Cleavage … and Fracture

Did that catch your eye?

Yes, this is a geology post. I’m thinking about cleavage because it came up in class today. I would really like to do a demonstration of cleavage by smashing a calcite crystal to bits in front of the class. Don’t think that’s going to happen, alas.

But since I was thinking about it, I thought I’d write about it. So…

Cleavage (in the geological sense) is the property of some minerals to break along specific planes in the crystal. These planes of weakness exist because the bonding between atoms making up the mineral happen to be weak there. Since all minerals have different compositions and arrangements of atoms, cleavage may or may not be present, and when present, can have lots of different forms.

Here. Look at this video of calcite being hit with a hammer. It starts as a rhombic crystal (like a leaning cube). When it gets hit by the hammer, it breaks into smaller rhombic crystals.

Those flat surfaces are the cleavage surfaces. In the case of calcite there are three planes, which cause the crystal to break into leaning cubes.

The mineral halite (or rock salt) also has three cleavage surfaces, but they’re all perpendicular to each other, resulting in little perfect cubes when the crystal is broken. Yeah, this video is kinda sucky, but you see that the broken crystal still has the planes at right angles to each other.

The number and relative orientations of these cleavages is one important property that is used to identify minerals.

Minerals like fluorite have four planes of cleavage.

Mica minerals have one plane of cleavage.

Some minerals have no cleavage. Quartz is the best known example of this. Instead of cleavage, such minerals exhibit fracture, which is often seen as a dished out depression. This is called concoidal fracture.

The fact that quartz has no cleavage is why it is used so often for making arrowheads. (Flint is a variety of quartz.) The fractures can be made small and side-by-side making for a very sharp edge.

And with that, I conclude my mini-lecture on cleavage. It definitely wasn’t what you thought it would be, wasn’t it?

Now, I really need to find a giant crystal of calcite that I can smash to bits.