INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1931
Literature Cited International Critical Tables, Vol. IV, p. 259, McGraw-Hills Ibid., Vol. V, pp. 88-91; Vol. VII, p. 303. Ibid., Vol. V., p. 101. I b i d . , Vol. V., pp. 178-9, 203-4. Ibid., Vol. V, p. 204. I b i d . , Vol. V I I , p. 238. I b i d . , Vol. VII, p. 239. Ibid., Vol. V I I , p. 240. (9)I b i d . , Vol. VII, p. 241.
(1) (2) (3) (4) (5) (6) (7) (8)
1926’
(10) (11) (12) (13) (14) (15) (IS) (17) (18) (19)
1413
Ibid., Vol. VII, p. 306. Lewis and Randall, “Thermodynamics,” p. 108, McGraw-Hill, 1923. Lewis and Randall, I b i d . , p. 173. Lewis and Randall, Ibid., p. 255. Lewis and Randall, Ibid., p. 298. Lewis and Randall, Ibid., p. 344. Lewis and Randall, Ibid., p. 347. Mehring, Ross, and Merz, IND. END.CAEM.,21, 379 (1929). Randall and Rossini, J . A m . Chem. Soc., 61,335 (1929). Trautz and Gerwig, 2. unorg. ullgem. Chem., 134, 409 (1924).
Structural Characteristics of Apatite-Like Substances and Composition of Phosphate Rock and Bone as Determined from Microscopical and X-Ray Diffraction Examinations‘ S. B. Hendricks, W. L. Hill, K. D. Jacob, and M. E. Jefferson BUREAUOF CHEMISTRY AND SOILS, DEPARTMENT OF AGRICULTURE, WASHINGTON, D . C.
Animal bone, free from organic matter, is a carbonate American continental phosphate rock consists essentially of submicrocrystalline fluorapatite, C:al~F2(PO4)6, apatite, Ca~oCos(Poa)s.HzO, isomorphous with fluorcan be prepared by which contains some excess fluorine and a small amount apatite. Oxyapatite, Cal~O(P04)6, of sodium. The compounds Calo(OH)z(PO,),, Cas(H2O)Z- heating hydroxyapatite or bone to constant weight at The apatite-like diffraction pattern of tri(P04)6,and CaloCOs(P04)e.H20 form extensive series of solid 900” C. solutions with fluorapatite, some members of which occur calcium phosphate hydrate is destroyed if the comin geologically recent rocks. The phosphate rock of Curacao pound is heated to constant weight at 900’ C. These Island is essentially a hydrate of tricalcium phosphate, various compounds in natural or artificial mixtures Cas(H20)n(PO&. Hydroxyfluorapatite, Calo(F,0H)~(P04)e,can be identified by means of their x-ray diffraction in which only about half of OH- is replaced by F-, occurs patterns. on Christmas, Nauru, and Ocean Islands.
HE phosphate rock deposits of the world have usually been described as consisting of various amorphous and microcrystalline apatite-like minerals. Lacroix (25) considered certain French phosphorites to consist essentially of colloidal tricalcium phosphate, collophane, crystalline staffelite (francolite), and crystalline dahllite (podolite). Schaller (58) concluded that phosphorites consist of the following compounds in solid solutions:
T
Dahllite Francolite Collophane Fluorapatite Hydroxyapatite
9Cs0~3P~Oa~CaCOs~HzO 9Ca0.3PzOrCa(F1,COa).H?O 9Ca0.3PzOsCaCOs~HzO xHzO SCa0.3PzOs~CaFz 9CaO.3PzOvCa(OH)r
+
Rogers (35) writes: “The principal constituents of phosphorites or so-called phosphate rocks and also of fossil bones is an amorphous substance with properties sufficiently characteristic to be entitled to recognition as a distinct mineral. I t may be called ‘collophane.’ ” He gives as the approximate formula of collophane: 3Ca3(PO4)~1-2Ca(C03,Fz,S04,0).xHz0
A number of poorly defined minerals have been described as occurring in phosphate rock. Some of them (17) are: Voelkerite Dahllite Podolite Staffelite A fibrous form Collophanite
1
CaloO(POd8
3Cas(POi)z.2CaCOa.HzO 3Cas(PO4)zCaCOz
~C~S(PO~)Z.C~COXC~F~.HZO of francolite (3Cas(P01),)~.(2CaF?C03) y2Hz0 (also known‘as-pyroclasite nauruite, pyroguanite. sombrerite, monite, fluocollophanite: and lewistonite)
Received July 27, 1931.
