How do crystals form?

https://theconversation.com/how-do-crystals-form-213202

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How do crystals form? – Alyssa Marie, age 5, New Mexico

Scientifically speaking, 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 are found naturally in the ground and can’t be broken down further into different materials other than their constituent atoms. Rocks are mixtures of different minerals. All minerals are crystals, but not all crystals are minerals.

Most rock shops sell mineral crystals that occur in nature. One is pyrite, which is known as fool’s gold because it looks like real gold. Some shops also feature showy, human-made crystals such as bismuth, a natural element that forms crystals when it is melted and cooled.

A dark gray rock with a large concentration of shiny yellow material covering part of its surface. — Pyrite in black shale rock from a quarry in Indianapolis, Ind. James St. John/Flickr, CC BY

Why and how crystals form

Crystals grow when molecules that are alike get close to each other and stick together, forming chemical bonds that act like Velcro between atoms. Mineral crystals cannot just start forming spontaneously – they need special conditions and a nucleation site to grow on. A nucleation site can be a rough edge of rock or a speck of dust that a molecule bumps into 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 to form crystals.

Rock salt, which is actually a mineral called halite, grows this way. So does 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.

White rock terraces around a vent in the earth — Travertine ledges at Mammoth Hot Springs in Yellowstone National Park in Wyoming. Terraced pools form due to deposition of travertine from the hot spring fluids as they cool and release carbon dioxide. USGS

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 and lower a string into it, then leave it for several days, the water will slowly evaporate and leave the Epsom salts behind. As that happens, salt crystals precipitate out of the water and grow crystals on the string.

Many places in the Earth’s crust are hot enough for rocks to melt into magma. As that magma cools down, mineral crystals grow from it, just like water freezing into ice cubes. These mineral crystals form at much higher temperatures than salt or travertine precipitating out of 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 rocks’ age. This dating method is called geochronology – literally, measuring the age of materials from the Earth.

One of the most valued mineral crystals for geochronologists is zircon, which is so durable that it quite literally stands the test of time. The oldest zircons ever found come from Australia and are about 4.3 billion years old – almost as old as our planet itself. Scientists use the chemical changes recorded within zircons as they grew 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, that form when layers of molecules accumulate as the mineral grows. These rings can tell scientists all kinds of things about the environment in which they grew. For example, changes in pressure, temperature and magma composition can all result in growth rings.

White rectangular feldspar crystals with faintly visible growth rings are prominent against grey granodiorite rock. — Feldspar crystals with growth rings in granodiorite rock near Squamish, British Columbia. Natalie Bursztyn, CC BY-ND

Sometimes mineral crystals grow as high pressure and temperatures within the Earth’s crust change rocks from one type to another in a process called metamorphism. This process causes the elements and chemical bonds in the rock to rearrange themselves into new crystal structures. Lots of spectacular crystals grow in this way, including garnet, kyanite and staurolite.

Red mineral cubes embedded in a pale grey rock — Garnets, which are usually but not always red, typically form in metamorphic rock. Paul Starosta via Getty Images

Amazing forms

When a mineral precipitates from water or crystallizes from magma, the more space it has to grow, the bigger it can become. There is a cave in Mexico full of giant gypsum crystals, some of which are 40 feet (12 meters) long – the size of telephone poles.

Especially showy mineral crystals are also valuable as gemstones for jewelry once they are cut into new shapes and polished. The highest price ever paid for a gemstone was $71.2 million for the CTF Pink Star diamond, which went up for auction in 2017 and sold in less than five minutes.

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