|Dolomite has almost exactly the same structure as calcite except
that layers of magnesium and calcium atoms alternate. The alternation means
a complete unit cell of dolomite is not the same as a cleavage rhombohedron.
In the view at left, calcium atoms are green and magnesium are orange.
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 dolomite structure with carbonate
radicals. Carbonate radicals centered on the front edges are lightest,
those on the rear edges darkest.
The cleavage unit cell 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. here calcium octahedra are green and magnesiums are orange. Why yes, the thickness of the magnesium sheet is less because of the smaller radius of the Mg ion, thank you for asking.
Created 22 Sept 1997, Last Update 22 Sept 1997
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