Carbon Dioxide is a gas on earth, except in the form of dry ice, but it will be a mineral in the outer solar system. It has the same symmetry as pyrite, and virtually identical structure. In both cases, the cation has a face centered cubic arrangement, but the anions are canted, reducing the symmetry. The difference is that in pyrite the surrounding anions are all equally bonded to the central iron, creating an octahedral coordination. In carbon dioxide, two oxygen atoms flank a central carbon in a straight line. Since the molecules are covalently bonded, it makes no sense to define coordination polyhedra, even if the geometry is very similar to pyrite. Below is an oblique view of the carbon dioxide unit cell. Oxygen atoms are in blue, with foreground atoms darker.
Carbon Dioxide, looking along a unit cell edge. Light colored molecules are in the central plane of the cell. The top and bottom molecules have their tops tilted into the diagram; the middle row has the bottom tilted inward.
Created 22 Sept 1997, Last Update 02 May 2011
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