Snowflake crystal structure: Photograph of a snowflake revealing its hexagonal six-sided crystalline structure. This crystalline structure makes ice a " mineral. The molecules of water that form each tiny ice crystal naturally arrange themselves into a hexagonal six-sided structure. The result will be a snowflake with six sides or six arms. Ice crystals are " minerals " because they are naturally occurring solids with a definite chemical composition and an ordered internal structure.
The newly-formed ice crystal snowflake is heavier than the surrounding air and it begins falling. As it falls towards Earth through humid air, more water vapor freezes onto the surface of the tiny crystal.
This freezing process is very systematic. The water molecules of the vapor arrange themselves so that the hexagonal crystal structure of ice is repeated. The snowflake grows larger and larger as it falls, enlarging the hexagonal pattern. Although all snowflakes have a hexagonal shape, other details of their geometry can vary. These variations are produced by different temperature and humidity conditions through which the snowflake falls.
Other conditions produce flakes with wide flat arms. Other conditions produce thin, branching arms. These different shapes have an unlimited number of variations, each representing the conditions of temperature and humidity and water vapor the snowflake encountered during its fall.
This brings those electrons a little closer. It also gives the oxygen a relative negative electric charge. The two hydrogen atoms end up a tad positive, in terms of charge. Alone, the structure of a water molecule resembles a wide V. But when multiple H 2 O molecules find themselves close to one another, they begin to pivot so that their electrical charges pair up. Opposite charges attract.
So a negative hydrogen aims itself towards a positive oxygen. The shape that tends to result: a hexagon. It stems from the hexagonal — six-sided — structure of most ice crystals. And hexagons team up. They link with other hexagons, growing outward. Each hexagon contains a lot of empty space.
Warmer H 2 O molecules in the liquid phase are too energetic to settle into a rigid hexagon. As a result, the same number of H 2 O molecules occupy 9 percent more space as solid ice than they do as liquid water. Depending on the temperature, these hexagons join with each other and grow in different ways. Sometimes, they make needles.
Others may form branch-like dendrites. All are beautiful. And all have their own unique story of crystal growth. It may not be immediately clear, but they are all symmetrical in a similar kind of way. Why do they have such a pattern? And if they all have such a similar pattern, why is it so inconceivable that two snowflakes be identical?
To answer both questions, you have to know how a snowflake forms. Snow is not simply a frozen droplet of water falling from a cloud. What makes a snowflake different is that it forms slowly, and that it grows in the cloud. A snowflake is born when water vapor travels through the air and condenses changes from a gas to a solid on a particle.
There it forms a slowly growing crystal. There are two basic ways that the vapor can condense. Each way plays a big role in the shape that the snowflake will eventually take.
If one replaces "soul" with "complex biochemistry of living organisms," Kepler was essentially correct in his thinking. There is no genetic blueprint that guides snow-crystal development. Their growth is determined by relatively simple physical rules—far simpler than the chemistry of living organisms—yet complex shapes emerge spontaneously.
Kepler realized that the genesis of complex patterns and structures from simple precursors was a worthy scientific question, and it is one that scientists are still investigating today. The advent of x-ray diffraction techniques in the s illuminated crystalline structures, helping to lay the foundations of the field of crystallography, and soon revealed the sixfold symmetry of the ice-crystal lattice.
The lattice structure helped to explain the sixfold symmetry of snow crystals, but by itself it does not explain the complex crystal morphologies. Figure 3. It takes only a magnifying glass to appreciate the beauty of snow crystals, but truly detailed images require a microscope. In , Robert Hooke used his early microscope, with little more magnification than a modern hand lens, to sketch the forms of snowflakes bottom.
Wilson A. Bentley, a Vermont farmer who assembled a masterful collection of more than 5, images from the late s through the s, pioneered the use of a camera with a microscope and followed a painstaking process of hand-trimming negatives to improve background contrast top left. Bentley's magazine publications popularized the adage that no two snowflakes are alike. More advanced imaging techniques were developed by Nakaya in the s.
He was also the first to grow synthetic snowflakes in the laboratory top right. Nakaya used a rabbit hair to anchor the crystal, as the oils on the hair would encourage growth from a single nucleation site rather than permitting frost to grow evenly along the hair. Physicist Ukichiro Nakaya of the University of Hokkaido in Japan brought 20th-century scientific methods to bear on this problem in the s in a remarkable series of observations and experimental investigations.
After observing and documenting the range of natural snow-crystal types, Nakaya realized that laboratory experiments were necessary to investigate under what conditions the different crystal types appeared. Nakaya developed several techniques for growing isolated snow crystals in test chambers and soon found that a crystal's morphology was mainly a function of the temperature and humidity of the air. Just below freezing, at around -2 degrees, thin platelike crystals appeared.
Under slightly colder conditions, around -5 degrees, slender needles were the preferred shape. At degrees, the largest and thinnest plate-like crystals formed, while below degrees, the crystals grew mainly as short columns. At all temperatures, Nakaya found that simple prismlike crystals formed when the humidity was low and growth was slow, whereas higher humidity yielded faster growth and more complex structures. Subsequent work has additionally shown that smaller crystals have generally simpler shapes, while larger crystals are more complex.
