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.