The results of chemical analysis indicate that bone is either carbonate apatite (Ca10C03(P04)6),hydroxyapatite (Ca,,. (OH)2(P04)6), and calcium carbonate; or tricalcium phosphate and calcium carbonate (11, 13). Fossil bone has been described as collophane (35). Since the compounds just mentioned are closely related chemically, it is difficult to identify them in naturally occurring sub-microcrystalline materials. The small particle sizes of the non-phosphatic impurities render difficult the interpretation of chemical analyses. This makes it practically impossible to determine from chemical and microscopic analyses alone the correct distribution of fluorine and carbon dioxide between the phosphate minerals and possible fluoride and carbonate impurities. The use of inaccurate methods for the determination of fluorine (22) and the difficulty in accurately determining combined water in phosphate rocks has frequently, indeed usually, led to incorrect conclusions concerning the nature of the compounds present. The authors have used microscopical, analytical, and x-ray diffraction methods for determining the nature of the com. pounds present in phosphate materials; the application of the three methods usually leads to an unambiguous answer. Microscopical examination is of great value for the identification of minute quantities of crystalline substances, but the finely divided character of many of the compounds present in phosphate rock limit the applicability of this method. Chemical analyses are open to the objections mentioned previously. X-ray powder diffraction photographs are of some value for the identification of impurities present in amounts greater than about 2 per cent. They are of far
INDUSTRIAL AND ENGINEERING CHEMISTRY
1414
Silica was determined by the modified Berzelius method developed by Hoffman and Lundell (18). Fluorine was determined by the volatilization method (33, eo), which has been used extensively in analyses of phosphate rock (22, 32). Although it is the most satisfactory method available for the determination of fluorine in the usual commercial grades of phosphate rock, it apparently gives results that are about 6.5 per cent too low, even under the most favorable conditions. The results in Table I have been corrected by this amount. A volatilization method had been used earlier by Carnot (5) in his extended researches on phosphate minerals (6). Free lime was determined by the method of Lerch and Bogue (27). The optical examinations were made with a petrographic microscope. The amounts of impurities were usually estimated by comparison with standard samples, but in some cases counts were made.
greater value in characterizing the principal crystalline constituent in the material under examination. A number of apatite-like compounds were synthesized in order that their diffraction patterns might be obtained and compared with those of phosphate rocks. These compounds probably delineate the region of solid solutions, including the various poorly d e h e d minerals mentioned in the foregoing. Phosphate Material Used
APATITE-The chlorapatite, sample 634, was a museum specimen from the Bamle Mines, Kragero, Norway. The fluorapatite, sample 905, was from Quebec Province, Canada.
