Humans have marveled at crystals for millennia. Viridescent emerald, rich amethyst or blinding diamonds dazzle us with their ordered structure, geometric harmony and perfect symmetry. And less flashy minerals are crystalline, too, as are snowflakes, grains of table salt and the molecules of DNA that help make us who we are.
A crystal is simply any solid material made up of atoms arranged in an orderly, repeatable pattern. This imperceptible atomic order is recapitulated in the visible form of the material; for example, a cube of table salt is made up of repeating squares of sodium and chloride ions.
“You don’t see a lot of order and perfection in the natural world. It’s kind of lumpy and bumpy,” said Jeffrey Post, mineralogist and curator-in-charge of gems and minerals at the Smithsonian National Museum of Natural History. “But crystals have an order and perfection that is almost magical.”
Scientists today can “look” into a crystal’s internal order with a technique called X-ray crystallography or X-ray diffraction, in which X-ray beams bounce, or diffract, off a crystal’s atoms and reveals the atomic structure within.
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 advancements of the 20th century. “X-ray diffraction is the most important probe into looking at the structure of solids,” Post said. “It opened up the world in a whole new way.”
Looking for clues in ice and snow
When the ancient Greeks first pulled colorless, glassy quartz out of the snowcapped Alps, they called it “krystallos,” meaning ice. While humans came to understand that the perfect structures were not frozen water, the name stuck, and early scientists began to suspect that a crystal’s faultless outer form might be a product of an internal ordering of its building blocks. In 1611, Johannes Kepler, best known for describing the movement of planets around the Sun, suggested that the six identical sides of a snowflake resulted from a dense, orderly packing of miniscule water spheres (the atom had not yet been discovered and named).
But there was no way yet to confirm this hypothesis. Though Galileo and others developed modern microscopy in the late 17th century, not even the most powerful light microscope could show scientists a crystal’s internal lattice. 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 atoms to probe these structures and learn something about them,” said Post.
By the turn of the 20th century, the identity and behavior of the atom was coming into focus. German engineer Wilhelm Röntgen discovered X-rays in 1895, and scientists started to use the newfangled electromagnetic radiation in experiments. In 1912, German physicist Max von Laue guessed these rays might have a short enough wavelength to bounce between a crystal’s atoms. He beamed X-rays at a crystal of copper sulfate positioned 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 beaming radiation at crystals of table salt. From the intricate, repetitive patterns that appeared “they immediately knew that these X-rays were being diffracted by the atomic arrangement within the crystal,” Post said. The younger Bragg determined the angles at which the X-ray waves scatter off a crystal’s atom when diffracted, and the duo built three-dimensional models based on their calculations.
The Braggs won the Nobel Prize for Physics in 1915, and the scientific community swiftly took X-ray crystallography and ran with it. “It’s astounding how quickly the whole world grasped on to this method,” Post said. Only a year or so later scientists Paul Scherrer, Peter Debye and Albert Hull developed a technique that allowed for X-ray diffraction of powdered 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 makeup of any crystal they come across.
From DNA to Mars
It’s not just quartz or salt that can be analyzed with X-ray diffraction — the atomic structure of any of the many crystalline materials that make up the Earth can be revealed with the technique. “This was a completely new insight into the nature of crystals, and therefore most solid materials,” Post explained. Any material that has a symmetrical, repetitive molecular structure can be identified with the technique, including many organic and biological models. In 1953, British chemist Rosalind Franklin created the first “image” of DNA using X-ray crystallography. Its radiating “X” shape inspired James Watson and Francis Crick’s groundbreaking publication of the molecule’s double helix structure.
Another British chemist named Dorothy Crowfoot Hodgkin used X-ray crystallography to solve the structures of several further essential biological molecules, including cholesterol, vitamin B12, penicillin and insulin. Throughout the 20th century, scientists in the fields of geology, biology and chemistry all used the technique to unveil and catalog the molecular structures of scores of materials. Crystallography has even been deployed on Mars — in 2012, the Curiosity rover performed X-ray diffraction of sand from the surface of the red planet, finding a similar mineral makeup to the volcanic soils of Hawaii.
At the museum, Post and his colleagues use X-ray diffraction in several different ways, first and foremost to identify the myriad specimens that pass through the collections. “We have thousands of different kinds 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, and then a computer matches its atomic structure in a database of known crystal materials. “We’re always getting new specimens and people borrow samples for research, so it’s a routine thing we do to determine what we have.” Though Post has performed these steps countless times, he still marvels at every X-ray diffraction pattern he sees. “It’s a little miracle in my mind,” he said. “Every phenomenon in the world starts with atoms, and X-ray diffraction gave us the tool to understand them.”
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