10 ChemMatters | OCTOBER/NOVEMBER 2017
How Crystals Form
SOME OF THE LARGEST CRYSTALS EVER FOUND were recently uncovered in an underwater cave in Mexico. A mining company drained the water
from an area it was exploring, revealing breathtaking
crystals of selenite (CaSO4 • 2H2O). The largest crystals
found in the cave were 12 meters long and weighed
These crystals did not form overnight, but rather
formed slowly over thousands of years. Despite their
massive size, these crystals are quite fragile—they
deteriorate in air and can be scratched with a fingernail. For a stunning view of these crystals, visit the You-Tube video, titled “World’s biggest crystal cave” (https://
Sedimentary rocks form a thin
layer over the surface of the earth, so they
are the most commonly encountered rocks.
Sedimentary rocks are formed when
existing rocks are worn down
by erosion or other mechanical
processes. The sediment then
settles and seals to form a
new type of rock. They bind
together through physical
processes, such as pressure under layers of the
earth or water, or chemical
processes, in which the
varying components react
with one another. Sandstone,
coal, and limestone are common sedimentary rocks.
Metamorphic rocks form
because of chemical changes that
occur in rocks at high temp-
eratures and pressures deep
underground. These rocks
start out as igneous or sedi-
mentary and then undergo
a metamorphosis, or
a sedimentary rock,
can be converted
into marble, a meta-
morphic rock, if it is
subjected to enough heat and pressure. Both
are mainly composed of CaCO3, but marble
has a much harder and stronger structure.
Slate, a metamorphic rock, begins as shale, a
sedimentary rock. Slate is much sturdier and
is commonly used as a building material (for
floors or roofs) because of its durability but
also because of its attractive appearance.
Igneous rocks come from volcanoes
and are formed when liquid rocks cool and
solidify. Think of a volcano as a giant pressure cooker that is so hot it melts rock. If this
liquid rock stays below the ground, it is called
magma, and if it oozes above the ground
it is called lava. When lava cools, it freezes
and turns back into a solid. It may seem odd
to refer to a rock as a frozen substance, but
freezing is the process of changing from a liquid to a solid, which is exactly what lava does
at room temperature.
A common type of igneous rock is pumice.
When molten rock is deep within a volcano
it is under immense pressure, keeping many
gases, such as water (H2O) and carbon dioxide (CO2), dissolved within. When this rock
is violently ejected into the atmosphere, the
pressure suddenly drops, causing a
rapid release of these gases, much
like when bubbles form when you
open a can of soda. Because the
rock simultaneously cools, these
bubbles freeze in the rock. The
escaping gases create a porous structure,
making pumice one of the few rocks that
Not all types of igneous rock form suddenly. Granite is one type of igneous rock that
forms underground as magma cools slowly.
If molten rock cools below ground, it happens slowly, which allows time for minerals
If you are interested in rocks and miner-
als, many diverse career opportunities
potentially await you. The extrac-
tion of rocks and miner-
als from the earth is
industry. Engineers at
mining companies locate
new sources of under-
ground mineral deposits.
Once the ore is out of the
ground, gemologists cut
minerals into stones and
polish them for jewelry.
Geologists have the job of
predicting earthquakes and
exploring volcanoes. Each of these
occupations requires an intimate knowledge
of chemistry, which provides the foundation
for understanding the nature of rocks and
Nelson, S. Mineral Chemistry. Department of Earth
and Environmental Sciences, Tulane University, Sept
30, 2013: http://www.tulane.edu/~sanelson/eens211/
mineral_chemistry.htm [accessed Aug 2017].
Minerals, Rocks & Rock Forming Processes.
Indiana University, Bloomington: http://www.
Rocks and Minerals: Everyday Uses. Museum of
Natural and Cultural History, University of Oregon:
[accessed Aug 2017].
Brian Rohrig is a science writer who lives in
Columbus, Ohio. His most recent ChemMatters
article, “Espresso, Café Latte, Cappuccino… A
Complex Brew,” appeared in the April/May 2017 issue. WW
Cave of the Crystals in Naica, Chihuahua, Mexico