Cleavage

Cleavage refers to the tendency for crystal to break along certain directions. Each 'direction' is a plane that intersects the surface. Although cleavage can be difficult to identify, the trained eye learns to recognize it by the way light is reflected from the broken surface of the crystal.

	Irregular Surface: Light rays coming from one direction are scattered in different directions, so the surface looks dull.
	Flat Surface: Light rays coming from one direction are all reflected in the same direction, so they the surface looks shiny like a mirror.
	Cleavage Surface: Since the apparently irregular surface is really made of flat, parallel surfaces, light rays are reflected in the same direction, so the surface looks shiny like a mirror, and gives a 'flash' when turned in the direction that catches the light in just the right way.

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Here are some photos and movies of specimens from the lab kit that show you what to look for in identifying cleavage.

Specimen C

Here you can see the mirror-like surface of specimen C with the sky reflected in it. As you hold the sample, rotate it slightly in all directions until you see a flash of light reflected from the flat cleavage plane. This specimen has 'perfect' cleavage in 1 direction. For a larger view, right click the photo or click and hold the left mouse button and select "view image"

Specimen D

Here is specimen D in a position where the cleavage plane is not apparent. For a larger view, right click the photo or click and hold the left mouse button and select "view image"

Here is the same specimen rotated slightly to the right to catch the 'flash' of lighe from the flat cleavage surface. This mineal also has 'pefect' cleavage, but in this case there is more than one direction of cleavage. To see the others hold the crystal on each of the other two sides and rotate it until you can see the 'flash'. For a larger view, right click the photo or click and hold the left mouse button and select "view image"

Specimen A

	Here is specimen A in a position where the cleavage plane is not apparent. For a larger view, right click the photo or click and hold the left mouse button and select "view image"
	Here is the same specimen rotated slightly to the right to catch the 'flash' of lighe from the flat cleavage surface. This mineal also has 'pefect' cleavage, but in this case there is more than one direction of cleavage. To see the others hold the crystal on each of the other two sides and rotate it until you can see the 'flash'. This mineral has perfect cleavage in one direction and good cleavage in another. It is difficult to find the second one on some specimens. For a larger view, right click the photo or click and hold the left mouse button and select "view image"
	This is an mpg movie that shows the two direction of cleavage in speciman A. Watch for the slight 'flash' as the crystal is rotated first in one direction, the moved to show the surface shown in the above photos.

Specimen J

	This is specimen J. It is more difficult to see the flat surfaces of the cleavage planes in this mineral because the cleavage is 'poor'. For a larger view, right click the photo or click and hold the left mouse button and select "view image"
	But when it is rotated to catch the light reflected from the tiny flat surfaces that are like the treads of steps, it becomes more apparent. If you look at this specimen with the magnifying lens (loupe) you will see that the surface is actually composed of many little 'blocky' steps. The steps are the result of two directions of cleavage, as shown in the illustration at the beginning of this page.. If you look very closely with the loupe you can estimate the angle between the two cleavage directions. For a larger view, right click the photo or click and hold the left mouse button and select "view image"
	In this short mpeg video clip you can see the surface appear to 'light up' when the specimen is rotated so that the light is reflected off of the parallel cleavage surfaces.

Specimen B

Not all flat surfaaces represent cleavage planes. Some of them are crystal faces that result from the growth of the crystal rather than from it being broken. Specimen B is an unbroken crystal. But if you look at the bottom (the side opposite the pointed end) you will see that it is broken in an irregular way. Although there may be flashes of light from the broken surfaces, they do not all 'light up' from the reflections at the same time, in the same orientation the way the crystals that show cleavage do. For a larger view, right click the photo or click and hold the left mouse button and select "view image"