Pegma-whats? What-matites?

Pegmatites! One of the more common sources for large, beautiful minerals are these igneous features. But what are pegmatites? In brief, they are the dregs of a magma. The last fluids to crystalize.

What was that? All the magma doesn’t crystalize at once? Nope! Have you ever put left overs in the fridge and taken them out to find that some of the fat on top has turned to a solid, while underneath there is a pool of runny liquid? Although it isn’t the most appetizing analogy, pegmatites are composed of that runny liquid. In the case of magmatic fluids, compatible elements form the earliest mineral crystals as the magma cools. As cooling continues, there is less energy to keep incompatible elements in liquid form, so they will come together to form mineral crystals of a different mineral.

In the end, the last magmatic fluids that are left are the oddball elements that don’t easily make minerals, and the fluid needs to find a place to go since all the real estate is taken up by the minerals that have already formed. The result of this real estate crunch is that these fluids tend to push their way through the recently formed rock in either a dike or a sill. Once they’ve made enough space for themselves, the lowered pressure lets them cool slowly, forming what can be incredibly large and often very pretty, pegmatites.