The Journal of Physical Chemistry, Vol. 83, No. 4, 1979 475
Calcium Phosphate Precipitation
Formation of Calcium Phosphates in Moderately Supersaturated Solutions i. P.
Feenstra and P. L. de Bruyn"
Van' t HoffLaboratory, Transitorium 3, Paduaiaan 8, Ufrecht, The Netherlands (Received September 12, 1978; Revised Manuscript Received September 12, 1978) Publication costs assisted by the University of Ufrechf
The precipitation of calcium phosphates was studied by a method which allowed a desired supersaturation to be built up under well-defined conditions. Special attention was paid to the stage preceding the growth of nonstoichiometric apatitic crystals. Growth curves at pH 6.7,7.4, and 8.5 and at 26 "C are presented. Analysis of these experimental data indicates octacalcium phosphate as an intermediate phase in the apatite formation process. Light scattering measurements suggest the formation process to be initiated by a heterogeneous nucleation step.
Introduction In the past many investigators have studied the precipitation of calcium phosphates from supersaturated aqueous solutions. Knowledge of the mechanism(s) involved is of importance for water softening and waste water treatment processes as well as for the understanding of the growth of bone mineral which contains a major constituent that resembles hydroxyapatite (HAP), Ca5(PO4),OH. At high supersaturations the formation of HAP was shown to proceed by the precipitation and subsequent conversion of an amorphous calcium phosphate (ACP).' The transformation of ACP to HAP is believed to occur by an autocatalytic, solution-mediated, recrystallization process and to be followed by a so-called proliferation period of apatitic Many assumptions have been made as to the chemical nature of this precursor phase, for example, tricalcium phosphate (TCP),5-7octacalcium phosphate (OCP),6-10and brushite (DCPD)ll were suggested as likely candidates. A t the present time it would seem reasonable to consider ACP as a noncrystalline or glasslike material of which the composition is pH dependent.12 Structural analysis of this material reveals it to possess a t most short-range order. At relatively low supersaturations, however, no precursor phases appear to be involved in the process of HAP formation.1°~13 The formation of ACP is detected by a gradual development of apalescence12in the solution after a certain lag time. The dependence of this lag time on concentration, ionic strength, and pH was reported by Termine and Posner.14 The dependence of the lifetime of ACP on the above-mentioned parameters was studied by Termine, Peckauskas, and Posner.15 It appeared that both the lag time for the formation and the lifetime of ACP are increased at lower supersaturations. At very low concentrations the appearance of ACP could only be established by observation of the Tyndall effect or UV turbidity measurements.l3 In this paper we report a study of the formation of calcium phosphates from supersaturated solutions at a constant temperature of 26 " C , an ionic strength of 0.15 mol/L, and at three pH values viz, 6.70,7.40, and 8.50. In this composition domain HAP is the thermodynamically stable phase. In order to establish as accurately as possible the concentration of soluble species in the early stages, the initial concentrations of calcium and total phosphate (pT) were chosen so that the amount of ACP formed was always very small compared to the total amount of calcium phosphate precipitate formed at the conclusion of the precipitation process. In fact at pH 6.70 and 7.40 the detectible changes in solution pH were practically only due 0022-3654/79/2083-0475$01 :OO/O
to the growth (proliferation) of the apatitic phase. Our first objective is to study the dependence of the induction period on the degree of supersaturation and pH. This experimental parameter, to be defined operationally later, is related to the ACP lag time and lifetime referred to earlier. In order to obtain reproducible and meaningful induction times (7)a procedure is developed by which a desired degree of supersaturation could be realized under well-defined mixing conditions. This approach it, also complemented by light scattering measurements on dust-free supersaturated solutions. A second objective of this investigation is a study of the kinetic changes during the early stages in the development of HAP by a continuous recording of the rate of uptake of alkali at a fixed pH. The nature of the solid phase formed during this period will be traced by X-ray diffraction, infrared spectroscopy, scanning electron microscopy and chemical analysis. Experimental Bection The precipitation experiments were performed in a reaction vessel (see Figure 1)developed by a Vermeulen et a1.16 To achieve a desired supersaturation a solutiion of Ca(N03)2(0.20-1.15 M) and a solution of KH2P0, and K2HP04(0.12-0.74 M) were adjusted to the elected final pH and then slowly injected under nitrogen pressure with a Gilson peristaltic pump or a MPL plunger pump and injection devices16 into an aqueous solution of the same pH and containing 0.15 M KNO,. The initial calcium to phosphate molar ratio (Ca/P) in the mixed solution varied from 1.55 