The crystal structure of kyanite, Al 2 SiO 5 , has been refined by three-dimensional least-squares techniques. Refinement was based on observed data measured using a single-crystal counter diffractometer, and reduced the disagreement factor, R , for 981 hkl reflections to 8.6%. The triclinic kyanite structure proposed in 1929, based on a distorted close-packed oxygen framework, was found to be fundamentally correct. Aluminum cations fill 40% of the octahedral interstices in such a way that half of the occupied octahedra form single chains parallel to the c axis similar to those in andalusite and sillimanite. The arrangement of aluminum cations yields 10 independent shared octahedral edges ranging in length from 2.485 Å to 2.633 Å (±.01 Å). The corresponding Al–Al separations vary from 2.88 Å to 2.763 Å. Silicon atoms occupy 10% of the tetrahedral interstices. The Si–O distances range from 1.611 Å to 1.644 Å (±.005 Å), and the average of eight independent Si–O distances is 1.628 Å.