Fracture and Cleavage
Bill Long 11/10/04
Breaking Up is Hard to Do
One of the things that minerologists are interested in is how minerals break. That is, in order to know the full properties of a mineral, you have to try to destroy it in order to "learn" about it. Is that perverse? Well, if it exhibits "cleavage," you will not actually destroy it, and if it "fractures," you might not have to destroy the whole thing, so I suppose my first reaction was overstated. Then again, concern for the way that something breaks may be analogous to saying that in order to be a good pyschologist or psychiatrist, you have to know how people "break." But are people like rocks? You don't need to sing Simon & Garfunkle to believe that sometimes they (we) are. Thus, it seemed to me that mineralogical speech of "rock split" might give us a helpful vocabulary to understand the dynamics of how people "split" or "break up" when things don't go well. Let's begin with the minerals.
Don't worry. I'll get to the sexual meaning soon enough. First, however, let's start with its classical meaning in crystallography. Cleavage is either the plane along which crystals can be easily split, the property of splitting along such planes or the action of splitting the crystal along the lines of natural breakage. Cleavage is about splitting or breaking of minerals. But the significant point is that if a mineral exhibits cleavage, and not all do, then a part of the mineral can break off and the broken parts retains a smooth edge or crystal shape. The smoothness of the part has to do with whether the cleavage is perfect, good, fair or poor. These terms for the quality of cleavage are imprecise and approximate, however. It is a matter of judgment when something passes from good to fair.
In addition, the quality of cleavage must be supplemented with an awareness of the forms into which a mineral might cleave. For example, something might cleave in a basal form (a "horizontal" or "peeled" form, such as mica), in a cubic form (such as halite or galena), in an octahedral form (like fluorite or carbon), in a prismatic form (like aegrine or other members of the pyroxenes), or in a rhombohedral manner (such as calcite), among others. Finally, you need to be aware of the direction of the cleavage. Since a mineral is three dimensional, most will have six sides, though more complex minerals may have more. A one-directional cleavage will affect two sides, etc. Thus, if you see notations about cleavage where it says "Good, 2 Directions," it means that the cleavage quality is rather smooth over four sides of the mineral, with two sides exhibiting no cleavage.
Phew. That was a long one. As a reward, you get to learn about cleavage as "the cleft between a woman's breasts as revealed by a low-cute decolletage," as the OED has it. I suppose this use of the word has had a certain staying power because of its vividness: we can just imagine the "plane" along which the globularly-shaped "crystals" of the breasts can be "split." The first usage of cleavage in this manner in English? 1946. Place? None other than Time Magazine. The quotation? "Low-cut Restoration costumes...display too much 'cleavage' (Johnston Office trade term for the shadowed depression dividing an actress' bosom into two distinct sections)."
What is interesting about cleavage is that it only happens in a mineral if there is a natural "fault line" in the mineral--i.e., if the bonds holding together the stone are of unequal strength. When this happens, the mineral might split smoothly, creating two rather than one attractive surfaces. This is to be contrasted with fracture, to which we now turn.
Fracture is the characteristic mark left when a mineral chips or breaks. While cleavage is the break of a crystal face that results in a new, somewhat smooth face being formed, fracture is the chipping of the stone. All materials fracture, even those that have cleavage, but not all stones have cleavage. Fracture happens to minerals that are roughly equal in bonding strength throughout the whole.
There are about seven terms generally accepted for the ways in which minerals may fracture. I will close this essay with a list of these terms and their definitions, and then pick up on them in the next essay. Conchoidal fracture is the breaking in a semi-circular or smooth, curved surface that gives the appearance of a seashell or conch. As a result this is sometimes called shelly fracture. Quartz and glass exhibit this. Subconchoidal fracture is somewhere between conchoidal and even fracture, where even fracture means a smooth surface without the geometric sharpness of cleavage. Uneven fracture, to no one's surprise, is a rough surface and earthy or crumbly fracture is what its name suggests--it crumbles like an ill-made scone. Finally, splintery fracture means that elongated, sharp or acicular edges remain while hackly fracture are rough jagged edges that remain.
As a parting word, I also need to note, without further comment, that parting is another word used by mineralogists to describe a (rather rare) type of breakage something between cleavage and fracture. Since I feel we really have learned something in this essay, I will ignore whatever that previous sentence might mean and move on to a humanistic reflection on cleavage and fracture.
Copyright © 2004-2010 William R. Long