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The molar absorption coefficients for the OH stretching vibrations of 16 hydrothermally prepared hydroxyfluorapatites Ca5(PO4)3(OH)xF1-x have been determined. The coefficients vary with composition, reaching a maximum at x = 0.8. At low OH concentrations, such as they are typical for most natural fluorapatites, the absorption coefficient is fairly constant with a value of about 2.4x106 cm/mole OH. With this absorption coefficient the OH concentration of a sample of natural fluorapatite from Durango, Mexico, has been determined from the infrared spectra. It was found that about 2 % of the ions in the channels of the apatite structure are OH ions. This would correspond to 0.04 weight % H2O. Measurements on single crystals and powder specimens gave almost identical results.

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