreciable change in the unit cell dimensions. The use of polarized radiation enables much more detailed information to be obtained about the carbonate groups, such as a preferred orientation with respect to the c-axis of the apatite crystal. The most useful mode to examine is the out-ofplane deformation of the carbon atom at about 870 cm-‘. This mode will only absorb when the direction of the electric vector of the incident radiation is perpendicular to the plane of the carbonate group. Examination of this mode in francolite, Fig. 3, allows the following deductions to be made. First, there is some orientation of the carbonate; this is in agreement with the collected birefringence data by GEIGER (1950). Second, only one absorption band is seen. This is not the result which would be expected from the structure proposed by MCCONNELL (1952), which involves carbonate groups in two different environments. However, this result is not conclusive because it would be possible for the carbonate groups in the two different environments to have the same absorption frequencies. Lastly, by measuring the actual magnitude of the absorption in the two directions of the polarizer, it is possible to calculate the angle 8 between the perpendicular from the plane of the carbonate ion and the c-axis, provided, of course, that the absorption 19
282
J. C. ELLIOTT
is due to one type of carbonate group at a constant inclination to the c-axis.This angle is 38’ (estimated error 5O), calculated from the equation, dichroic ratio= 2cor2B given by ELLIOTT et al. (1948), which is applicable in this case. This is to be compared with the only other estimate of this angle, namely 42’ calculated by TRAUTZ (1960) from the optical constants of francolite and calcite. The two results are in agreement within the limits of the experimental errors of the methods used. Examination of the same spectral region in the section of enamel, Fig. 2, shows that there are two absorption bands, and because this is a non-degenerate mode, this has been interpreted as indicating the presence of carbonate groups in two different environments (ELLIOTT (1961), EMERSONand FISCHER(1962)). From the actual dichroic measurements the carbonate ions would be approximately parallel and perpendicular to the c-axis. It should be emphasized however, that, although this is the most likely interpretation of this doubling, it is not the only one possible. It cannot be deduced from these measurements whether the carbonate groups are within or absorbed on the crystal lattice, as both would give orientation. It is very unlikely that the observed orientation could arise from carbonate groups present in an amorphous phase. Acknowledgements-The author would like to acknowledge the permission given by Dr. C. H. Bamford, Head of the Courtaulds Research Laboratory, Maidenhead, for the use of the infra-red spectrometers and the help given by Dr. E. M. Bradbury in making the spectra of the sections. Thanks are also due to Dr. G. F. Claringbull and Mr. P. G. Embrey, Department of Mineralogy, The British Museum (Natural History) for supplying the mineral samples used. This work has been supported by a grant from the Medical Research Councii. REFERENCES EITEL, W. 1924. ilber Karbonatphosphate der Apatitgruppe. Schr. kdnigsb. gelehrt. Ges. Naturw. KI. 1, 159. ELLIOTT,A. and A~~BROSE, E. J. 1947. Nature, Lond. 159, 641. ELLIOTT,A., AMBROSE,E. J. and TEMPLE,R. B. 1948. J. Chem. Phys. 16,877. ELLIOTT,J. C. 1961. The Infra-red Spectrum of the Carbonate Ion in Carbonate-Containing Apatites. Abstract in J. dent. Res. 40, 1284. EMERSON,W. H. and FISCHER,E. E. 1962. Infra-red Absorption Spectra of Carbonate in Calcified Tissues. Arch. oral Biol. (In press). FORD, M. A., PRICE, W. C., SEEDS,W. E. and WILKINSON,G. R. 1958. J. Opt. Sot. America48,249. GATEHOUSE,B. M., LIVINGSTONE, S. E. and NYHOLM,R. S. 1958. J. Chem. Sot. p. 3137. GEIGER, TH. 1950. Beitrsge zum Problem der Karbonatapatite. Schweiz. mineralog. petrog. Mitt. 30, 161. MCCONNELL, D. 1952. The crystal chemistry of carbonate apatites and their relationship to the composition of calcified tissue. J. dent. Res. 31, 53. POSNER,A. S. and DUYCKAERTS, G. 1954. Infra-red study of the carbonate in bone, teeth and francolite. Experentia. 10, 424. TRAUTZ,0. R. 1960. Crystallographic studies of calcium carbonate phosphate. Annals of the New York Academy of Sciences. 85, 145. WALLAEYS,R. 1954. Silicon, Sulphur, Phosphates. Colloquium of the International Union of Pure and Applied Chemistry. Miinster. Verlag Chemie.