What are Crystals? (Geology)
Crystals Simply Explained
The term crystals is widely used to describe rough or polished rocks, minerals and tumbled stones. The use of this word is likely to have come about because most but certainly not all of these materials are crystalline. A naturally occurring solid that's crystalline means it's made up of crystals. If a solid is crystalline it's a mineral.
So what are crystals in the true sense of the word?
Crystals are made up of atoms arranged in an orderly repeating pattern that extends in all three spatial dimensions. As atoms connect together they form molecules. When molecules come together they form a crystal.
The arrangement within the crystal is known as a crystal lattice. The process of crystal formation through crystal growth is called crystallisation.
Almost everything in the universe is made up of atoms. Atoms are far too small to be seen with the naked eye, more than one million would easily fit onto a pinhead. 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 image demonstrates an orderly repeating arrangement of atoms in a crystalline solid.
An easy way to explain the way atoms come together to form a crystal is to imagine a full box of eggs. All the eggs are neatly arranged and evenly spaced out in perfect 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 grow when a liquid cools and solidifies. The longer the cooling process the more time they have to grow. Crystals can also form from the precipitation from water. Water can only hold a certain amount of dissolved minerals and salts. As the quantity of the mineral increases it's no longer possible for it to remain dissolved in the water. The particles then come together to form a solid.
Obsidian which is a volcanic glass is a non crystalline or amorphous solid. This is because molten lava cooled so fast there wasn't time for crystals to grow.
Granite on the other hand is known for its large crystals. This is because the magma from which this rock forms cooled very slowly deep beneath the surface of Earth for 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 billions of atoms combined to form a crystal lattice. With the atoms 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. This is seen as flat faces 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. In fact this shape correctly 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 amorphous solids or mineraloids. Examples include obsidian, shungite, opal, moldavite and pearl.
If a material is not crystalline it's a rock not a mineral. Minerals are made up of crystals, rocks are made up of different minerals.
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. Both pictures are clickable and redirect to the original non-compressed photo.