Humans have marveled at crystals for millennia. Viridescent, rich amethyst or blinding emerald diamonds dazzle us with their ordered structure, geometric harmony and perfect symmetry. And the less flashy minerals are also crystalline, just like the snowflakes, grains of table salt, and DNA molecules that help make us who we are.
A crystal is simply a solid material made up of atoms arranged in an ordered and repeatable pattern. This imperceptible atomic order is recapitulated in the visible form of matter; for example, a cube of table salt is composed of repeating squares of sodium and chloride ions.
“You don’t see a lot of order and perfection in the natural world. It’s a little bumpy and bumpy,” said Jeffrey Post, mineralogist and curator in charge of gems and minerals at the Smithsonian’s National Museum of Natural History. “But the crystals have an almost magical order and perfection.”
Scientists today can “examine” the internal order of a crystal using a technique called X-ray crystallography or X-ray diffraction, in which beams of X-rays bounce, or diffract, off the atoms of a crystal and reveal the atomic structure inside.
This April marks 110 years since German scientist Max von Laue took the first steps in the development of X-ray crystallography, considered by many scientists to be one of the most important scientific advances of the 20th century. “X-ray diffraction is the most important probe for examining the structure of solids,” Post said. “It opened up the world in a whole new way.”
Searching for clues in ice and snow
When the ancient Greeks first mined colorless, glassy quartz from the snow-capped Alps, they called it “krystallos”, which means ice. As humans came to understand that perfect structures were not frozen water, the name stuck and early scientists began to suspect that a crystal’s flawless outer shape might be the product of an internal order of its constituent elements. In 1611, Johannes Kepler, best known for describing the motion of the planets around the Sun, suggested that the six identical sides of a snowflake resulted from a dense and orderly packing of tiny spheres of water (the atom had not yet been discovered and named).
But there was no way to confirm this hypothesis yet. Although Galileo and others developed modern microscopy in the late 17th century, even the most powerful optical microscope could not show scientists the internal lattice of a crystal. The wavelength of visible light is longer than the spacing between atoms and unable to squeeze between a tightly packed crystal to reveal its atomic structure. “They needed something that would have been about the same wavelength as the spacing between the atoms to probe these structures and learn something about them,” Post said.
At the turn of the 20th century, the identity and behavior of the atom became obvious. German engineer Wilhelm Röntgen discovered X-rays in 1895, and scientists began using the new electromagnetic radiation in experiments. In 1912, German physicist Max von Laue speculated that these rays might have a short enough wavelength to bounce between atoms in a crystal. He transmitted X-rays to a crystal of copper sulphate placed in front of a photographic plate. When von Laue and his colleagues developed the film-like plate, an ordered ring of dots appeared.
Soon, British father and son William Henry Bragg and William Lawrence Bragg began their own experiments emitting radiation onto table salt crystals. From the intricate, repetitive patterns that appeared, “they immediately knew that these X-rays were being diffracted by the atomic arrangement in the crystal,” Post said. The young Bragg determined the angles at which X-ray waves scatter off a crystal atom when diffracted, and the duo built three-dimensional models based on their calculations.
The Braggs won the Nobel Prize in Physics in 1915, and the scientific community quickly took X-ray crystallography and ran away with it. “It’s amazing how quickly the whole world has taken to this method,” Post said. About a year later, scientists Paul Scherrer, Peter Debye, and Albert Hull developed a technique that allowed X-ray diffraction of the powder crystal rather than a large chunk, and used the method to elucidate the structure of graphite. Today, scientists can take an X-ray diffraction pattern, measure the angles of each scattered X-ray beam, and use computer software to determine the atomic composition of any crystal they encounter.
From DNA to Mars
It’s not just quartz or salt that can be analyzed by X-ray diffraction – the atomic structure of any of the many crystalline materials that make up the Earth can be revealed with this technique. “It was a completely new insight into the nature of crystals, and therefore most solid materials,” Post explained. Any material that has a symmetrical and repeating molecular structure can be identified with the technique, including many organic and biological patterns. In 1953, British chemist Rosalind Franklin created the first “picture” of DNA using X-ray crystallography. Its radiant “X” shape inspired James Watson and Francis Crick’s groundbreaking publication on the duplicate structure helix of the molecule.
Another British chemist named Dorothy Crowfoot Hodgkin used X-ray crystallography to solve the structures of several other essential biological molecules, including cholesterol, vitamin B12, penicillin and insulin. Throughout the 20th century, scientists in the fields of geology, biology, and chemistry have all used this technique to uncover and catalog the molecular structures of dozens of materials. Crystallography has even been deployed on Mars – in 2012 the Curiosity rover performed X-ray diffraction of sand from the Red Planet’s surface, finding a mineral composition similar to volcanic soils in Hawaii.
At the museum, Post and his colleagues use X-ray diffraction in several different ways, first and foremost to identify the countless specimens that pass through the collections. “We have thousands of different types of minerals and hundreds of thousands of specimens,” Post explained. Museum staff run a tiny sample of a material through an X-ray diffraction machine, then a computer compares its atomic structure against a database of known crystalline materials. “We’re always getting new specimens and people are borrowing samples for research, so it’s a routine thing we do to figure out what we have.” Although Post has performed these steps countless times, he still marvels at every X-ray diffraction pattern he sees. “It’s a small miracle in my mind,” he said. “Every phenomenon in the world starts with atoms, and X-ray diffraction has given us the tool to understand them.”
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