to 1.70 but was mostly kept at 1.67 f 0.03. The initial calcium concentration was kept between 6.0 and 8.5 mmol/L at pH 6.70, between 3.1 and 4.6 mmol/L at pH 7.40, and between 1.10 and 1.54 mmol/L at pH 8.50. In all experiments solution concentrations were known to within an accuracy of 0.5%. The design of the reaction vessel is such that the iniected solutions mix with the KNO, solution at a very high speed. The time required to build up a desired supersaturation varied from 7 to about 40 min. The pH of the solution was monitored by a one-way automatic titrator (Mettler pH-stat). The concentration of C02-free KOH solutions added to the relaxing system by the titrator ranged from 0.025 to 2.00 M without affecting the shape of the precipitation curves. The same injection system used for the calcium- and phosphate-bearing solutions was also applied for the alkali addition. Ingold type 465-35 combination electrodes, which were calibrated every 4 h with Electrofact buffer solutions, were used to measure the pH of the solution. To avoid the 0 1979 American Chemical Society
476
The Journal of Physical Chemistry, Vol. 83, No. 4, 1979
T. P. Feenstra and P. L. de Bruyn
TABLE I : Dissociation Constants and Solubility Products dissociation constants, rnol/L H,PO, (20) H2P0,-(21) HPO,Z- ( 2 2 ) CaH2PO4' (23) CaHPO,' ( 2 3 ) Capo,- (23) CaOH* (24) a
solubility products
7.112 X 6.339 X 4.3 x 10-13 3 . 9 0 6 ~l W Z a 1.825 X lCJ3a 3.45 x 1 0 - 7 a 5.00 X lo-'
OCP (25) TCP ( 2 6 ) HAP (27) DCPD (28)
1.25 x lo-'' (mol/L)8 1.15 x 1 0 - 2 9 ( m 0 1 / ~ ) 5 6.3 x 10-s9 ( m 0 1 / ~ ) 9 2.1 x lo-' (mol/L)z
In the literature these values are given as association constants.
Jlr
I
t
A
Figure 1. Sketch of experimental setup: (A) pH-stat, (B) recorder, (C) automatic buret, (D) double-walled reaction vessel with baffles (E), (F) combination electrode, (G) pump, (H) titrant solutions.
initiation of nucleation by artifacts, this calibration was not doce during the induction period (Le,, between the attainment of the initial supersaturation and the crystal growth period). It was noted experimentally that during this period introduction of foreign material could seriously affect the results. If due to malfunction of the electrode the pH did drift during this stage it was noted that the length of the induction period was not affected. The automatic titrator did not add alkali to the solution as a result of a shift in pH. In most experiments except those with very long induction periods the maximum deviation due to drift never exceeded 0.01 pH units. Those experiments in which larger deviations were found were not used in the analysis. The ionic strength of the precipitating solution was maintained constant by the presence of 0.15 M KNOB. The total volume of the solution was 3 L and the temperature was maintained at 26.0 f 0.2 "C. All reagents were of analytical grade and water was twice distilled before use. Uptake of COz was prevented by continuously bubbling nitrogen gas through the solution. The stirring speed proved to have no influence on the form of the reaction curves when it was varied from 300 to 1000 rpm. All experiments were therefore done at a speed of 550 rpm. Aliquots (50-250-mL samples) were withdrawn for structure and chemical analysis. The precipitates were separated from the solution phase by centrifugation and dried at 50 "C or by filtration through 100-nm millipore paper and subsequent freeze-drying. The samples were not washed. Debye-Scherrer patterns of the solid material were recorded photographically with an Enraf-Nonius diffractometer employing Cu K a or Cr radiation and the photographs were scanned with a microdensitometer. Infrared spectroscopy was done with a Hitachi EP1-G3
spectrophotometer. The scanning electron microscope was a Cambridge S4 Stereoscan. Light scattering experiments were performed in a Fica 50. Prior to scattering measurements the samples were made dust-free by filtration through 100-nm millipore filters followed by centrifugation at 20 000 rpm in a Beckman preparative ultracentrifuge. Qualitative tests for the retention of calcium on the filters proved to be negative. The calcium content of the precipitate (after complete dissolution) was established by atomic absorption spectrophotometry with addition of strontium to diminish interference by phosphate. In the determination care was taken of matrix effects due to pdl and phosphate. Total phosphate (pT)was determined spectrophotometrically at 720 nm with the molybdenum-blue method.17 Calculations of supersaturation and solute concentrations were performed by an iterative methodls taking into account the complexes CaH,P04+,CaHPO,O, Capo4-, and also CaOH+. Activity coefficients were calculated with the formula proposed by Davies.lg The equilibrium constants and solubility products used in the calculations are listed in Table I. The supersaturation (II) with respect to a given calcium phosphate solid phase is defined as follows: ionic activity product (IP) in solution n= (1) solubility product (I&) where the following IP's may be considered:
IPDCPD= [Ca2+If2[HP042-If2
In these expressions f,represents the activity coefficient of a z valent ion.
