Tag Archives: hexagonal

The Science of Snowflakes

If you’ve ever seen a photo of a snowflake up close, you know how beautiful and intricate they can be. People say “no two snowflakes are alike”–this is true, because of the way snowflakes grow. Each snowflake grows according to the crystal structure of ice and the conditions it experiences as it falls to the ground.

From Wikipedia, click for link.

What is an ice crystal?

A snowflake is a single crystal of ice. Many substances are crystalline, but most of the ones we encounter are polycrystalline, or composed of many crystals. Some examples of crystalline materials are metals, bone, ceramics, and jewels. Different kinds of crystals grow in different ways– some are cubic and some are hexagonal. (You can see great crystals in the Smithsonian Natural History Museum in Washington, DC. Some pictures are in this link.) Table salt, for instance, is cubic.

Salt crystals, via Tony Wong on Flickr.

Ice has a hexagonal crystal structure, which is why snowflakes have hexagonal symmetry. A snowflake tends to grow along six vectors (or directions) separated by 60 degrees each. Some particle of dirt nucleates, or initiates, the beginning of a snowflake. The exact mechanism is not known. But once growth has been initiated, the snowflake grows. (If you have ever made rock candy, you create a supersaturated mixture of sugar in water. Then you add a sugar crystal, onto which more sugar crystal grows.)

Snowflakes are unique

Snowflakes are so varied because each snowflake experiences a slightly different environment. Tiny differences in temperature, pressure, and moisture change how each snowflake grows. In the snowflake above, you can even see flaws in tiny parts of the hexagonal symmetry. Even across a snowflake, tiny differences change how the crystal grows. Snowflakes as big as a dime have been documented, but theoretically, there is no size limit.

Learn more!

One of the first photographers of snowflakes was Wilson Bentley. He photographed over 5,000 flakes from his home in Vermont in the 1800 and 1900s. The children’s book Snowflake Bentley describes his life and work.

Ken Libbrecht, a professor of physics at Caltech, also maintains an awesome website about his research on snowflakes. In his lab, he studies how to grow snowflakes, to better understand the conditions under which they form. He has grown crystals up to an inch wide. His Field Guide to Snowflakes is a beautiful and informative resource on snowflakes, accessible to all audiences.

Fun Science: Crystals Everywhere!

I went on a trip to DC last fall. Almost accidentally I ended up in the Natural History Smithsonian Museum. Wow! Especially worthy is the section on minerals. I assume there are other museums with such displays, but I hadn’t been to one. The Hope diamond is displayed also in the minerals section, but fancy jewels I can’t touch are way less interesting than all the minerals and natural crystals.

I find crystals fascinating because they tell you so much about the microscopic structure of the material. Where else in life can you just look at an object and see what it does down to the nanometer? So naturally the camera came out. Below are a few favorites, and some comments about what we can infer from the pictures.

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Fluorite (CaF2): As you can see, Fluorite has a cubic crystalline structure. Fluorite can come in basically any color. Color can be due to impurities, exposure to radiation, or defects in the crystalline structure. Fluorite was originally so named due to fluorescent properties; fluorite can fluoresce in a variety of colors depending upon the impurities present.

IMG_2139Beryl (Be3Al2(SiO3)6): You might be more familiar with other names for Beryl, such as aquamarine or emerald or morganite. Beryl is naturally clear, but takes on color in the presence of impurities. Emerald, for example, has chromium or vanadium present. Aquamarine coloration results when the Fe2+ oxidation state is present. Fe3+ results in yellow coloration. You can see in the image above that beryl has a hexagonal crystal structure. You can also see that this is one big hexagonal crystal, unlike the population of cubes in the fluorite picture. This tells us a lot about how the crystal grew. If the crystal grew very fast, there would be a number of columns, because crystallization would be faster than the time for the mineral components to diffuse to one specific column. So this crystal grew pretty slowly.

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Wulfenite (PbMoO4): Wulfenite is often found around lead deposits, which makes sense since it contains lead. It has a tetragonal crystal structure, and tends to be yellow or orange or brown in color. You can see that the crystals are much smaller in this picture than the beryl crystal. Clearly these crystals grew quickly from many nucleation sites. The size to which crystals tend to grow is a property of the crystal too; some only form a ton of small crystals, some form a few very large ones. It depends whether it is lower energy to just form another crystal, or if it is lower energy to allow diffusion to an already established crystal. This is related to thermodynamics. Wulfenite seems to favor lots of small crystals. Some wulfenite has a really cool property called piezoelectricity; when there is the right kind of pressure on the crystal, an electric charge accumulates.

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Manganese dioxide (MnO2): This manganese dioxide has grown in a dendritic fashion. It might look like frost or snowflakes, which grow in similar ways. These dendrites are very fractal, a favorite topic of mine. Here diffusion was definitely limited, so crystals grew where the materials were present.

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Chalcedony (SiO2): Chalcedony is a type of silicon dioxide, which is the chemical composition of most sand. Chalcedony is composed of two different silicon dioxide minerals: quartz and moganite. Quartz and moganite have different crystalline structures which grow together at a fine scale in chalcedony, which is probably why it looks far less geometric than the other crystals I’ve shown. Agate is a type of chalcedony.