This is a confusing one for students. It can sometimes be a challenge for folks who already have their PhD’s in geology.
How do you distinguish among crystal habit, cleavage, and fracture in minerals?
I talk a bit about cleavage and fracture in this older blog post. As a brief review:
Cleavage is a planar direction within a mineral along which it is likely to break. This is determined by the crystal structure – the way the atoms making the mineral bond together. Where bonds are weak, the crystal might break. The best example of this is the micas. These form in sheets that split easily apart. Cleavage tends to result in flat, relatively smooth surfaces.
Fracture is what you get when a mineral breaks on a surface that’s not already naturally weak. Fractured surfaces tend to be jagged, and sometimes scooped out. Think about a chip in a piece of China. The result is a rounded surface, sometimes with sharp edges.
Cleavage and fracture have a lot to do with what a crystal looks like when you find it. But there’s one other important thing that affects what minerals look like.
Habit is the natural shape of a mineral’s crystals before any cleavage or fracture occurs. Check out the examples of different crystal habits on Wikipedia.
Some crystals are long and pointy. Some are shaped like disks. Some are little cubes. What’s interesting (and sometimes confusing) is that a mineral’s habit may have nothing to do with it’s pattern of cleavage or fracture.
For example, I posed this question to my students the other day:
The trick here is to understand that while both the habit and cleavage are dictated by the crystal structure, they are both about totally different things. The crystal habit forms as the mineral is growing, and is dictated by how the individual atoms in the crystal come together. After the crystal is formed in whatever shape it’s going to be, there may be weak planes in the crystal along which it can easily break: Cleavage.
However, as is the case for quartz, there may be no planes of weakness and therefore no cleavage, despite a lovely crystal shape. When there are no planes of weakness and mineral can only break by fracturing.
Here’s a lovely fluorite crystal (CaF2).
That’s one possible habit for fluorite. Here’s another:
But all fluorite, despite it’s crystal habit, has four directions of cleavage:
This is because the crystal habit is determined as the crystal grows, and cleavage occurs when the crystal breaks.
Crystal habit, cleavage, and fracture in minerals are all related to the crystalline matrix, but they are all in reference to different physical properties of the mineral.