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How are crystals formed? – Alyssa Marie, 5 years old, 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 strictly ordered pattern, like bricks in a wall. “Bricks” can be cubes or more complex shapes.
I’m an Earth scientist and teacher, so I spend a lot of time thinking about minerals. These are solid substances that occur naturally in the soil and cannot be broken down into different materials other than the atoms that make them up. Rocks are mixtures of different minerals. All minerals are crystals, but not all crystals are minerals.
Most rock shops sell naturally occurring mineral crystals. One of these is pyrite, known as fool’s gold because it resembles real gold. Some stores also carry fancy, man-made crystals like bismuth, a natural element that forms crystals when melted and cooled.
Why and how do crystals form?
Crystals grow when similar molecules approach and stick together, creating chemical bonds between atoms that act like Velcro. Mineral crystals cannot begin to form spontaneously; They need special conditions and a nucleation site to grow. The nucleation site could be a rough edge of the rock or a speck of dust that a molecule hits and sticks to, starting a crystallization chain reaction.
At or near the Earth’s surface, many molecules dissolve in water flowing through or over the ground. If there are enough similar molecules in water, they will separate from the water as solids; this is called precipitation. If there are nucleation sites, they will stick there and start forming crystals.
This is how rock salt, which is actually a mineral called halite, grows. So does another mineral called travertine, which sometimes forms flat outcrops in caves and around hot springs, where water causes chemical reactions between the rock and air.
You can make “salt stalactites” at home by growing salt crystals on a string. In this experiment the string is the nucleation site. When you dissolve Epsom salts in water, lower a rope into it, and let it sit for a few days, the water will slowly evaporate, leaving the Epsom salts behind. When this happens, salt crystals precipitate from the water and crystals form on the string.
Many parts of the Earth’s crust are hot enough for rocks to melt and turn into magma. As this magma cools, mineral crystals grow from it, just as water freezes into ice cubes. These mineral crystals form at much higher temperatures than salt or travertine that precipitates from water.
What can crystals 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 dating method is called geochronology; literally measuring the age of materials on Earth.
One of the most valuable mineral crystals for geochronologists is zircon; Zircon is so durable that it literally stands the test of time. The oldest zircons ever found come from Australia and are approximately 4.3 billion years old, almost as old as our planet itself. Scientists use the chemical changes zircons record as they grow as a reliable “clock” to figure out how old the rocks containing them are.
Some crystals, including zircons, have growth rings, like the rings of a tree, formed by the accumulation of layers of molecules as the mineral grows. These rings can tell scientists all kinds of things about the environment in which they grew up. For example, changes in pressure, temperature, and magma composition can all result in growth rings.