|The structure of calcite is not very complex but can be hard
to visualize. It is sometimes described as a "modified NaCl"
structure, in about the same sense that an antique that has had every part
replaced is still an antique!
On the left is the structure of NaCl, with sodium atoms purple and chlorine atoms green (although since both have the same atomic arrangement, it hardly matters.) On the right is calcite, with calcium yellow, oxygen blue and carbon gray. We can see that there are rows of alternating Ca and CO3 units, just like Na and Cl alternate in halite.
Of course, calcite is not cubic. The carbonate groups break up the cubic symmetry in several ways. First, their three-fold symmetry axes line up with only one of the symmetry axes of the cube (in red). Second, they alternate in orientation (shown by the two shades of gray). Most important, the wide spacing of the carbonate groups stretches the atomic planes and distorts the cube into a rhombohedron.
|At left is the more familiar rhomobohedral unit cell. The
calcium ions have a distorted face-centered cubic arrangement. Recall that
a rhombohedron is a cube distorted along one of its three-fold axes. Ions
on the nearest face are yellow, with light and dark green denoting ions on
planes further to the rear.
Only one CO3 unit is shown, in the center of the rhombohedron. The others would be centered in the middle of each edge.
The radial purple lines are in the plane of the carbonate radical and connect to the midpoints of three edges related by threefold symmetry.
|This view shows the complete unit cell with all carbonate
radicals. Carbonate radicals centered on the front edges are lightest,
those on the rear edges darkest.
This is the cleavage unit cell but it is not a true Bravais lattice cell since not all the radicals point in the same direction.
Another way to visualize the structure of calcite is that the calcium atoms are octahedrally coordinated with the oxygen atoms in the carbonate anions.
It makes almost as much sense to consider calcite as derived from corundum or hematite rather than halite. Above is a layer of carbonate ions with gray symbolizing carbon and the surrounding oxygens colored yellow, green and blue. Calcium atoms (purple) sit atop half of the octahedral interstices and others (shown as if the sheet were transparent) occupy the remaining interstices on the underside of the sheet.
Compared to corundum or hematite, half the octahedral interstices are vacant. We can get away with this because carbon ions bind the oxygens as well. Also, in real calcite the carbonates aren't close-packed, owing to the large radius of calcium ions. Below is a more realistic view.
Created 22 Sept 1997, Last Update 22 Sept 1997
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