Crystals are everywhere. With few exceptions the solid inorganic substances that make up our world are crystalline. This includes rocks, minerals, ceramics and metals.
The fascination with crystals is easy to understand. A crystal has a mesmerizing effect as light reflects and refracts from its interior surfaces. The fortuneteller’s crystal ball appears depthless and mysterious as it draws one’s gaze into its interior. Many crystals, especially gems, have been cut, faceted and polished to take advantage of the natural order of atoms that define the crystal state.
Gemstones such as emerald, ruby, sapphire and diamond have been hoarded and coveted as wealth since biblical times. Famous crystals such as the Hope diamond and the British crown jewels draw tens of thousands of visitors every year to gawk at the specimens under glass and high-tech security.
Despite the wide appeal of these large crystals, most of the crystals that surround us are not big. Take the crystals in a piece of granite.
Here the visual appeal is not individual crystals, but rather the pattern. Countless crystals define each piece in endless variety. A quick trip to a granite store reveals the panoply of patterns, colors and swirls. Although most of those are not true granite in the geological sense, they all have the common characteristic of a matrix of silicate crystals.
Most of the crystals in our environment are too small to see with the naked eye. Some are visible only with electron microscopes. Most inorganic solid substances are crystalline. Some other examples are ceramics, metals and concrete. A notable exception is glass, which is quasi-crystalline.
What makes a crystal is an orderly arrangement of atoms.
Because of the electromagnetic properties of atoms, they tend to form chemical bonds according to the number of unpaired electrons in the highest energy level. A lesser-known but equally important characteristic of atoms is that they tend to fill up space according to size as well.
As an example, the crystal structure of table salt (sodium chloride) aligns alternating large sodium and small chlorine ions in a three-dimensional array known as octahedral coordination. This is when each sodium ion atom is surrounded by eight chloride ions, and vice versa in a three-dimensional grid.
Octahedral coordination is a common structure in many metals as well as other halides and sulfides such as calcium fluorite (fluorite) and iron sulfide (pyrite). But in silicate minerals a tetrahedron of oxygen atoms surrounding a single silicon atom is the basis for 99 percent of Earth’s rocks.
Although we do not often think of metals as being crystalline, the crystal structure of metals is a critical factor in determining their strength. For example, the difference between pure iron metal and steel is the presence of other atoms such as carbon and transition metal atoms such as chromium, iron and vanadium.
These alien atoms act as disruptions in the arrangements of atoms in the metallic crystals that prevent planes of weakness. Large carbon atoms in carbon steel act like pegs to keep the atoms from sliding along these planes.
The crystal structure of stainless steel is apparent in close-up inspection as elongated crystals that appear as fine lines.
Although the arrangement of atoms is too small for direct observation, the patterns become visible when X-rays reflect from the atoms to reveal their structure. The external shape of crystals such as quartz and optical properties such as the brilliant fire of diamonds and the double refraction of calcite all reflect the repeating internal structure.
Richard Brill is a professor of science at Honolulu Community College. His column runs the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.
