Lattice parameters and cation distribution of solid solutions of calcium and lead hydroxyapatite

Abstract

Summary. Solid solutions of calcium hydroxyapatite (CaOHA) and lead hydroxyapatite (PbOHA) of the formula Ca,0.x Pbx (PO4)6 (OH)2 were prepared by coprecipitation followed by heating at 800~ in a stream of CO2-free water vapor of 1 atm. The samples were apatitic in the range 0 < x < 6 and contained lead phosphates as a second phase at higher Pb/Ca ratios. Lattice parameters and cation distribution of the apatitic samples were determined by X-ray diffraction. The lattice parameters varied linearly with x in the range considered, whereas all Pb 2+ were located in the sixfold position for cations. There was a miscibility gap in the apatite series of solid solutions in the range 1 < x < 4, whereas apatites in the range 6 < x < 10 were not stable under the conditions of preparation. It is concluded that apatites in the range 4 < x < 6 represent a minimum in the free energy of solid solutions between CaOHA and PbOHA.

Preparation of Lead–Calcium Hydroxyapatite Solid Solutions by a Wet Method Using Acetamide

“…6(B pH after Aging (11) for PbHap. PbCaHap has been obtained at pH 6.5 (9), 11 (10), and 12 (8,11). Therefore, it is novel that PbCaHap was produced at such a low pH of 3.92}5.22 in the present study.…”

“…These sites are established in being occupied respectively by smaller and larger cations in Hap solid solution containing two kinds of cations (3). It has been reported that Pb> and Ca> ions of PbCaHap prepared by the co-precipitation method are located respectively in (II) and (I) sites (3,8,12). Consequently, PbCaHap with X .…”

Feasibility study for the detection of lead substitution sites in the hydroxyapatite crystal structure using high-resolution electron microscopy (HREM) at optimum focus

“…In this way the centrosymmetry of the structure is preserved because of the statistical distribution of the hydroxy groups above and below the mirror plane. The lattice parameters used have been determined by Verbeeck, Lassuyt, Heijligers, Driessens & Vrolijk (1981) for a 60% lead substitution (a~ = a2 = a3 = 9"668, c = 7"096 ~). For EDLA the occupancy factor was set to 0"6 for Pb atoms and 0.4 for Ca atoms in all Ca sites.…”

“…Ca(l) atoms are on the fourfold positions at (1/3, 2/3, 0.001) and the Ca(2) atoms are on the sixfold positions at (0.2464, 0.9938, 1/4) (Kay, Young & Posner, 1964). Engel (1973), Verbeeck, Lassuyt, Heijligers, Driessens & Vrolijk (1981) and Andres-Verges, Higes-Rolando, Valenzuela-Calahorro & Gonzalez-Diaz (1983) have shown from X-ray powder diffraction patterns that the Pb 2+ ions in (Ca, Pb) hydroxyapatite solid solutions occupy mainly the Ca(2) sites and the crystallinity of the solid decreased for the highest values of substitution.…”

Detection of non-hexagonal symmetry in an apatite-structure-related mineral (nasonite)

“…Often, the mixed-crystal structural details and properties cannot be obtained from endmember properties [an example is the interactions of F-, CI-and OH-reported by Sudarsanan & Young (1978)]. Engel, Krieg & Reif (1975), Verbeeck, Lassuyt, Heijligers, Driessens & Vrolijk (1981) and Andres-Verges, Hijes-Rolando, ValenzuelaCalahorro & Gonzalez-Diaz (1983) have shown from X-ray powder diffraction patterns that the Pb 2÷ ions in (Ca,Pb) solid solutions occupy mainly the Ca2+(2) sites and that the crystallinity of the solid decreases for the highest values of substitution. Nasonite [Pb6Ca4(Si207)3C12] is a mixed (Ca,Pb) mineral showing 60% lead substitution and with strong structural and chemical relations with pyromorphite and the apatite group.…”