Nakaya displayed all his data in what is now called the snow-crystal morphology diagram , which displays the crystal shape as a function of temperature and humidity see Figure 2. After 75 years, we still cannot explain many of the features seen in this simple diagram. In particular, the odd temperature dependence of the crystal morphology, exhibiting an almost oscillatory behavior between plates and columns over just a few degrees, is still largely an unsolved puzzle.
The morphology diagram can handily explain two immediately interesting features of snow crystals—why they all look so different and why the six branches on a stellar crystal all look alike. The explanation stems from Nakaya's observation that ice growth is remarkably sensitive to temperature and humidity.
As it blows about inside the clouds, a developing snow crystal experiences ever-changing temperatures and humidity levels during its travels. Each change in its local environment alters the way the crystal grows.
Its growth may be platelike or columnar, faceted or branched, all depending on the conditions it sees. Because the sensitivity to temperature and humidity is so great, even modest variations inside a cloud cause large changes in growth behavior.
After numerous twists and tumbles during its travels, the final structure of an individual crystal can be quite complex. Furthermore, the route each growing snowflake takes is itself a highly random walk, influenced by the chaotic whorls and eddies that are ever present in the atmosphere. It is all but impossible for two snowflakes to follow exactly the same path though the clouds, so the likelihood of finding two identical snowflakes is basically nil.
Luckily for snowflake watchers, nature has conspired to make a stunning variety of crystal forms. Although each snowflake follows a different path, the arms of an individual stellar crystal travel together. The six arms all undergo the same changes in conditions at exactly the same times. As a result, the branches seem to grow in synchrony, simply because they each experience the same growth history. So Thoreau's "creative genius," capable of designing snow crystals in an endless variety of beautiful and symmetrical patterns, can simply be found in the ever-changing winds.
Going beyond the morphology diagram, much progress in understanding snow crystals has come from work in crystallography and metallurgy done by many scientists over several decades, as the foundations of modern materials science were being laid throughout the 20th century. The semiconductor industry provided considerable impetus in these fields, as suddenly the ability to produce large crystals—which required an understanding of their growth dynamics—was a business necessity.
Figure 4. The temperature dependence of snow-crystal forms shown in the morphology diagram can be brought to life with lab-grown snow. These small crystals were all grown as they fell freely in a chamber held at an intermediate level of supersaturation, but at varying temperatures. The left panel shows a montage of crystals grown at -2 degrees Celsius, the middle at -5 degrees and the right at degrees.
Photographs courtesy of Kenneth G. The formation of facets—flat crystalline surfaces—is a nearly ubiquitous phenomenon in crystal growth. Faceting plays a major role in guiding the growth of snow crystals.
Once a cloud droplet freezes, the expanding crystal develops facets because some crystalline surfaces accumulate material more slowly than others. Condensing molecules are especially attracted to rounded surfaces that are rough on atomic scales, because such areas present greater available molecular binding.
Molecularly flat regions—the facet surfaces—have fewer dangling chemical bonds and are thus less favorable attachment sites. After a crystal grows for a while, only the slow-growing facet surfaces remain. The crystal eventually becomes faceted, regardless of its initial shape.
The molecular bonding to the crystal lattice determines which surfaces grow slowly, and thus which lattice planes become facets. The process of faceting is how the geometry of the water molecule is transferred to the geometry of a large crystal. Different mineral crystals have different facet structures, depending on the details of their molecular lattices. When faceting dominates snow crystal growth, the resulting hexagonal crystal has six side faces, called prism facets, capped by top and bottom surfaces, called basal facets.
This is the basic shape of small or slow-growing snow crystals. Visible remnants of this form can often be seen at the centers of larger, more complex snow crystals, revealing their simpler initial shapes. Under some conditions, water molecules will attach more readily to the prism surfaces than the basal surfaces, producing thin plates of ice. In other circumstances, the molecules attach to the basal facets, resulting in columns.
In either case, faceting is one of the most important mechanisms for producing different shapes and patterns. Faceting cannot be the whole story, however, or all snow crystals would be shaped like simple hexagonal prisms, which is certainly far from the case.
Something else happens when the crystal size is large—typically about half a millimeter across—or when the growth is rapid. Then a crystal may sprout branches because of a well-known growth effect called the Mullins-Sekerka instability , or simply the branching instability.
This process largely explains how complex, flowerlike snow-crystal structures can arise spontaneously from nothing more than freezing water vapor. As snow crystals grow, they use up the water vapor in their immediate surroundings, and it takes a certain amount of time for additional molecules to diffuse through the air to reach the crystal.
Snow-crystal growth is therefore said to be diffusion limited , and different regions on a crystal effectively compete for available resources. If a spot on a crystal—for example, one of the points on a hexagonal plate—sticks out farther into the air, then water molecules will preferentially collect on that point, simply because the diffusion distance is slightly shorter.
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