0
-
0
/c',Oxygntom 2.1 above /reflexion plane
I \
Oxygen ions on reflexion plane
Oxyg n ions 1.3 abom and below reflexion plane
!/
d
Oxygen
Calcium ions
Fluorine ions
ol
COS group
@
PhosphoruJ ions
Carbon of CO, qroup
B
A
Fi~iure 1 --representation of structure of fluorapatite as projection on (0001) ( 3 1 ) representation of structure of carbonate apatite a5 projection on (0001) ~~
A-Partial B-Partial
Vol. 23, No. 12
PHOSPHATE ROCK-A large number of samples of different grades and types of phosphate rocks from deposits in the United States and various West Indian and Pacific Ocean Islands were examined, but detailed data were obtained for only eight samples. With the exception of sample 1011 which was a prospect sample from a deposit near Garrison, Powell County, Mont., these were obtained from shipments of commercial material. The Florida land-pebble phosphates, samples 912 and 947, were from deposits near Mulberry and Brewster, in Polk County. Sample 906, Tennessee brownrock phosphate, and the Tennessee blue-rock phosphate, sample 930, were from deposits near Wales, Giles County, and Gordonsburg, Lewis County, respectively. The Wyoming phosphate, sample 948, was mined near Cokeville, Lincoln County. Bom-sample 1100 was prepared by steaming commercial naphtha-extracted bone, sample 1123, under pressure of approximately 40 pounds per square inch (2.8 kg. per sq. cm.) for about 225 hours. SYNTHETIC CALCIUM PHOSPHATES-TriCalCiUm phosphate, sample 1095, was prepared by slowly adding a solution of pure trisodium phosphate to a solution containing an excess of calcium nitrate. The precipitate was washed with a saturated solution of tricalcium phosphate until the filtrate was free of nitrates. The salt was dried a t 50" C. Hydroxyapatite, sample 1117, was prepared by hydrolyzing tricalcium phosphate with neutral ammonium citrate solution (21). Experimental Details
With the following exceptions, the chemical analyses were made by methods generally used for analysis of minerals and rocks (16, 16):
~
The x-ray powder diffraction photographs were made in small cylindrical cassettes (radii, 3.45-3.50 cm.). The photographs, reproduced as Figures 2-4, were obtained by use of CuK radiation. Other photographs were made with CUR radiation, with FeK radiation, and with MoK ala2 radiation.
Since the several compounds studied had approximately the same lattice dimensions as fluorapatite, it was possible to calculate the masses ( M ) associated with the units of structures from density ( p ) determinations:
M = VNp where V = volume of unit of structure and N = Avagadro number The densities were determined by suspending the finely ground material in pure methylene iodide and heating under moderate vacuum in order to dispel occluded air.' The suspension was then centrifuged, and acetone was added until the mass of the material remained suspended after prolonged centrifuging. I n the case of the materials examined, this method would probably give a minimum value for the density. Structural Characteristics of Apatite-Like Substances
The crystal structure of the well-defined mineral, fluorapatite (CaloFz(PO&), has been determined by NBray Szab6 (31). A partial representation of the structure is shown as Figure 1A. The PO4 groups have regular tetrahedral forms and are independent. The fluorine ions are co-planer with and equidistant from 3 calcium ions a t a distance of about 2.35 A. The calcium ions are of two types, 4 being surrounded prismatically by 6 oxygen ions of closest approach and 6 by irregular polyhedrons of 5 oxygen ions and 1 fluorine ion.
A somewhat different structure for fluorapatite has been suggested by Mehmel (SO). In this structure the fluorine
INDUS'TEIAL A N D ENGINEERIXG CHEMISTRY
December, 1931
ions axe displaced hy c from the positions determined by X'8ray Szab6. A fluorine ion is equidistant from 6 calcium ions surrounding it at the corners of an octahedron. The calcium and oxygen ions are arranged as just described.
Teniieirer brows phosphate mek
pebble phosphate rock
Florida
Mniilana phosphate
rock
Calcium fluoride
Fieure 2---Porrder-Diffracflon Phofoi3rsphs (CuK Radlatloo) of Crystalline Fluorapatlteand of Some Arnerluln Phosphate Rocks Calcivm fluoride is pcerent in Montana phosphate rock
It was considered best to check the crystal structure of apatite, since the previous structnres had hcen determined in rather indirect manners. Replacement of fluorine, calcium, or phosphorus ions gives rise to a number of compounds isomorphous with, and perhaps structurally similar to, apatite. The predominant effect of heavy atoms, in some of these compounds, upon the intensities of the x-ray diffraction maxima allows their ready allocation in the units of structure. This is the case for the lead ions in pyromorphite, PbIoC1, (PO&, and for the lead and arsenic ions in mimetite, Pbir C12(AsO&. The arrangements of these ions closely approximate that of the calcium, chlorine, and phosphorus ions in the structure of fluorapatite suggested by Mehmel; fur mimetite the arrangement is as follows: 4Pb BPb 0Aa 2CI
e,
el
e.