Results and Discussion The Precursor Phase(s). With the automatic titrator a large number of experiments were run in which the development in time of the solid phase from a supersaturated solution at three different pH values (6.7,7,4, and 8.5) was recorded. Representative examples of these extent of reaction-time curves taken at 26 "C are displayed in Figure 2 (pH 6.7 and 7.4) and Figure 3 (pH 8.5). The extent of reaction a ( t )where 0 Ia I1,was evaluated by dividing the amount of alkali added at time t by the amount necessary to reach a value for IIHAp = 1. This reference point was chosen in the evaluation of a because at the pH values employed in this study HAP is the thermodynamically most stable solid phase. We note at the two lower pH values (Figure 2) a measurable lag in time in the development of the precipitate. In contrast to the behavior at pH 8.5 (Figure 3), subject to the experimental accuracy, a is seen to remain zero for a finite period of time. On going to lower calcium
Calcium Phosphate Precipitation
The Journal of Physical Chemistry, Vol, 83, No. 4, 1979 477
T A B L E 11: Parameters w h i c h D e f i n e t h e P r e c i p i t a t i o n Boundaries at
T = 26 C
a
PH
ionic strength, mol/L
total Ca, mmol/L
total PO, (Pt),
6.70 7.40 0.50
0.164 0.150 0.149
6.14 3.28 1.40
3.88 1.97 0.847
mmol/L
“1
aH,PO,
9
3.98 X 1.51 x 10-9 5.14 X Time
””[
x 10-3
x 4.13 x 10-4
t
'jel
r(
1.74 9.60
lo-’’
(rnin 1
081-
aCaZ +, m o l / L
mol/L
1200
i
--
ILL 250
OCi
1
1
500
750
Time [ min
1
Figure 2. Growth curves at pH 6.70 (11) and 7.40 (I). Initial concentrations were Cat = 4.21 mmol/L, p , = 2.50 mmol/L (I) and Cat = 6.38 mmol/L, pt = 3.96 mmol/L (11). The HP04*- concentrations were 1.68 mmol/L (I) and 1.48 mmol/L (11), T = 26 OC.
01 -
I
400 -. Time ( m i n 1 Figure 3. Growth curve at pH 8.50 and 26 OC plotted as
0
2 00
a vs. time.
and phosphate concentrations than those involved in the example at pH 8.5, the form of the a-t curves at this pH is also seen to approach that of the curves shown in Figure 2. In turn when the relevant concentrations are raised in experiments at pH 6.7 and 7.4 the trend shown in Figure 3 is noted. Presumably under these conditions ACP develops in the early stages of the reaction. A striking detail of the precipitation behavior at pH 6.7 and 7.4 is the observation that the shape of the reaction curves becomes nearly identical when equal concentrations of HP042-exid in the solution. From the above and similar curves at other initial supersaturations a characteristic “induction time” 7 may be derived. As illustrated in Figure 3 this parameter locates that time (starting from t = 0) at which the slope (da/dt) reaches a maximum value. A plot of 7 vs. pT (total phosphate concentration) a t pH 7.4, Ca/P = 1.67 f 0.02 and 26 OC is shown in Figure 4. Although the absolute value of 7 a t the higher phosphate concentrations were observed to vary somewhat in magnitude on repeating a set of measurementr, the hyperbolic shape of the curve was maintained and most importantly the same limiting pT
i
0
20
18
22
24
26
28
t o t a l p h o s p h a t e concentrotion (Pi) in m m o l / l
Flgure 4. Induction times (minutes) at pH 7.4 as a function of the total phosphate concentration. The ionic strength is 0.150 mol/L. Ca/P = 1.67. Time required to build up the supersaturation is always less than 18 min.
