How Do Crystals Form?

Video Transcript

"A crystal is a solid example of a mineral that has flat surfaces that meet at regular angles. Now, if you look at a typical rock specimen, like granite, for example, you can see minerals in it, but they may or may not be beautiful, well-formed crystal. In order to have crystals that are spectacular like this specimen of fluoride here, you have to have certain conditions that are met. The first thing to remember is that crystal formation is very, very slow, so you have to have a geologic process that allows time for the crystal to develop. And then secondly, you have to have room for it to grow. In other words, the fluoride crystals here did not develop inside of a solid mass of rock. It was a little cavity, probably filled with liquid, in which gradual precipitation, which means forming solids from liquid to a plate. Now, there are three specific ways or places where we get crystal formation. One of them is exemplified by the granite that we looked at just a moment ago. Granite forms from molten rock, which we term magma. And as that cools very slowly and gradually, you start to get individual crystals. And they reach a size that makes them big enough to be seen with the naked eye, but because it's a molten mass, they all sort of inter-penetrate, and you don't get real spectacular, well-formed crystals. Although, in some cases, if you have a cavity or an opening that would be filled with gas, for example, or liquid, that might take place. A second way in which crystals form is from precipitation in liquid, as we already mentioned. We looked at fluoride a moment ago. Here's another example: rock salt. If we look at the oceans of the earth, about three and a half percent of the oceans by mass contains dissolved materials. And sodium chloride, ordinary table salt, is the one that's most abundant. And if you get high concentrations of that, which normally will be caused by evaporation of water in hot, dry areas, you will get evaporates produced like this rock salt. And if you look at it closely, you can see some crystal surfaces. It's not a perfect example of salt crystals, but some of these have real nice cubic crystals. The other way in which crystals can form -- I don't have a fancy example here, but it's one that we're all familiar with. It happens inside clouds. When you have the condensation of the moisture in the atmosphere and the production of snowflakes. So every time that snowflakes fall from the sky, we're looking at little crystals of ice that are formed by the third process, which is condensation."