DCBICCS
Degracs 240 0 141 0
Lhgrccs 0
120 90
148 0
YO YO
1415
slag (N), can be made by lieatiiig hydroxyapatite or carbonate apatite to constant weight at 900" C. Strictly speaking, it probably is not isomorphous with point group Csr, but nevertheless its diffraction pattern closely resembles that of hydroxyapatite (Rgure 3). There are some slight differences in intensities of reflections that perhaps can he seen from a close inspect,inn of the photographs in Figure 3. The rrreight associated with the unit of structure, which is calculated from the density of oxyapatite ( P = 3.17) and the lattice dimensions of fluorapatite, is 1014 (calculated for (PO&, 987, in poor agreement because of dec.rcase in the volume of the unit of structure (note Figure 3). Althongh Bassett (3) clearly demonstrated the presence of Ca3(POa)2-xH20as a solid phase in the system CaO-P,OsH,O, there has subseqiiently been discussion concerning the possibility of the formation of a solid solution series hetxeen CaKPO,.xH,O and Ca,o(OIl)r(P04)6,as suggested earlier by Cameron, Seidell, and Bell (4). The diffraction pattern of tricnlcium phosphate, sample 1095, is closely siiiiilar to that of fluorapatite (Figures 2 and 4). The filler details of the pattern are obscured by tile increased line width caused by the very fine graining of t.he rnaterial (particle size 10-4 to 1 0 V cm. on the edge). The chemical analyses, Table I, show that the molecular ratio CaO:P20sis very close to 3 : l . The x-ray photographs indicate that the material is homogeneous and specifically demonstrate the absence of crystalline acid calcium phospliate impurities in amounts greater than 2 per cent. The weiglit associated with the unit of structure as determined from the density, p = 3.01, is 952. The material must be E hydrate of tricalcium 'phosphate, Ca~(IQg)z~xHzO, structurally similar to fluorapatite, Ca,, Fs(PO6)a. It is most probahle that the formula is C ~ V (H,O)dPOSs, molecular weight 966.4. For structural reasons such a compound would he expected to form a complete series of solid solutions with Ca,o(OH)t(P04)8, as has been observed by Bassett.
Chlorhpatite
Hydroxyapatite
Steamed-bonc caibonnte apatite
0
At this junoture it is unjustifisble to conclude that fluorapatite has a structure similar to mimetite, since the two suhstances might not he structurally isomorphous. Since the remainder of this work does not depend upon differentiating between these possibilities, the matter has been left undecided. The replacement of the fluorine ions in fluorapatite hy the larger and heavier chlorine ions changes both the intensities of interference maxima and the lattice dimensions. The effectsare readily noticeable on the powder-diffraction photomaohs ., . of chloraDatite in comDarison with fluoranatite (Figures 2 and 3). Hydroxyapatite, Calo(OII)2(PO&,is the only congruently soluble compound in the system CaO-PsOOsHzO (9,88). The x-ray powder diffraction photographs of this material are closely similar to, but distinguishable from, those of Auomptite. This is to be expected, since both the diameter and the scattering power of the hydroxyl ions are approximately the same as those of the fluorine ions. Oxyapatite, CaiaO(PO&, a compound found in Thomas
Ignited-bone oxyspstite
Ignited hydro5ypapatitcoxyapatm
Figure 3-Powder-i)tffraetion Phofo&rLrapha (CuK Radiation) of Cvetalllne Chlorapaftte. Bone. and Some Synthetic Apatlta-Llke Subsrancea
The hydrated tricalcium phosphate is changed to Car (PO& by heating to constant weight a t 900' C. The x-ray powder diffraction pattern of the anhydrous material ia markedly different from that of fluorapatite (Figure 4). This behavior upon heating is a characteristic distinguishing the hydrated tricalcium phosphate among the apatite-like compounds.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1416 ANALYSIS
-
Table I-Total
PI06 F CI
cor
SOa NarO
Analyses of S o m e P h o s p h a t e Rocks a n d S y n t h e t i c Material6 SAMPLE AND TYPEOF MATERIAL
1221, crvst.
