The primitive hexagonal cell requires that a=b and that c is unique. The alpha and beta angles are restricted to 90 degrees and the gamma angle is 120 degrees. The hexagonal close packing is tied with the cubic close packed structure for the most efficient packing. Many pure elements crystallize in the hexagonal close packed structure including zinc and titanium. There is an ideal c/a ratio of around 1.633 that is adopted by many of these structures, but zinc has a much higher ratio of around 1.83. Why might this be?