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Point Defects on Calcite

This movie of 30 individual frames shows point defects on the cleavage plane of calcite imaged in water. If you watch different parts of the image you can see individual defects appear, disappear, and move around.

One possible explanation is that these are defects in the hydration layers above substitutional defects in the crystal. Substitutional defects in calcite (say Fe for Ca) might be hard to see directly because the ionic radius is only 25% smaller. However, because water is highly polar, the water ordering should be sensitive to the charge state of the underlying ions.

If this hypothesis is correct, these defects should be sensitive to imaging force. Almost all previous AFM imaging of calcite in liquid was done in contact mode using larger forces. With higher force, the tip will push through the hydration layers. This could explain why these defects have not been observed previously. Our use of AC mode, small cantilevers with low thermal noise, and amplitudes of a few angstroms have enabled this result.

Imaged with a 440 kHz, 35 µm lever with 3 Å of amplitude. Original data rate: 13 s/frame. Z Scale: 3 Å.

 


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