This paper describes a straightforward method for the identification of the Miller indices associated with the face of a crystal of low symmetry. The method relies on orienting a crystal face parallel to the sample holder in the goniometer of a powder diffractometer. An intense diffractogram resulting from a single family of lattice planes parallel to the crystal face is obtained. The application of silicon powder to the surface of the crystal provides a means to correct 2 0 values for any

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... ignment of the crystal face. Calculated 2 0 and intensity values are used to assign the reflections from the corrected single-crystal diffractogram to a particular set of related lattice planes. In this paper are listed the corrected experimental 2 0 and intensity values for a particular face of a large single crystal of monoclinic hydroxylapatite and the theoretical 2 0 and intensity values for the hOO set of planes. Comparison of these parameters leads to the unambiguous assignment of that particular crystal face to the hOO Miller index.