"Breakage sounds like a smashing topic!"
"It is, Lou! And very important in identifying minerals!"
"OK, break it down for me, Lulu!"
"Very simple! There are two major ways in which minerals break when you give them a hit: (1) they can break irregularly, in which case we say they 'have fracture'; or (2) they can break along parallel, planar (flat) surfaces, in which case we say they 'have cleavage'! Look at the two pictures!"
"Looks simple!"
"Well, yes and no! Many minerals break regularly (along flat, parallel planes) on some surfaces, but on other surfaces they break irregularly! But as long as the mineral has any tendency at all to break regularly, the irregularity is ignored and the mineral is said to have cleavage! Look at the next two pictures! Both minerals have cleavage!"
"Why can't anything stay simple!!??"
"C'mon, Lou! You can handle it! So, to continue. Fracture is fracture, but there are different kinds of cleavage. I'll show you pictures of a few!"
"And here's some more!"
"Wow indeed! Let's look at a few types of cleavage more closely so we can understand cleavage better! We'll start with cubic cleavage!"
"First of all, consider a cube! It has three sets of parallel, planar, square faces (surfaces): a horizontal set (top and bottom), a front-and-back set, and a side set!"
"Now let's look at the three sets of faces separately! Notice that each set is at right angles to the other two sets! That is, the three sets of faces are mutually perpendicular!"
"That's what cubic cleavage means: any mineral fragment bounded by three parallel, mutually perpendicular sets of surfaces! Another way of saying it is: the mineral has cleaved along three sets of parallel, planer surfaces at right angles to each other. Look at the next picture! It's not a cube, but its surfaces have the same relationship as the surfaces of a cube!"
"I get it! So any of these fragments are "cubic" even though they're not cubes! And of course, unlike a cube, the faces don't have to be square!"
"Right! The same is true with rhombohedral cleavage! The fragment doesn't have to BE a rhomb! But its surfaces must have the same relationship as those of a rhomb! It has broken along three sets of parallel, planar surfaces NOT at right angles to each other. For octahedral cleavage, there are four sets of parallel, planar surfaces!"
"If a mineral has platy or micaceous cleavage, it means it breaks along one set of parallel planes into thin, flat pieces!"
"Many minerals break into fragments bounded by two or more sets of intersecting planes which are parallel to a common axis. The remaining surfaces are irregular. This type of cleavage may be called prismatic cleavage!"
"All very enlightening!"
"OK!! Now that you are in an enlightened state, it's time to see if you can recognize which type of breakage some selected minerals have!"

© 2010, David Leveson/Revised by G. Rocha and Michelle O'Dea