My Science School

Rocks are mixtures of different minerals. All minerals are crystals, but not all crystals are minerals. These solid substances are found naturally in the ground. But do we know how they are formed?

How do crystals form?

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.

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. 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 zircon ever found come from Australia and are about 4.3 billion years old - almost as old asour planet itself. Scientists use the chemical changes recorded within zircon as they grew as a reliable "clock" to figure out how old the rocks containing them are some crystals, including zircon, 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, changesin pressure, temperature and magma composition can all result in growth rings. 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.

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 jewellery 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. (The author works at University of Montana.) THE CONVERSATION

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Researchers have succeeded in printing robotic hands with bones, ligaments and tendons for the first time. Using a new laser scanning technique, the new technology enables the use of different polymers.

Additive manufacturing or 3D printing is the construction of a 3D object from a 3D digital model. The technology behind this has been advancing at great pace and the number of materials that can be used have also expanded reasonably. Until now, 3D printing was limited to fast-curing plastics. The use of slow-curing plastics has now been made possible thanks to a technology developed by researchers at ETH Zurich and a MIT spin-off U.S. start-up, Inhabit. This has resulted in successfully 3D printing robotic hands with bones, ligaments and tendons. The researchers from Switzerland and the U.S. have jointly published the technology and their applications in the journal Nature.

Return to original state

 In addition to their elastic properties that enable the creation of delicate structures and parts with cavities as required, the slow-curing thiolene polymers also return to their original state much faster after bending, making them ideal for the likes of ligaments in robotic hands.

The stiffness of thiolenes can also be fine-tuned as per our requirements to create soft robots. These soft robots will not only be better-suited to work with humans, but will also be more adept at handling delicate and fragile goods.

Scanning, not scraping

In 3D printers, objects are typically produced layer by layer. This means that a nozzle deposits a given material in viscous form and a UV lamp then cures each layer immediately. This method requires a device that scrapes off surface irregularities after each curing step.

While this works for fast-curing plastics, it would fail with slow-curing polymers like thiolenes and epoxies as they would merely gum up the scraper. The researchers involved therefore developed a 3D printing technology that took into account the unevenness when printing the next layer, rather than smoothing out uneven layers. They achieved this using a 3D laser scanner that checked each printed layer for irregularities immediately.

This advancement in 3D printing technology would provide much-needed advantages as the resulting objects not only have better elastic properties, but are also more robust and durable. Combining soft, elastic, and rigid materials would also become much more simpler with this technology.

Picture Credit : Google