Cinnabar (HgS) superficially looks like a cubic close-packed array of sulfur atoms with mercury filling the octahedral voids. Closer inspection shows that the octahedra are slightly flattened along a three-fold axis, giving it a rhombohedral symmetry (a rhombohedron is a cube distorted along a three-fold symmetry axis. The thickness of a regular octahedron (perpendicular to opposing faces) is 0.816 times the edge length. The spacing of the layers of sulfur atoms in cinnabar is 0.764 times the edge length of the Hg-S polyhedron. So the Hg-S polyhedra are flattened by only about 6% compared to a regular octahedron.
Created 22 April 2013, Last Update 05 Jun 2013
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