Deep beneath the geysers and hot springs
of Yellowstone Caldera lies a magma chamber produced by a
hot spot in the earth’s mantle. As the magma moves towards
the Earth’s surface, it crystallizes to form young,
hot igneous rocks. The heat from these rocks drives
groundwater towards the surface. As the water cools, ions precipitate out
as mineral crystals, including quartz crystals from silicon
and oxygen, feldspar from potassium, aluminum,
silicon, and oxygen, galena from lead and sulfur. Many of these crystals have signature
shapes— take this cascade of pointed quartz,
or this pile of galena cubes. But what causes them to grow into these
shapes again and again? Part of the answer lies in their atoms. Every crystal’s atoms are arranged
in a highly organized, repeating pattern. This pattern is the defining
feature of a crystal, and isn’t restricted to minerals— sand, ice, sugar, chocolate, ceramics,
metals, DNA, and even some liquids have
crystalline structures. Each crystalline material’s atomic
arrangement falls into one of six different families: cubic, tetragonal, orthorhombic,
monoclinic, triclinic, and hexagonal. Given the appropriate conditions, crystals will grow into geometric shapes that reflect the arrangement
of their atoms. Take galena, which has a cubic structure
composed of lead and sulfur atoms. The relatively large lead atoms are arranged in a three-dimensional
grid 90 degrees from one another, while the relatively small sulfur atoms
fit neatly between them. As the crystal grows, locations like these
attract sulfur atoms, while lead will tend to
bond to these places. Eventually, they will complete the grid
of bonded atoms. This means the 90 degree grid pattern
of galena’s crystalline structure is reflected in the visible
shape of the crystal. Quartz, meanwhile, has a hexagonal
crystalline structure. This means that on one plane its atoms
are arranged in hexagons. In three dimensions, these hexagons are
composed of many interlocking pyramids made up of one silicon atom
and four oxygen atoms. So the signature shape of a quartz
crystal is a six-sided column with pointed tips. Depending on environmental conditions, most crystals have the potential to form
multiple geometric shapes. For example, diamonds, which form deep
in the Earth’s mantle, have a cubic crystalline structure and can
grow into either cubes or octahedrons. Which shape a particular
diamond grows into depends on the conditions where it grows, including pressure, temperature,
and chemical environment. While we can’t directly observe growth
conditions in the mantle, laboratory experiments have shown some
evidence that diamonds tend to grow into cubes at
lower temperatures and octahedrons at higher temperatures. Trace amounts of water, silicon,
germanium, or magnesium might also influence a diamond’s shape. And diamonds never naturally grow into the
shapes found in jewelry— those diamonds have been cut to
showcase sparkle and clarity. Environmental conditions can also
influence whether crystals form at all. Glass is made of melted quartz sand, but it isn’t crystalline. That’s because glass cools
relatively quickly, and the atoms do not have time to arrange
themselves into the ordered structure
of a quartz crystal. Instead, the random arrangement
of the atoms in the melted glass is locked in upon cooling. Many crystals don’t form geometric shapes because they grow in extremely close
quarters with other crystals. Rocks like granite are full of crystals, but none have recognizable shapes. As magma cools and solidifies, many minerals within it crystallize at the
same time and quickly run out of space. And certain crystals, like turquoise, don’t grow into any discernible geometric
shape in most environmental conditions, even given adequate space. Every crystal’s atomic structure has
unique properties, and while these properties may not have
any bearing on human emotional needs, they do have powerful applications
in materials science and medicine.