-
value ( 7 m ) was always obtained. It is possible that impurities in the potassium nitrate are responsible flor the deviations in 7 values. Experiments were also conducted in which after a period of 72 h no evidence of precipitate formation could be detected by the pH stat. The PT values of these tests were always smaller than the limiting value derived by extrapolation from the 7-concentration curves. These limiting concentrations could be determined with an accuracy of 2 9i . The asymptotic pT values help to define a critical supersaturation for the formation of a metastable precursor phase and thus in the location of a “kinetic” precipitation boundary. Although details of the nucleation process involved are not known it is possible to establish the composition of the precursor phase as demonstrated first by McGregor and Brown29and further elaborated upon by Walton et alS5We represent the composition of this electroneutral metastable phase by the general formula Ca(HP04)3x-2(P04)z-2x and note that for this compound, Ca/P = l/x. Based on eq 1 we may define the supersaturation for this solid
= ~ C ~ ~ ~ P O ~ / ~ H P O ~ ~ 2 ~ ~ ~ P(2) O ~ / ~ P O ~
On introducing the relevant phosphate equilibria this expression simplifies to yield W
p
=
(3)
where K and K’are composite thermodynamic equilibrium constants. If we now assume, as proposed and argued by McGregor and Brownz9 and Walton et al.,5 that the metastable precipitation boundary is fixed by the condition that I I b is a constant, then on taking the logarithm of the above expression we may write log
(aCaaOH2)
= (constant) - x log aHaPO4 (4)
From this equation, given the experimental solution concentrations the factor x and thus the composition of the metastable phase may now be evaluated.
478
The Journal of
phvsical Chemistry,
Vol. 83. No. 4. 1979
T. P.
Feenstra and P. L. de Bruyn
TABLE 111: Results of the Chemical Analysis of the Solid Phase in Experiments a t p H 6.7 and 7.4
pH 6.70
PRECIPITATE
pH 7.40
11-
=
1l"Ap
7.45 x 10'0
T= 26.0
t," min
Ca/P
5 26 147 44 2
1.51 1.53 1.46 1.58
T= 24.3
t.' min 6 16 27 89
=
350 x 10'0
Ca/P 1.38 1.41 1.39 1.44
Time corrected for induction time
PH
Figure 5.
:
Comparison 01 precipitation boundaries with the solubili
..
botherms 01 HAP, TCP. OCP. and DCW. CaIP = 1.67. Ionic strength is 0.15 mol1L.
Table I1 lists all the pertinent information which when introduced in eq 4 yields for the molar ratio Ca/P ( = l/x) a value of 1.31 f 0.03. This result suggests therefore that, subject to the chemical conditions under which our investigation was conducted, an OCP-like precursor phase was present. The significance of this analysis (based on induction times) is illustrated in Figure 5 which gives plots of log pT vs. pH. The resultant curves describe the boundary lines separating the stability regions of the various phosphate-hearing solid phases (HAP, TCP, OCP, and DCPD) from their saturated solutions (n = 1). The dashed curve describes the metastable phase boundary line derived from the ahove analysis. We note that this metastable boundary line parallels that of the stable OCP boundary line. Walton et al? applied this analysis to similar precipitation experiments and found Ca/P = 1.49 f 0.02 and suggested TCP as a possible precursor phase. The study of these investigators was, however, conducted at a higher temperature (37 "C) and a t much lower ionic strengths (3.1-7.3 mmol/L). McCregor and Brown used a similar analysis to show the presence of OCP in child hone. Supporting evidence for the presence of an OCP-like precursor was found in an X-ray and infrared analysis of the solid phase in an experiment a t pH 6.70. In this experiment the calcium and phosphate concentrations were about twice those used in other experiments. A solid sample was taken in the early stage (low a value) of the precipitation process. When viewed with Cu radiation this sample showed a strong X-ray diffraction line a t 18.6 A which is known to he characteristic of OCP."' In the infrared spectrum the bands a t 3570 and 631 cm-' which belong to the stretching and libration modes of the hydroxyl group in HAP" were absent but a t 910 and 865 cm I the two peaks characteristic of OCP;30 although weakly developed, were present. Kinetics of the Precipitation Process. During the establishment of the desired final supersaturation and the evaluation of the induction period, samples were taken in the reaction vessel. These samples were partly used for light scattering studies and partly to record any subsequent changes in pH. They were not stirred but those destined for light scattering observations were made extremely dust-free (see Experimental Section). The calcium concentrations in these samples varied between 2.07 and 4.15 mmol/L at pH 7.4 and between 4.70 and 7.90 mmol/L at pH 6.7.
...
...
b.
.
!
- 2
*..
i L.
0
- -
1 w
. .
