What is a Crystal? (Geology)

Crystals Simply Explained

The geological definition of a crystal is a naturally occurring solid made up of atoms arranged in a regular repeating pattern.

In recent years the word 'crystals' has been adopted to describe rough or polished rocks, minerals and tumbled stones used for their metaphysical healing properties.

This has caused a significant amount of confusion when talking about crystals in the geological sense of the word.

When used in the metaphysical sense, the word encompasses anything used for its healing or spiritual properties.  In some articles man-made materials such as opalite and goldstone are also referred to as crystals.

This overlap in terminology has made it particularly difficult for those trying to get to grips with the difference between crystals (in the geological sense) and rocks and minerals.

The word is likely to have been adopted because most but certainly not all crystals are crystalline.  A naturally occurring solid that's crystalline means it's made up of crystals.

A naturally occurring solid that's crystalline will usually be a mineral.  A naturally occurring solid that's non-crystalline will most likely be a rock. As with everything in life, there are always exceptions.

So, what is a crystal in the true geological sense of the word?

A crystal is made up of atoms arranged in an orderly repeating pattern that extends in all three spatial dimensions.  When two or more atoms chemically bond together, they form a molecule.  When molecules stack together, they form a crystal.

The arrangement of atoms within a crystal is known as a crystal lattice.  

Virtually everything in the universe is made up of atoms.

Atoms are way too small to be seen with the naked eye. 

Using A.I technology I asked how many atoms would fit onto a pinhead.  This is the answer I was given;

To give you a rough estimate let's consider the size of a typical pinhead which is approximately 1.5 millimeters in diameter. Assuming we're dealing with atoms that are approximately 0.1 nanometers in diameter we can make some calculations.

First let's calculate the volume of the pinhead. The volume of a sphere can be determined using the formula V = (4/3) * π * r^3, where r is the radius. With a pinhead diameter of 1.5 mm the radius would be 0.75 mm (or 0.75 * 10^-3 metres).

Plugging the values into the formula we get V = (4/3) * π * (0.75 * 10^-3)^3 ≈ 1.767 * 10^-9 cubic metres.

Now let's consider the atomic volume. Assuming atoms are spherical we can calculate their volume using the formula V = (4/3) * π * r^3, where r is the radius. If we approximate the atomic radius to be 0.1 nanometres (0.1 * 10^-9 meters) we can calculate the atomic volume as V = (4/3) * π * (0.1 * 10^-9)^3 ≈ 4.189 * 10^-29 cubic metres.

To estimate the number of atoms that can fit on the pinhead we can divide the volume of the pinhead by the volume of a single atom: (1.767 * 10^-9) / (4.189 * 10^-29) ≈ 4.216 * 10^19.

Therefore, as a rough estimate, it's possible for around 4.216 * 10^19 atoms to fit on a pinhead but do note that this is a rough calculation and can vary depending on the assumptions made.

I hope you followed that!  I then asked what the figure 4.216 * 10^19 equates to.  This was the answer; 

42,160,000,000,000,000,000 (42 quintillion, 160 quadrillion). 

That gives you an idea of the size of an atom. 

According to the late Professor Stephen Hawkings approximately fourteen billion years ago the entire universe would have been the size of a single atom.

The following diagram is an example of an orderly repeating arrangement of atoms within a crystalline solid.  When it's explained that a crystal is made up of an orderly repeating arrangement of atoms this is what it means. 

An easy way to explain the way atoms come together to form a crystal is to imagine a full box of say twenty four eggs.  All the eggs are perfectly arranged and evenly spaced out in parallel rows and columns.  This could be described as an orderly repeating arrangement of eggs.

The size of the formation is dependant on the size of the box.

Likewise the size of a crystal is dependant on how much time it has to grow and the amount of space that's available.

Crystals generally form when liquids cool and solidify.  The longer the cooling process the more time they have to grow.

As a liquid cools and solidifies the particles within it arrange themselves in an ordered and repeating pattern.  This process of crystal formation through crystal growth is called crystallisation.

Crysallisation creates the crystal structure. 

Crystals can also form from the precipitation from water.  Water can only hold a certain amount of dissolved minerals and salts.  As the quantity increases it's no longer possible for them to remain dissolved.

Once the solution reaches its maximum capacity the particles come together and arrange themselves in a repeating pattern.  In doing so they form a solid crystal. 

The particles coming together to form a solid is known as precipitation.

Obsidian, which is a volcanic glass, is a non-crystalline solid.  This is because the molten lava from which it was produced cooled so fast there was no time for crystals to grow.

Granite is a rock known for its large crystals.  This is because the magma from which it formed cooled very slowly over millions of years.

Crystals have smooth surfaces known as faces and straight edges.  Whilst some are large enough to be seen with the naked eye an example being quartz, others are microscopic.

Some crystals are so small it's even difficult to see them under high magnification.

Irrespective of size, the one thing that remains unchanged is crystals within the same mineral varieties have exactly the same crystal structure.  Should this change the result will be a different mineral.

The smallest piece of quartz is made up of quadrillions of atoms combined to form a crystal lattice.  With them coming together in an orderly repeating arrangement they form a crystalline solid.

Quartz is made up of silicon and oxygen atoms.  An atom is the smallest unit of matter that forms a chemical element.

When sodium and chlorine atoms join together in an orderly repeating pattern they form molecules. The molecules stack together to form a crystal. Once combined, the crystals form a chemical compound called sodium chloride. This mineral called halite is better known as salt.

Another type of crystal is a snowflake.  Snow is almost pure crystallised water.

The repeating three dimensional pattern of the atoms defines the external shape of the crystal.  These flat faces are arranged in geometric forms.  Geometric means patterns made up of straight lines and shapes such as squares, triangles or rectangles.

There are seven different arrangements of crystal symmetry.  All minerals belong to one of these groups.  All crystals are symmetrical because they're built up of repeating geometric patterns.

The cube shapes often seen in pyrite and also the mineral fluorite are crystals.  This shape which is known as isometric is one of the most common and simplest shapes for a crystal.

When the arrangement of atoms within a naturally occuring solid do not form an orderly repeating pattern it will not be crystalline.  Solids void of a significant crystal structure are known as mineraloids. 

If a naturally occurring solid is non-crystalline, it will likely be a rock not a mineral.  Minerals are made up of crystals, rocks are made up of various minerals.

Article Pictures

Our first picture is a cluster of quartz crystals. Photo courtesy of Stan Celestian.

The second is cube-shaped fluorite crystals on quartz.  Courtesy of Ron Wolf.  The final photo of gold crystals is also courtesy of Stan Celestian.

All pictures are clickable and redirect to the original non-compressed photos.