by Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.
How do crystals form? – Alyssa Marie, age 5, New Mexico
Scientifically, the term “crystal” refers to any solid that has an ordered chemical structure. This means that its parts are arranged in a precisely ordered pattern, like bricks in a wall. The “bricks” can be cubes or more complex shapes.
I’m an Earth scientist and a teacher, so I spend a lot of time thinking about minerals. These are solid substances that occur naturally in the earth and cannot be further broken down into different matter other than their constituent atoms. Rocks are mixtures of different minerals. All minerals are crystals, but not all crystals are minerals.
Most rock crystal stores sell minerals that occur in nature. One is pyrite, which is called fool’s gold because it looks like real gold. Display crystals are man-made like bismuth, a natural element that forms crystals when melted and cooled in some shops.
Why and how crystals form
Crystals grow when similar molecules come close together and stick together, forming chemical bonds that act like Velcro between atoms. Mineral crystals cannot start forming spontaneously – they need special conditions and a nucleation site to grow on. A nucleation site can be a rough rock edge or dust speck that a molecule hits and sticks to, starting the crystallization chain reaction.
At or near the Earth’s surface, many molecules are dissolved in water that flows through or over the ground. If there are enough molecules in the water that are alike, they will separate from the water as solids – a process called precipitation. If they have a nucleation site, they will stick to it and start forming crystals.
Rock salt, which is actually a mineral called halite, grows this way. Another mineral called travertine, which sometimes forms flat ledges in caves and around hot springs, where water causes chemical reactions between the rock and the air.
You can make “salt stalactites” at home by growing salt crystals on a wire. In this experiment, the string is the nucleation site. When you dissolve Epsom salts in water and lower a wire into it, then leave it for several days, the water will slowly evaporate and leave the Epsom salts behind. As this happens, salt crystals precipitate out of the water and crystals grow on the string.
Many places in the Earth’s crust are hot enough to melt rocks into magma. As that magma cools, mineral crystals grow from it, just like water freezes into ice cubes. These mineral crystals form at much higher temperatures than salt or travertine precipitating from water.
What crystals can tell scientists
Earth scientists can learn a lot from different types of crystals. For example, the presence of certain mineral crystals in rocks can reveal the age of the rocks. This method of dating is called geochronology – literally, measuring the age of materials from Earth.
One of the most valuable mineral crystals for geochronologists is zircon, which is so durable that it literally stands the test of time. The oldest zircons ever found come from Australia and are around 4.3 billion years old – almost as old as our own planet. Scientists use the chemical changes recorded within zircons as they grow as a reliable “clock” to determine the age of the rocks they contain.
Certain crystals, including zircon, have growth rings, like the rings of a tree, which form when layers of molecules accumulate as the mineral grows. These rings can tell scientists all sorts of things about the environment in which they grew. For example, changes in pressure, temperature and composition of magma can lead to growth rings.
