The Crystal-Clear Guide to Crystalline Structures

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Crystals are some of the most interesting structures to appear naturally throughout the world. In fact, they’re utterly peculiar in the way they’re shaped and how they reproduce. How do crystalline structures form exactly?

Like any other mother in the natural world, crystals “gift” their characteristics to their “offspring.” The comparison to living organisms was so compelling that in 1966 chemist Graham Cairns-Smith published a paper arguing that that crystals may have played a role in the development of the first-ever genes and the rise of multicellular life itself.

Cairns-Smiths’ idea about crystals live on, even if testing them remains extremely difficult or even impossible with current technology. Some evolutionary biologists believe the rise of multicellular life wasn’t just a function of chemistry, but also of geology and other environmental factors.

What are the different types of crystal systems? And why have they so thoroughly captured the attention of school-aged scientists-in-training, enterprising chemists, and laymen and women throughout the world?

What Are the Properties of Crystalline Structures?

Crystalline solids, or crystals, are known as “true solids.” This means they have a “long-range” three-dimensional order throughout their structure. This ordered “lattice” manifests thanks to the constituent parts within crystals — molecules, ions or atoms — arranging themselves in a symmetrical and repeating pattern (such as triangles and squares) throughout the entire crystalline structure.

Most of us run into crystals every day of our lives. Here are some examples:

  • Table salt
  • Sugar
  • Snowflakes
  • Graphite
  • Dry ice
  • All minerals
  • Stalactites and stalagmites
  • Quartz, diamonds and other gemstones

These and other crystal structures demonstrate what’s known as “cleavage” — that is, they break apart along straight lines and planed surfaces.

If you’ve ever grown your own rock candy at home, you either knowingly or unknowingly created one of the most common types of crystalline structures.

All types of crystal systems also demonstrate anisotropy. This means qualities like tensile strength, conductivity, and refractive index (how much light passes through) vary according to the direction in which the force is applied.

How do Crystalline Structures Form?

All minerals are naturally occurring and inorganic, and all minerals form crystals. It is possible to have two minerals with the same chemical makeup but very different crystalline structures. We call these instances polymorphs.

Crystals represent the highest form of physical order. This is compared to “amorphous structures,” which lack a repeating pattern.

So how do crystals form?

The process known as “crystallization” can occur naturally or it can be induced under controlled circumstances. In a natural setting, crystals form in a variety of environments — including when liquids cool down and begin to harden. They can also arise when water evaporates from a mixture, such as when saltwater evaporates.

Some molecules tend to form crystals because they are inherently unstable and need to share electrons with their “neighbors.” This results in the telltale repeating pattern as more atoms link with one another.

The most common crystals in nature are those that form the bedrock of planet earth. Crystals appear in rocks in all sizes and shapes here, ranging from smaller than a millimeter to multiple feet in diameter. These crystalline rock deposits in the earth tend to form when magma cools under different combinations of temperature and pressure. These are known as “magmatic” or “metamorphic processes.”

What Are the Seven Types of Crystal Symmetry?

There are seven distinct types of crystalline solids which appear throughout nature. Each one is defined by a different type of symmetry:

1. Hexagonal

In hexagonal crystalline structures, three of four axes appear on a single plane and have equal measurements. Each intersects with the other at a 60-degree angle. The fourth and final axis has a different measurement from the others and intersects with them at a right angle.

Examples include four-sided pyramids, 12-sided pyramids and four-sided prisms. In nature, these appear as emeralds, zincite, apatite, aquamarine and others.

2. Cubic

Also called “isometric” crystal systems, this is where three equal axes intersect with one another at right angles to form a square inner structure.

Cubes, rhombic dodecahedrons, and octahedrons are all examples of cubic systems. In nature, these take the form of diamonds, pyrite, garnet, gold, silver and others.

3. Tetragonal

With tetragonal systems, two axes appear on the same plane and have identical measurements. The primary axis is either longer or shorter. All axes intersect at right angles. They have a rectangular inner structure.

Examples include four-sided prisms, pyramids, eight-sided pyramids and double pyramids. You can find these in nature in the form of zircon, rutile, wulfenite apophyllite and others.

4. Trigonal

Trigonal crystalline structures are also called “rhombohedral” systems. These are sometimes included alongside hexagonal crystals. Looked at in cross-section, trigonal structures reveal three sides while hexagonal structures reveal six. Trigonal crystals have a triangular inner structure.

Examples include rhombohedra, three-sided pyramids, three-sided prisms and others. You can find these in nature as amethyst, jasper, calcite, quartz, citrine and many others.

5. Triclinic

Triclinic systems have three axes measuring different lengths and each one inclines toward the other at a non-perpendicular angle. These take the form of “paired faces” within nature. Examples include amazonite, rhodonite, turquoise, aventurine feldspar and others.

6. Monoclinic

Monoclinic crystalline structures have three noncongruent axes. Two appear at right angles to one another, with the last inclined, forming a parallelogram interior structure.