1 p
Figure 6. Scanning electron micrographs at pH 6.70 (a) t = 26 min (corrected fw induction time): (b) f = 147 min (cwected fw induction
time).
In the light scattering experiments no change in the scattering intensity could he detected even after 14 days had elapsed from the time of sampling. Those samples not made dust-free and not stirred yielded induction times longer than those recorded in the stirred reaction vessel. The light scattering experiments indicate that a heterogeneous nucleation step is responsible for the initiation of the precipitation process. All evidence gathered in this investigation and those of other studies would suggest that this heterogeneous nucleation stage is not succeeded by a simple growth step. Termine and E a n e P recognize a period (A) during which ACP exists in the absence of any detectible crystalline phase which is followed by a conversion (period R) to an apatitic phase and concluded by a crystal growth period (C) which is also accompanied hy a change in stoichiometry. If we compare our precipitation curve at pH 8.5 with the plot of turbidity as a function of time presented by Termine and Eanes'* we can also identify these three different stages. The presence of ACP in region A (see Figure 3 ) was verified by X-ray diffraction. X-ray diffraction patterns of solid samples taken in region C indicate the solid phase to he poorly crystallized HAP:" Gradually, however, the patterns became sharper and revealed more detail thus indicating an increase in particle size and crystallinity. In all of these samples the characteristic OCP line a t 18.6 A could not he detected even when employing Cr instead of Cu radiation. Infrared spectra of these precipitates were compared with those described by Fowler, Moreno, and Brown:?O The OHstretch vibration at 3570 cm-' was always very weak. A broad diffuse peak around 875 c m ~which ' may he ascribed to HPO?. was always observed. An absorption peak a t 1900 em.', which is characteristic of carbonate" was never noted. Results of the chemical analysis of some representative samples in region C are given in Table 111. Scanning electron microscopy showed a structureless precipitate to he present initially (Figure 6a) hut with time a spongy texture composed of platelets developed as shown
Calcium Phosphate Precipitation
The Journal of Physical Chemistry, Vol. 83, No. 4, 7979 479
in Figure 6b. Similar electron micrographs were obtained Acknowledgment. The authors acknowledge Messrs. R. previously by Meyer et in seeding experiments. J. Stol and J. Dousma for stimulating discussions. We also If the major part of the a-t curves at pH 6.7 and 7.4 were thank Mr. J. Pieters for assistance in the scanning electron to describe mainly the growth of an apatitic phase, it is microscopic study and Miss M. Schipper for performing possible that the polynuclear growth model of N i e l ~ e n ~ ~ the chemical analysis. We are indebted to Mr. W. A. den could be applied. In this model the growth is described Hartog for the illustrations and to Miss Henny Miltenburg by the rate at which two-dimensional (surface) critical for typing the manuscript. nuclei are being created and then grow laterally upon a crystal face. A correction is applied for the interpeneReferences and Notes tration of the various growth centers. We attempted to (1) E. D.Eanes, I. H. Gillessen, and A. S. Posner, Nafure(London), 208, fit the experimental "growth" curves at pH 6.7 and 7.4 with 365 (1965). short induction times to this model. The Ilo chronomal (2) A. L. Boskey and A. S. Posner, J. Phys. Chem., 77, 2313 (1073). (3) E. D. Eanes and A. S. Posner, Trans. N. Y. Acad. Sci., 28, 233 (1965). of this model was found to describe the curves at pH 6.7 (4) E. D. Eanes and A. S. Posner, Mat. Res. Bull., 5, 377 (1970). and the I, chronomal, the curves at pH 7.4 quite satis(5) A. G. Wabn, W. J. Bodin, H. Furedi, and A. Schwartz, Can. J. Chem., factory. These fits according to the theory indicate that 45, 2695 (1967). (6) E. D. Eanes and A. S. Posner, Calcif. Tissue Res., 2, 38 (1968). the critical nuclei at pH 6.7 were composed of 28 and at (7) A. S. Posner, Physiol. Rev., 49, 760 (1969). pH 7.4 of 16 ions. It is, however, debatable what