-~ ma-
947, Florida pebble
CaO MgO
Vol. 23, No. 12
912, Florida pebble
906, Tennessee brown
930, Tennessee blue
948, W,YOmrng
1011,
Montana
943, 634, 451, Ocean Curacao' chlorIsland lsland apatite
1117, terial hyfrom droxy Curacao. apatite rock
905, fluorapatite
%
%
%
%
%
%
%
%
%
%
%
%
47.20 0.20 31.28 4.04 0.01 3.70 1.22 0.59
49.05 0.08 35.37 3.95 0.003 1.48 0.30 0.15
47.61 0.10 34.39 3.77 0.01 1.20 0.61 0.22
45.38 0.36 30.97 3.80 0.01 2.18 1.27 0.37
46.22 0.08 30.19 3.54 0.03 4.13 1.35 0.64
44.65 0.00 27.63 6.98 0.01 0.75 0.13 0.16
64.08 0.00 40.32 2.97 0.01 1 06 0.00 0.49
49.50 1.70 40.66 0.38 0.14 2.73 0.72 1.23
52.97 0.29 40.50 0.17 4.13 0.00 0.00 0.22
52.40 0.46 40.30 3.26 0.16 1.51 0.00
50.96
34.40 0.32 28.13
1.60 0.00
0.00
...
0.00
0.69
0.45
0.56
1.01
3.08
... 0.60 ... ... ... ...
0.00
0.09
0.00
0.54
1.95 103.72
8.30 100.72
0.11 0.37 0.43 0.19 0.35 KrO 7.11 9.13 7.48 7.59 7.19 Si01 1.17 1.22 1.05 0.97 1.19 Alto* Fe9Oa (total 1.69 0.70 3.42 3.42 0.87 Fe) TiOr 0.060 0.05 0.088 0.10 0.064 None None 0.00 2.09 0.52 S (pyrites) S (volatiles as 0.01 0.02 0.00 Trace 0.17 Has) 0.012 0,017 0.066 0.015 0.004 MnO 0.009 0.009 0.005 0.005 0.12 CrrOs 0.01 0.01 0.00 0.005 0.12 VIOi Ignition loss 3.34 3.78 5.99 2.79 7.52 at 10000 c. 103.44 104.51 104.77 102.63 104.31 Total COS, oxygen equivalent 3.78 3.14 2.79 5.40 5.62 of F and C1 Total cor100.63a 99.49 100.65 99.37 99.650 rected a With certain assumptions concerning behavior of Fe&.
0.22 17.30 0.99
trace 0.40
0.04 0.50
...
0.10 1.16 0.98
2.16
0.20
0.45
0.18
.... ..
... ...
... ...
2.15 103.13
...
... ... ... ... ... ...
2.94 102.47
... ... ... ... ... ...
5.19 103.24
... ... ... ... ... ...
0.48 101.18
... ...
0.67
38.47
...
...
... 0.13 ... ... .... ..
... ...
% 47.78
. .. .. ..4 6.. :..k .3.09 ...
%
%
49.99 0.11 38.68
44.58 3 28 38.42
.. .. .. .. .. .. ... 01.54 .98 1.86
0.12
0.62
. . . . . . . . . . 0.47 .. 0.09 ... 0.13 0.27 .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . . . . .. .. .. .. .. .. .. .. .. .. .. ..
37.61 10.97 8.56 102.18 99.77 99.14
5.43 97.45
3.68
2.31
2.92
0.98
2.92
1.50
3.09
0.00
0.12
...
99.45
100.16
100.32
100.20
100.80
99.22
99.09
99.77
99.02
95.91
Inorganic Composition of Bene
It has long been known that the CaO:PzOs ratio in bone is greater than that required for tricalcium phosphate (I, 9, 10, 84,34). The presence of 2-5 per cent COzrequires a distinction to be made among the following possibilities: (1) Ca8(PO&.xH20,with amorphous or crystalline CaCOs. ( 2 ) Cato(OH)2(PO&, with amorphous or crystalline CaCOs. (3) Carbonate apatite, or carbonate hydroxyapatite.