Crystal shapes with monoclinic structures include prisms with tapered ends and basal pinacoids. Naturally forming monoclinic structures include azurite, gypsum (a critical ingredient in drywall), moonstone, serpentine, lazulite and others.

7. Orthorhombic

Also simply called “rhombic” systems, these feature three axes all measuring different lengths. All axes are at right angles to one another. Crystalline shapes with this structure include rhombic prisms, pyramids, double pyramids and pinacoids. Examples in nature include alexandrite, tanzanite, topaz and others.

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What Are the Main Crystal Structural Groups?

Crystals belong to one of four main structural categories based on their physical characteristics and the nature of their chemical bonds. These crystal categorizations include:

1. Molecular Crystals

These are crystals whose structures contain identifiable molecules but fairly week bonds between them. Molecular crystals may appear as organic compounds, as gases (including krypton, argon, xenon and other “rare gases”) or as solids (as with dry ice, which is solid CO2).

Forces like hydrogen bonding and van der Walls forces keep these crystals together through non-covalent interactions. This means the atoms share, rather than transfer, electrons between themselves. These shared electrons result in a weaker bond than found in ionic or covalent crystals, meaning molecular crystals have relatively lower melting points.

2. Metallic Crystals

Metals and metal alloys are known for being good thermal and electrical conductors thanks to the mobile “free” electrons around the ion. Metallic solids are highly malleable. Most metals, but not all, are denser than nonmetals.

Metals maintain their solid structural bonds thanks to electrostatic interactivity between ions and their electron clouds. Adjacent atoms overlap their electron clouds, meaning electrons can move between one atom and another anyplace within the crystal. This is unlike covalent or ionic crystals, where electrons are shared (or “donated”) outright.

3. Ionic Crystals

Known as salts, ionic crystals comprise of ions with opposite charges from one another. A positive ion is called a “cation” and a negative ion is known as an “anion.” Ionic crystals have high levels of conductivity that increase as their temperature rises.

The atoms within this type of crystal receive their bonding force through electrostatic forces. This means a positively charged and a negatively charged proton are attracted to one another. Ionic crystals have high melting points — some close to 2,000 degrees Fahrenheit. Even so, they readily dissolve in water to form a conductive fluid filled with free ions.

4. Covalent Crystals

Diamonds and silicon are examples of covalent crystals. These tend to be brittle and hard. These are also called “network solids” because all the atoms bond with all the others by sharing electrons at their corresponding lattice points.

Diamonds, zinc sulfide, amethysts, rubies, and other covalent crystals are essentially very large molecules unto themselves. They have high melting points, they do not conduct electricity, and they do not dissolve in water.

What Else Is Worth Knowing About Crystals?

There are two other crystal classifications worth knowing about: Piezoelectric crystals and ferroelectric crystals.

Piezoelectric crystals enter a state called “dielectric polarization” when exposed to an electrical field. In a nutshell, this means the positive charges align along the electrical field and the negative charges do not. This results in the “piezoelectric effect,” where the application of a force to a material results in an electrical field, and vice versa. It’s the phenomenon that turns the human voice into an electrical signal within a smartphone, for example.

Ferroelectric crystals change their very shape when subjected to electrical voltage. In the presence of outside force or stress, they also produce electricity. Transducers, filters, oscillators and ultrasound machines make use of this effect.

As we alluded to with our rock candy example, the crystallization of a material can be induced or it may occur naturally. Examples of natural crystallization include:

  • Snowflake formation
  • Honey crystallizing in a jar
  • Gemstone formation
  • Stalactite and stalagmites

Artificial crystallization uses any number of techniques to bring this process about on-demand. These include solvent layering, sublimation, solvent evaporation, solution cooling, or adding an anion or cation. Some biochemical particles, such as proteins, are more difficult to crystallize artificially than other inorganic or organic molecules.

Crystals Make Modern Life Possible

It’s not a stretch to say that crystals make the modern world go ‘round. We mentioned smartphones and imaging equipment already because ultrasound machines and sonar wouldn’t work without piezoelectric crystals. Neither could liquid crystal display (LCD) screens. Solar panels use crystalline structures to convert sunlight into electricity.

Plus, the chips in smartphones and computers all make use of semiconductors. A semiconductor is what we call crystals with circuits etched within them. Devices with gyroscopes, accelerometers, and compasses also require micro-manufacturing using a crystalline substrate. In fact, any digital electronic device — from wristwatches to the most powerful computers — typically contain a “resonance circuit” derived from a quartz crystal which synchronizes its operations.

Now that you know a bit more about crystals, you’ll probably start seeing them everywhere! And who knows what other technologies we’ll get to explore in the future based on these and other fascinating chemical interactions.

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Category: Chemistry

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Article by: Megan Ray Nichols

Megan Ray Nichols is a freelance science writer and science enthusiast. Her favorite subjects include astronomy and the environment. Megan is also a regular contributor to The Naked Scientists, Thomas Insights, and Real Clear Science. When she isn't writing, Megan loves watching movies, hiking, and stargazing.