Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Ikaite, CaCO3.6H20, is a water rich form of calcium carbonate, and dehydrates to calcite in warm or dry environments. It consists of a split pentagonal pyramid of water molecules surrounding a central calcium atoms. Two of the apices of the coordination polyhedron share oxygen atoms with a carbonate ion. The coordination polyhedron plus the carbonate contain a single formula unit and has an overall neutral charge, so the only forces holding the mineral together are hydrogen bonds between hydrogen atoms on one polyhedron and oxygen atoms in the carbonate group of another. It's pretty weakly held together. The diagram below is halfway between the a and c axes, showing polyhedra pointing alternately right and left. Background polyhedra are lighter in color.
Depending how you orient the pyramid-carbonate pair, you can picture the unit as a raindrop, an ice cream cone, or the Millennium Falcon.
Below: same view, showing hydrogen atoms in red. Only the front layer is shown. The calcium is bounded on one side by a carbonate ion and on the other by a cap of water molecules. Depending on the orientation, you can consider it a teardrop (carbonate pointing up), the Millenium Falcon (sideways), or a parachute or ice cream cone (pointing down).
Here we see the bonds to the hydrogens. Water molecule bonds are purple, hydrogen bonds to other oxygens are orange.
The view below, slightly oblique to the a-axis, shows alternating layers of polyhedra pointing right and left.
Same view as above, showing the bonds to the hydrogens. Water molecule bonds are purple, hydrogen bonds to other oxygens are orange. Only foreground bonds are shown.
Another view, 201
View looking down the a axis, showing layers alternately pointing right and left.
View down the a axis showing hydrogen atoms and bonds.
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.
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Created 15 October 2009, Last Update