signif(8) H. Newesely, Arch. Oral. Biol., Spec Suppl., 6, 174 (1961). icance should be attached to the results of this model (9) H. Furedi-Milhofer,B. PurgariE, Lj. BreEeviE, and N. PavkoviE, Calcif. Tissue Res., 8, 142 (1971). analysis, because of the assumptions involved in setting (10) G. H. Nancolias and 5. TomaiiE, J. Phys. Chem., 78, 2218 (1974). up the model and also because the existence of precursor (11) M. D. Francis and N. C.Wsbb, Calcif. Tissue Res., 6, 335 (1971). phases is not accounted for in the theory. (12) J. D. Termine and E. D. Eanes, Calcif. Tissue Res., 10, 171 (1972). (13) A. L. Boskey and A. S. Posner, J . Phys. Chem., 80, 40 (1976). Conclusions (14) J. D.Termine and A. S. Posner, Arch. Biochem. Biophys., 140, 307 (1970). Light scattering experiments show that the first step in (15) J. D. Termine, R. A. Peckauskas, and A. S. Posner, Arch. Biochem. the formation of HAP from solutions supersaturated in Biophys.. 140, 318 (1970). calcium and phosphate at neutral and slightly alkaline pH (16) A. C. Vermeulen, J. W. Geus, R. J. Stol, and P. L. de Bruyn, J. Colloid Interface Sci., 51, 449 (1975). is the heterogeneous nucleation of ACP. Based on an (17) J. Murphy and J. P. Riley, Anal. Chim. Acta, 27, 31 (1962). analysis of measured induction times and of the structure (18) T. Feenstra, J. Chem. Ed., in press. of the developing solid phase convincing evidence has been (19) C. W. Davies, J. Chem. Soc., 2093 (1938). (20) R. G. Bates, J. Res. Natl. Bur. Stand., 47, 127 (1951). found that OCP is present as an intermediate in the (21) R. G. Bates and S. F. Acree, J. Res. Nati. Bur. Stand., 30, 129 conversion of ACP to an apatitic calcium phosphate. (1943). Additional, indirect evidence for the presence of OCP is (22) C. E. Vanderzee and A. S. Quist, J. Phys. Chem., 65, 118 (1961). (23) A. Chughtai, R. Marshall, and G. H. Nancollas, J . Phys. Chem , 72, furnished by the observed platelike habit of the precipitate 208 (1968). when viewed in the scanning electron microscope. It is (24) C. W. Davies and B. E. Hoyle, J. Chern. Soc., 233 (1951). well-known that OCP has a platelike habit.35 (25) E. C. Moreno, W. E. Brown, and G. Osborn, SoiiSci. SOC.Am., 24, I t has been known for a long time that quite often the 99 (1960). (26) E. C. Moreno, J. M. Patel, T. M. Gregory, and W. E. Brown, Internatlonal solid phase that first precipitates from a supersaturated Association of Dental Research Preprinted Abstracts of the 48th solution is not the thermodynamically most stable phase General Meeting, Abstract 183 (1970. under the experimental conditions. In fact this observation (27) Y. Avnimelech, E. C. Moreno, and W. E. Brown, J. Res. NaN. Bur. Sfand., Sect. A , 77, 149 (1973). forms the basis of the so-called Ostwald law of stages. (28) R. W. Marshall and G. H.Nancollas, J. Phys. Chem., 73, 3838 (1!369). Toschev and Gutzo$, have interpreted this law to predict (29) J. MacGregor and W. E. Brown, Nature(London),205, 359 (1965). that the most isotropic phase is likely to precipitate first (30) 8. 0. Fowler, E. C. Moreno, and W. E. Brown, Arch. Oral. Biol , 11, 477 (1966). from solution. This statement is in complete accord with (31) Powder Diffraction File No. 9-432, 11-184, Joint Committee of Powder the observation that ACP plays the role of precursor in Diffraction Standards, Swarthmore, PA, 1973. the calcium phosphate system. It is possible that ACP (32) R. Z. LeGeros, S. R. Contigugiia, and A. C. Alfrey, Calc. 7'i.s~.Iles., 13, 173 (1973). serves i3S a template for the heterogeneous nucleation of (33) J. L. Meyer, J. D. Eick, G. H. Nancollas, and L. N. Johnson, Cab. OCP which in turn might serve as a template for epitaxial Tiss. Res., 10, 91 (1972). growth of HAP. This latter possibility was already sug(34) A. E. Nielsen, "Kinetics of Precipitation", Pergamon Press, New York, gested by Brown et al.35in view of the great structural 1964, Chapter 4. similarity between OCP and HAP. Eanes and M e ~ e r ~ ~(35) W. E. Brown, J. P. Smith, J. R. Lehr, and A. W. Frazier, Nature (London), 196, 1048 (1962). suggested a similar mechanism based on other experi(36) S. Toschev and I. Gutzow, Krist. Tech., 7, 43 (1972). mental results. (37) E. D. Eanes and J. L. Meyer, Calc. Tissue Res., 23, 259 (1077).
