ChemMatters - October 2017

10 ChemMatters | OCTOBER/NOVEMBER 2017 www.acs.org/chemmatters

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

55 tons!

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:// www.youtube.com/watch?v=aGg2dXUSYoQ).

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
change. Limestone,
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 float.

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 to crystallize.

Career Paths

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
a multibillion-dollar
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
minerals.

industry.

SELECTED REFERENCES

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. indiana.edu/~geol105/1425chap5.htm [accessed Aug 2017].

Rocks and Minerals: Everyday Uses. Museum of Natural and Cultural History, University of Oregon: http://natural-history.uoregon.edu/collections/ web-galleries/everyday-uses-rocks-and-minerals [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
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