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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.

By applying the Weissenberg technique to small pieces of duplex crystals of FeA1 and FeA12, produced by directional eutectoid decomposition, it has been found possible not only to establish the previously unknown unit cell parameters of FeA12, but also the orientation relationship between FeA1 and FeA12. The unit cell of FeAI 2 can be described as an A-centred pseudo-monoclinic cell with a = 7.594, b = 16.886, c = 4.863 A, a = 89.55 ° ,/3 = 122.62 ° and 7 = 90-43°. The orientation relationship between both phases was found to be: [100]FeA12 II [i ll]FeA1 and (002)FeA12 II (01])FeA1. This relationship was confirmed by texture goniometry of larger directionally decomposed eutectoid samples in which the cubic FeA1 phase was frequently found to be twinned.

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