The results of chemical analyses and ossification experiments support the concept that the inorganic composition of bone is essentially that of carbonate apatite. Bassett (3) believed, however, that the results of his experiments and other evidence available a t that time were in best agreement with the second possibility mentioned in the foregoing. The x-ray observations of DeJong and others ( 8 , 3 7 )showed that the principal crystalline constituent of bone was an apatite-like substance. This is in agreement with any of the three possibilities given in the foregoing. X-ray observations on the mineral dahllite ( 4 l ) ,described as carbonate apatite, and usually considered to be similar to bone, indicate that it is closely similar to apatite. Naphtha-extracted bone (sample 1123) and bone steamed for 225 hours (sample 1100) each give an apatite-like diffraction pattern (Figure 3), with no indication of crystalline impurities in amounts greater thap 2 per cent. The particle size in naphtha-extracted bone is such as to give considerable broadening of the diffraction maxima. Prolonged steaming changes the Pz05:C02ratio (note Table I) and increases the particle size of the principal crystalline compounds of bone. The more clearly defined diffraction pattern of this steamed material closely resembles that of hydroxyapatite (Figure 3). The apatite-like diffraction pattern is retained when either the steam-extracted or the naphtha-extracted bone is heated to constant weight at 900" C. The behavior just described-the possible formation of oxyapatite-cannot be explained by Artini's article ( 2 ), unless tricalcium phosphate reacts a t 9000 C. with calcium oxide. The results of experiments designed to test this possibility are summarized in Table 11. Powder photographs of the ignited bone give no evidence of crystalline impurities, although cal-
cium oxide produced from amorphous calcium carbonatr should be crystalline. It is thus probable that bone is essentially a-carbonate apatite. Table 11-Free L i m e in Ignited Bone a n d S y n t h e t i c Mixtures [All samples were ignited at 900' C. for 1.5 hours (constant weight)] SYNTHETIC MIXTURES WITH CaCOa A N D TrjcalBONE cium Hydroxy- Naphthaphosphate, phosphate, extd., Steamed. 1095 1117 1123 1100 COI before ignition, % 4.31 4.42'3 3.09 1.50 8.52 7.87 8.46 25.6 PsOa:Con CaO equiv. of COI, % 5.50 5.64) 3.94 1.91 Free'lime found as CaO, % 5.47 5.30 ' 0.52 0.37 a 0.12 per cent COz in sample originally. b 0.14 per cent from hydroxy phosphate.
Chemical evidence supporting this conclusion, in addition to that to be found in the literature, is afforded by measurements of the citrate solubilities (19,ZO). The citrate solubility of commercial bone ash, expressed as the amount of P,Ob dissolved from 2-gram samples by 190 cc. of neutral ammonium citrate solution in 30 minutes a t 65" C., is 44 mg. (two Samples). This value is markedly smaller than that found for ignited (900" C., 236 mg.) and unignited tricalcium phosphate (241 mg.), synthetic hydroxyapatite (107 mg.), and steamed bone meal (202 mg.), but does not differ greatly from the value for phosphate rock (25 mg.), and ignited hydroxyapatite 900" C. (78 mg.). The mass associated with the unit of structure of the apatite-like constituent of bone ( p = 3.25 for naphtha-extracted bone) is 1030, in agreement with the formulas CaloCOa(P04)8 and Cal0CO3(PO4)6.H20. The value calculated, for Cal~(OH)~(P04)e is 1005, for C ~ S ( H ~ O ) Z (isP 967, ~ ~ ) for ~ CaloCOa(P04)6is 1031, and for CaloC03(P04)6~Hz0is 1049. It is thus very probable that normal bone is a carbonatephosphate substance, and for structural reasons it is probably CaloCOs(P04)rH~0. A possible atomic arrangement in the compound C a l r (COs)(P04)e.H20 is shown in Figure 1B as a projection on (0001). In order for microscopic neutrality and ionic packing to obtain, the atomic arrangement must simulate that of mimetite rather than of fluorapatite (31). It would be expected that carbonate apatite could form solid
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1931
1417
Table Iii---Microacopicai and X-Ray Diffraction Charactertsflcs of S o m e Phosphate Rocks X - R I Y DIIYRACTIOX P*TTLI(N
Snup~s
Tup. OP M ~ r ~ a r a ~
C a Y s r A L n N B IIPUO-ITSBY
RBMdP=S
Calcite Quartz Gypnum Fluorite
947
912
Low-grade Florida pebble Nigh-Grade Florida pebble
..
